Kopie Van Profielwerkstukcamielverschoor

Surface tension

Basilisk or Jesus Lizard makes use of surface tension to travel over water Section Paper Physics and Chemistry Camiel Verschoor 6VT

Content Page 1. Summary ............................................................................................................................................. 3 2. Introduction .......................................................................................................................................... 4 2.1 Explanation of surface tension....................................................................................................... 4 2.2 Effects of surface tension in everyday life ..................................................................................... 6 2.3 Influencing surface tension and the effect of this .......................................................................... 7 2.4 Methods to measure the surface tension....................................................................................... 8 2.5 The opposing force ........................................................................................................................ 9 2.6 The goal of my section paper ........................................................................................................ 9 3. The experiments................................................................................................................................ 10 3.1 Experiment report: The best cleaning product............................................................................. 10 3.2 Experiment report: Building a robot strider for humans ............................................................... 11 4. Results............................................................................................................................................... 12 4.1 Results of the Vrije Universiteit of Amsterdam ............................................................................ 12 4.2 Results of my own build water strider .......................................................................................... 13 5. Discussion ......................................................................................................................................... 15 5.1 Results of the Vrije Universiteit .................................................................................................... 15 5.2 Results of my own built water strider ........................................................................................... 15 6. Conclusion ......................................................................................................................................... 17 7. Evaluation .......................................................................................................................................... 18 8. Logbook ............................................................................................................................................. 19 9. Literary sources ................................................................................................................................. 20

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1. Summary This section paper describes the methods used to measure surface tension and the accuracy of these methods. This will be illustrated by two experiments that show the best cleaning brand and whether or not it is possible for humans to stand or walk on water. Before explaining the execution of these experiments first the following points are discussed:  The explanation of surface tension  Consumers of surface tension  The theoretical discoverer of surface tension  The ways to measure surface tension  Influencing surface tension and the effect of this  The methods of the above given experiments After presenting the results of the experiments, the results will be discussed and then I will be able to conclude what the outcome is of the performed experiments.

2. Introduction Have you ever wondered how lizards and several other animals can walk on water without perishing in the water? Have you ever wondered how Jesus could walk on water? Have you ever wondered if it is possible for humans to walk on water? Then you are reading the right section paper because this paper will be explaining this phenomenon widely. The reason for this is simply caused by some mysterious thing called surface tension. In this text the following points will be addressed: an explanation of surface tension, consumers of surface tension, the theoretical discoverer of surface tension, the ways to measure surface tension and influencing surface tension and the effect of this. Furthermore the experiments connected to answer the main question of this paper are explained. After that the results will be discussed and a conclusion is made. At the end of the paper I will look back on the process of writing this essay, what I learned and what I liked to be done better; in short, the evaluation of this assignment. I hope that you, the reader, will find this essay informative, useful and maybe, just maybe, a little fun to read.

2.1 Explanation of surface tension Surface tension is the property of the outer layer of a fluid that causes the surface of a liquid to be attracted to another surface. This appearance is caused by the intermolecular forces, van der Waals forces, between the liquid's molecules. The van der Waals force is a weak chemical bond which is the attractive or repulsive force between molecules. The force is caused by the difference in the polarity, the difference between the positive end and negative end, of the molecules. As example I pick a molecule which has a temporary polarity being approached by a non-polar molecule meaning that it does not contain polarity.

Figure 1: The symbol

(read as 'delta') means slightly.

As the molecule approaches the electrons of the right hand molecule will be attracted by the slightly positive side of the other molecule. This causes induced dipole in the approaching molecule which means that the right molecule also has polarity in such a way that the +, slightly positive, side is attracted by -, slightly negative, side of the approaching molecule.

Figure 2: The original molecule with a temporarily polarity induces the other molecule causing the molecules to have a stronger binding.

The electrons in both molecules keep moving therefore the slightly negative and positive ends keep on changing while they are attracted to each other.

Figure 3: Two molecules held together by the van der Waals force.

As long as the molecules are close to each other this process goes on in order to maintain the attraction between the molecules. The small attraction force between the molecules is called the van der Waals force.

Figure 4: A roster of molecules held together by the van der Waals forces

The van der Waals force is not limited to only two molecules. As long as the molecules are close enough to each other this process happens. In a liquid therefore all molecules pull their neighbouring molecules causing force in all direction. This means that all molecules in a liquid pull their neighbouring molecules resulting in that molecules on the surface of the liquid are pulled inwards by other molecules deeper in the liquid and are not attracted with the same force by the molecules of the neighbouring medium like air. This causes molecules at the surface have an inward force that is balanced by the resistance to compression of the liquid. The reason for this is the driving force to make the surface area smaller until it has smallest surface possible. Due to this phenomenon the surface molecules can handle a bigger force. The molecules in contact with another molecule have a lower state of Figure 5: Surface molecules energy then when if there was none. This means that the molecules can handle a bigger force on the surface contain more energy because they are not completely surrounded by other molecules. Normally there is a process where molecules move from the gas into the liquid and other way around, which is energy neutral. Due to the characteristic of a liquid to minimize its surface area, there are more molecules in the air then in the liquid if this occurs. Therefore, there are molecules forced to move inside the liquid due to lack of space. This is not energy neutral since the molecules in the air have less neighbouring molecules then in the liquid resulting in energy that causes the reduction of the surface area. To increase the surface area energy, work, is needed to break the chemical bonds in order to restore the balance between the liquid and the gas. This explains why several objects and animals with a higher density then water seem to float on water. The potential energy of these are hold by an resistant force of the chemical bonds. This leads to the following definition of surface tension: Gamma (γ) represents surface tension and is measured in energy per area (J/m2)

Another definition can be explained by the experiment shown in figure 6 where a fluid frame, made of wires, is stretched. In the frame the bow is moveable so the film can be pulled apart. In order to move the bow
s meter a force F is needed, this means work, force times distance, is needed. Due to the work a new surface comes into existence. The new surface is
A = h*
s * 2. The two in the formula is there because the new surface is formed on the front and back of the frame. From this I can conclude that the surface work is also: W = F *
s = F *
A / (2*h). Dividing it by the Figure 6: Film experiment. Note: If change of surface gives us the following formula for surface the materials are not affected by the tension: W /
A = F / (2*h) = γ. This means that the surroundings temperature the second definition is the following: surface tension cause the materials to cool down and if the film gets smaller it warms up showing the Gamma (γ) represents surface tension and is measured energy created by surface tension.

in force per length unit (N/m).

(The length is defined in 2 times the height in the experiment)

Due to the small lengths used in measurements scientists often use the unit dyne instead of centimetres. 0.001 N/cm equals 1 N/dyn. This means that the two definitions are closely related since force per unit length equals energy per difference in area.

2.2 Effects of surface tension in everyday life In everyday life we are continuously confronted with surface tension since it is a natural phenomenon. We can see the effects of surface tension in everyday life since several animals use this property and we live on a earth that consist of 70% out of water meaning it is nearly unavoidable. Most of the effects we face everyday happen with water. These are namely, the following:  Water dripping from a tap. The water is attracted by the gravity force, which stretches the liquid in two separate droplets. The surface tension gives the spherical shape to the two droplets.  Soap bubbles. They are made of surfactants, which reduces the surface tension of water making it possible to create soap bubbles.  Oil separating from water. This is caused by surface tension between dissimilar liquids (interface tension).  Water beading on a leaf. Water is not attracted by the wax and strongly to itself therefore the water clusters into droplets. Due to the surface tension these have a round shape.  The drops of wine on the side of a glass. Water and ethanol have a different surface tension causing complex interaction.  Flotation of objects in water. This occurs when non-wettable objects have a smaller weight then the forces arising from surface tension. Animals make also use from this phenomenon.

Figure 6: A water strider

Figure 7: When a water strider pushes its middle legs backward, momentum is produced in the water via hemispherical vortices, sending the animal forward.

Animals also make use of surface tension in order to stand and travel on water. The water strider (shown in figure 6) is one of the animals that make use of this force. To stand on the water the water strider makes use of the intermolecular forces to heave the gravity force. To create more stability the water repelled hairs on the legs of the water strider can hold air bubbles that contributes to the buoyancy of the water strider. Moving on water is a different phenomenon since therefore the water strider is pushing his legs backwards to produce a momentum in the water by hemispherical vortices (shown in Figure 7). Hemispherical vortices are cyclical movements under water, which give the water strider a forward force if he pushed his legs backwards.

2.3 Influencing surface tension and the effect of this In some situations the surface tension needs to be reduced causing the water to spread out over a bigger surface. The reduction of surface tension is caused by chemicals named surfactants. The process of reducing surface tension is a chemical reaction called saponification. This process is commonly used to clean the dishes because soap is a surfactant. The molecules of a surfactant consist of long carbon chains and hydrogen atoms. Soap molecules have a unique ability since on one side of the molecule has a configuration of atoms which like to be in water (hydrophilic) and on the other side a configuration that does not like to be in water (hydrophobic). Although the hydrophobic side does not like water it easily attaches to grease. In a cleaning process, the hydrophobic end of the molecule attaches itself to the grease on the dirty plate, letting water seep underneath. The dirty plate is cleaned since the dirty particles are pried loose due to the surrounded surfactant molecules and the flood of water is causing it to move.

Figure 8: Soap on a greasy plate.

The surface tension of water is reduced, when soap is added to the water. This phenomenon is caused by the hydrophobic side of a surfactant molecule since this end tries to squeeze their way through the water molecules to the surface. This separates the water molecules from each other due to the intervening surfactant molecule meaning that the distance between the water molecules increases. A larger distance between water molecules causes a weaker van der Waals force and therefore, a decrease of surface tension. Surfactants lower the surface tension making it possible to create bubbles since a lower surface tension protects a bubble from evaporation. It also prolongs the life time of an individual bubble. For this reason, we see bubbles if soap is added to water.

The temperature can affect the surface tension of liquids since higher temperatures cause molecules to move faster. The faster movement makes it harder for water molecules to stick together causing the forces produced by surface tension to be weaker. Therefore, it is important to measure the temperature of a liquid when measuring the surface tension in order to compare the results.

2.4 Methods to measure the surface tension There are various ways to measure surface tension since it manifests itself in many effects. The best method depends upon the nature of the liquid being measured, the conditions it is measured and the stability of the surface. In order to examine the best method for my section paper I have asked several experts on this subject. Firstly, the Du Noüy ring method is a traditional method to measure surface tension. The maximum force of the liquid on the ring is measured. The advantage of this is that the wetting properties of the liquid have little influence on the results. The disadvantage is that it is not accurate to measure the exact surface tension, which is what I want to measure in my experiments. Accurate results are needed in order to specify the maximum weight I can put on the surface of the liquid. Secondly, the Wilhelmy plate method is a method to check surface tension over long periods of time. This method's goal is to find out the force that is created due to the wetting. This method takes a long period of time therefore it is not suitable for my experiments. Thirdly, the bubble pressure method is a measurement technique to measure surface tension for a short period of time. The maximum pressure of each bubble is measured. The tensiometer makes use of this method and is a handy apparatus to measure surface tension. Accept measuring the surface tension it also measures the temperature of the liquid since that influences the surface tension. I personally prefer this method to measure the surface tension in my experiments since the results can be very accurate depending on the accuracy of the tensiometer. Fourthly, the capillary rise method measures the height solution reaches inside the capillary. The end of a capillary is immersed into the solution and the height is related to the surface tension. By measuring the height of the liquid in the capillary the surface tension can be measured. This method has as disadvantage that it is not accurate. Fifthly, the Sessile drop method determines the surface tension and density by placing a drop on a substrate and measuring the contact angle. The contract angle can be used to calculate the surface tension of the droplet. This measurement method is again not accurate enough for the exact experiments I want to do. Sixthly, test ink method is a new method to measure surface tension. There are several dyne pens and each has its own surface tension value. If the test ink disappears in three seconds the value of this pen is the surface tension of the liquid. This is done by wetting a piece of film with the liquid where you want to know the surface tension from. This method is also inaccurate and therefore not handy to use for my experiments. The best and the most accurate method is the bubble pressure method since this method is the most accurate method, an important criterion. The experts of Technex bv recommended this method for my two experiments.

2.5 The opposing force The force that opposes the surface tension is the gravitational force. This explains why some animals can walk on water and others not. The gravitational force is the attracting force that two masses practice. The gravitational force (in newton) can be defined in mass (in kilograms) times gravitational constant (in m/s/s).

Fg = m* g The forces created by surface tension heave the gravitational force if the mass of the object is not too heavy. Figure 9 shows the balance between the two forces.

Figure 9: The force (Fs) produced by surface tensions heaves the gravitational force (Fz).

2.6 The goal of my section paper The main and first goal of my section paper is to find out if it is possible to build a structure for humans, which let them float on water using the forces created by surface tension. The possibility of this I want to find out by building first a prototype of a water strider in order to see how this animal floats on water using the surface tension. Then I can calculate how big surface my structure has to cover in order for a human to stand on water. The second goal of my section paper is to find out what the best brand is to decrease the surface tension. This interests me since this will show if the price and quality difference is correct. If the outcome has big differences, I will be contacting a consumers' association in order to inform them about the results. Although my main goal I want to find out sounds impossible I will give it my best since I am interested in the abilities of this small force.

3. The experiments In this part of my section paper both the experiments are described by prepared experiment reports. This shows how I prepared my experiments for people that want to check if they measure the same results as I have.

3.1 Experiment report: The best cleaning product Aim To find out what soap brand influences the surface tension the most. Hypothesis Soap of Ajax influences decreases the surface tension the most since it contains more surfactant molecules per cubic centimetre. Materials Dynamometer Tap water Distilled water

Measuring glass Adjustable Labjack Plastic holder

Ajax Cillit bang Driehoek

Method Firstly, put 1968,75ml water in the measuring glass. Secondly, add 31,25ml of soap. Thirdly, measure the water its surface tension. Fourthly, add a known amount of soap into the measuring glass. Fifthly, measure the solution its surface tension. Lastly, repeat this experiment several times with different brands.

3.2 Experiment report: Building a robot strider for humans Aim To build a object like the robot strider for humans to travel to the other side of the canal using surface tension Hypothesis I think it is impossible for a human to stand on water using the forces produced by surface tension since our mass is too big and the forces created by surface tension are too small. Materials Wire Tin Soldering iron Method Firstly, take a sample from the canal Secondly, measure the surface tension of the water in the canal Thirdly, calculate the maximum mass per area the forces produced by the surface tension can hold. Fourthly, built a small prototype Fifthly, look if it is possible to build one, which can divide my mass on the water.

4. Results After preparing my experiments I did them and I made some useful measurements for my research questions. I came up with the following tables and photos of the experiments.

4.1 Results of the Vrije Universiteit of Amsterdam On Thursday the ninth of April I went to the Vrije Universiteit to do my measurements for the experiments I came up with. I had to use a different method since no one could supply a tensiometer therefore I used a method similar to the Du Noüy ring method. After some small instructions of the teacher Jos Kragtwijk I found the following results with the following measurement construction:

Figure 10: Note: The thin wire is to keep the plastic holder in balance

Figure 11: The 3 soaps.

Figure 12: The effect of surface tension.

Measurement information of the plastic holder Diameter plastic holder: 8 centimetre Mass plastic holder with thin wire 9,76 gram Mass plastic holder without thin wire 8,31 gram Perimeter plastic holder 25,1 centimetre Table of surface tension measurement results with water and soap

Name: Driehoek Cillit Bang Ajax Distilled water Canal water Tap water

Measurement 1 (gram) Measurement 2 (gram) 11,76 grams 11,26 grams 11,26 grams 10,51 grams 10,76 grams 11,01 grams 13,36 grams 13,26 grams 10,81 grams 10,31 grams 13,66 grams 13,16 grams

Table of calculated results Name: Force (N) Driehoek Cillit Bang Ajax Distilled water Canal water Tap water Concentration soap in the soap - water solutions Price Volume Surface tension (After adding the soap)

Measurement (gram) 10,76 grams 10,76 grams 11,01 grams 13,16 grams 10,31 grams 13,66 grams

Average: 11,4 gr. 10,8 gr. 10,9 gr. 13,2 gr. 10,5 gr. 13,4 gr.

Surface tension γ = F / (2*l) (N/m) 0,016 N 0,251 m 0,032 N/m 0,011 N 0,251 m 0,022 N/m 0,012 N 0,251 m 0,024 N/m 0,034 N 0,251 m 0,067 N/m 0,021 N 0,251 m 0,041 N/m 0,036 N 0,251 m 0,071 N/m 1/128 of the original concentration (31.25 ml soap and 1968.75 ml distilled water in every solution) Driehoek € 2,19 725 ml 0,032 N/m

Length (m)

Cillit Bang € 4,10 750 ml 0,022 N/m

Ajax € 1,30 750 ml 0,024 N/m

4.2 Results of my own build water strider These are the results I found while building my water strider since several calculations have to be made before constructing the water strider

Figure 13 and 14: Close ups of my water strider

Specifications of the water strider Mass Length in contact with water Lowest surface tension it can stand on Surface it can stand on Movie link

4 grams 68,4 centimeters 0.057 Newton per metre Water and distilled water http://www.youtube.com/watch?v=eFDqBs9AloY

Table of calculations Mass Force Length (that needs to be in contact with water) by a surface tension of 0,040 N/m (Canal water)

Human (me) 70 kg 686,7 N 17167 m

Prototype 0,004 kg 0,04905 N 0.981 m

5. Discussion After a long journey with many obstacles I finally found these results. The results are accurate enough for answering my research questions. However, the results can be more accurate if a more accurate method was available. Apart from this the results are accurate enough for the experiments I did. Here, I will discuss the following two things in this chapter: the problems I have experienced during my research and the results that were measured in experiments will be compared to known data.

5.1 Results of the Vrije Universiteit First of all, due to the scarce amount of tensiometers in the neighbourhood the bubble pressure method could not be used. The method I used instead was a variation on the Du Noüy ring method. I did several measurements on the same solutions in order to get a more accurate result. This method measures the surface tension by measuring the pulling force the solutions have on the plastic holder. Furthermore, there were not any difficulties while doing this experiment and I found some nice results presented in the table below. Table of calculated results Name: Force (N) Driehoek Cillit Bang Ajax Distilled water Concentration soap in the soap - water solutions

Length (m)

Surface tension γ = F / (2*l) (N/m)

0,016 N 0,251 m 0,032 N/m 0,011 N 0,251 m 0,022 N/m 0,012 N 0,251 m 0,024 N/m 0,034 N 0,251 m 0,067 N/m 1/128 of the original concentration (31.25 ml soap and 1968.75 ml distilled water in every solution)

The manual way of doing this experiments can have caused some less accurate results. Since surface tension is a really small force, the chance is bigger to make a mistake while measuring it. The known data say that distilled water has a surface tension 0,0728 N/m at a temperature of 20 degrees Celsius. Although my measured result is close to the known data, I can say that this method should not be used by specific measurements. Despite this I think the results are accurate enough to say that Cillit Bang is the best soap to clean your house. It decreases the surface tension with a factor of 2/3. However, Ajax is much cheaper and decreases the surface tension nearly with the same amount. Therefore, Ajax is the winner in the end since it is three times cheaper then Cillit Bang. In short, the advertisements of Cillit Bang are true but it is more advantageous to use Ajax looking at the price/quality difference.

5.2 Results of my own built water strider This experiment was the most challenging part of my section paper. I started off by just reconstructing a water strider making use of steel wire. From the descriptions found of this small insect I concluded that I needed large legs in comparison with the body. After soldering the feet onto the body of the water strider I had to check if the legs were straight enough since otherwise the contact surface is too small. After tuning the water strider it could float because of the surface tension. This is what I filmed in the movie link. However, after building the water strider I measured the surface tension of the canal water and from that result I can conclude that my water strider is too heavy and is not heaved by the surface tension.

Below you can see the results of the calculations and measurements: Table of calculated results Name: Force (N) Length (m) Canal water Tap water Mass Force Length (that needs to be in contact with water) by a surface tension of 0,040 N/m (Canal water)

0,021 N 0,036 N Human (me) 70 kg 686,7 N 17167 m

0,251 m 0,251 m Prototype 0,004 kg 0,04905 N 0.981 m

Surface tension γ = F / (2*l) (N/m) 0,041 N/m 0,071 N/m

The surface tension in the canal is too low to heave my water strider. The minimal surface tension my water strider can float on is 0,057 N/m. The calculations also show that it is impossible to build a structure only supported by the surface tension for humans because then there is more then 17 km of contact surface needed. On the other hand for small things like my metal water strider, if I would rebuild it with longer legs, it would be possible to stand on the canal water. My plan was to motorize the water strider so it could move around in water. This could not be done since the small vibrating motor including battery with a mass of 15,8 grams is simply too heavy. The canal water has a lower surface tension since the citizens of Amsterdam throw several things into the canal. Some of those contain surfactants, which lower the surface tension of the water in the canals is lower compared to tap water. Soaps for example minimize the surface tension with a factor of 2/3. This means that realizing a vehicle for humans supported by surface tension is impossible. Therefore we should stick with our ‘real’ invented floating objects like the boats.

6. Conclusion So I can conclude that due to my extensive research in this subject I can answer the two main questions of this section paper. First of all it is impossible to create a water strider like creature since the surface tension force is too small. Therefore, I can accept my first hypothesis since I predicted that it is impossible to build a structure for humans that only makes use of the surface tension. Secondly, this section paper answers also my second question of my section paper, namely, what the better brand is. The advertisements about Cillit Bang are totally true and it decreases the surface tension the most. On the other hand it is not the cheapest soap and therefore the cheapest soap, Ajax, is the overall winner. Therefore, I can accept my second hypothesis since I predicted Ajax to win this confrontation. So at last we now know that surface tension is the attractive property of the surface of a liquid.

7. Evaluation Here I will evaluate my section paper for physics and chemistry, looking back on how I eventually could improve for the next time. On the main I find I did not do well on this assignment. My planning was awfull and several problems occure while doing this assignment. These are the result of my bad planning and some unlucky incidents. The research question and the sub questions did not really bother me and after all I think I am proud of the end product I produced. Given that many things still went wrong. First of all, I started off too late since I already was late with choosing the subject and research question for my section paper. A better start was not possible, but I was able to find some interesting subjects on the website of the Vrije Universiteit and I came across the interesting subject 'surface tension'. After handing in my section paper form, research question and sub question, I started researching this subject and came across some information. This information was enough to prepare a couple of experiment and to write parts of my introduction. I handed in a concept of my section paper in December in order to look whether or not I was going the right way. The teacher's response was quiet positive but there was still a lot of work to be done. The busy weeks before the test week and the other assignments did not allow me to work on my section paper. During these days I tried to build my water strider for my second experiment and was trying to find out where to get the materials for my experiments. After contacting several companies, I started contacting university for additional information. Due to this I came in contact with a teacher of the Vrije Universiteit, 'Jos Kragtwijk'. He gave me clear information about the definitions of surface tension. Furthermore, I could not find the materials for my experiments anywhere and at last I contacted the teacher of the Vrije Universiteit. Three weeks after the deadline of the school I could finally do my experiments for my section paper. From the above things I can say that I should plan better the next time. This was not the only problem I had since I also experienced problem with understanding the definition of surface tension. After an explanation of my dad and the teacher of the university I completely understand the definition of this little force. After all, I have learned many different things about my subject and what happens if you cannot plan that well. Overall, I enjoyed doing this section paper since I explored a new interesting area of knowledge, which explains me why several things happen in everyday life. This assignment is now completed, so this paper is at its end (only the logbook and literary sources are left), so I hope after reading this paper, found this paper informative, useful and maybe, just maybe, a little fun to read.

8. Logbook When

How Long

What

31 October 2008

1 Hour

Brainstorming session Choosing the subject of my section paper and making the main research question and writing a email to inform the physics teacher.

6 November 2008

1 Hour

Researching the subject and collecting information.

15 November 2008

1 Hour

Making sub questions that tie up with the main research question and finding out what experiments to do.

26 November 2008

1.5 Hours

Collecting information answering the sub questions and starting to prepare 1 experiment. Created the front page of the section paper

14 December 2008

1 Hour

Writing the summary and started writing the introduction

15 December 2008

1 Hour

Writing part of the explanation of surface tension.

16 December 2008

2.5 hours

Writing the reports and finished the explanation of surface tension. Collected information regarding the other sub headings in the introduction.

17 December 2008 till

Remaining

11 April 2009

Hours

Remaining work

9. Literary sources Name:

Link:

Kennislink

www.kennislink.nl

Chemistry guide

www.chemguide.co.uk

Massachusetts Institute of Technology

www.web.mit.edu/1.63/www/Lec-notes/

Boussey control www.boussey-control.com Krüss www.kruss.info Unimeter www.unimeter.net Laude www.lauda.de Technex BV www.technex.nl Hyperphysics www.hyperphysics.phy-astr.gsu.edu Ilpi www.ilpi.com/genchem/demo/tension/ Efunda www.efunda.com Physics 5th edition by Giancoli Physics 5th edition by Resnick Jos Kragtwijk Vrije Universiteit Amsterdam teacher physics