01 How To Solve Problems

HOW SHOULD NUMERICAL PROBLEMS BE FACED 1- Read the problem and be sure you have found and understood the question (identify the question or questions) 2- Read it again but drawing a labelled diagram showing the situation. This is absolutely important for you to visualise what is happening. 3- Make a two-column chart listing all the data and unknowns. 4- Change if necessary all units into a consistent set: if you are using “amperes” (coulombs/second) check that times are in seconds and not in hours). Avoid working at the same time in metres, kilometres, hours and seconds. It is really messy and you will sure get to a meaningless result. 5- Now start thinking: what is this all about? Find what part of physics the problem deals with: electricity, pressure, uniform movement, projectile movement, etc. 6- Write down all the relevant equations you know or find on this topic (equations are usually and wrongly called “formulae”). 7- Now start solving: replace symbols by data, relate different equations, etc. You will need as many equations as unknowns you have. You will frequently find that you have two unknowns in every equation! Do not commit suicide!! Just think they form a system and proceed as you have learned in Maths. 8- Once you get to a result, see that it corresponds to what you have been asked for. If you were asked for a length, be sure the answer is not a speed. This means that you should be aware of the units. Many times this silly thing happens and you just do not notice. 9- Write the answer clearly and distinctly so that there is no doubt that it is really your result. 10- Pray to your God . Human sacrifices are not accepted (especially the teacher is not an appropriate victim). Before you do anything STUDY THE THEORY: It is not enough but is the only way to succeed. Addenda (just for 1P pupils) •

Use this procedure to solve the following problem:

A car moves at a steady speed along a straight highway. At 2:30 pm it is at 174 km from B.A. and at 3:45 it gets to Arroyo Seco (some 300 km from B.A.). Find its speed and the point at which it started moving if it leaved at 1:45 pm. • Solve the last two problems in the photocopy. Consider these two basic issues: a- There are two objects moving so you have two sets of charts (one corresponds to object one and the other one to object two) but one diagram. Identify these sets of data properly. b- The moment and place at which objects meet (tf and xf) is the same for both. c- Choose one frame of reference, be it town A or B, but keep it.