Measuring Motion Ch5.1 8th

Chapter 5 Section 1

Describe the motion of an object by the position of the object in relation to a reference point.  Identify the 2 factors that determine speed.  Explain the difference between speed and velocity.  Analyze the relationship between velocity and acceleration.  Demonstrate that changes in motion can be measured and represented on a graph. 



Between the 16th and 18th centuries

 Basis 

for modern science:

Ideas from: Copernicus  Galileo  Descartes  Sir Isaac Newton (1642-1727) 

 Describe

your position in the classroom using a reference point and a set of reference directions.  Write

your answer on a sheet of paper…



Watch an object in motion…



You are observing the object in relation to another object that appears to stay in place.



The object that appears to stay in place: Reference Point



When an object changes in position over time relative to a reference point: Motion

 North,

South, East, or West

 Earth’s

surface, trees, buildings…

 Moving

objects can also be reference

points  Ex:

Hot-air balloon relative to a flying bird

http://image.tutorvista.com/content/motion/frame-of-referenceexample.jpeg

 Speed:

the distance traveled by an object divided by the time taken to travel that distance.

 SI

Unit for speed: meters per second (m/s)

 Other

common units: km/h, ft/s, mi/h

 Most

of the time, objects do not travel at a constant speed

 It

is useful to calculate average speed

average speed = total distance/total time

 What

is your average speed if you take 0.5h to jog 4,000 m?

 If

the average speed of a car is 110km/h, how long will it take the car to travel 715km?

 8,000  6.5h

m/h



Distance vs. Time

 Distance  Time

is plotted on the y-axis

is plotted on the x-axis

 Straight,

speed

 Slope

diagonal line indicates constant

of the line is the average speed

http://www.golfranger.co.uk/images/distance_ti me.gif



Page 120 of text. Calculating average speed.

 Go

through Step 1 & 2 together

 Try

#1-3 ‘Now it’s Your Turn’

 Velocity:

the speed of an object in a particular direction

 Velocity

must include a reference direction (different from speed!)

 Example:

600km/h south, not 600km/h.

 Rate

of change of an object’s position

 An

object’s velocity is constant only if it’s speed and direction do not change

 Constant

velocity is always motion in a straight line

 An

object’s velocity changes if it’s speed or direction changes

 Change

in velocity:

 Car

traveling 60mi/h north speeds up to 70mi/h north

 Change  Car

in velocity:

traveling 60 mi/h north, stays at the same speed but begins to travel east



Combine velocities to find the resultant velocity

 Text 

example pg. 121 Fig. 4

A person’s resultant velocity on a moving bus 15 m/s east + 1 m/s east – 16 m/s east  15 m/s east – 1 m/s west = 14 m/s east 



Consider these statements (true/false?) 

If you slow down on your bicycle, you accelerate. (T/F?)

 If

you ride your bicycle at a constant speed, you cannot accelerate. (T/F?)

 Changing

the speed & changing direction of your bicycle are both examples of acceleration. (T/F?)



Acceleration: the rate at which velocity changes

 Velocity

changes if speed changes, direction changes, or if both change.

 Therefore,

an object accelerates if any of these changes occur



Positive acceleration: increase in velocity

 Negative

acceleration: decrease in

velocity  How

fast velocity changes = acceleration

 Average

acceleration = final velocity – starting velocity / time is takes to change velocity



a = v2 - v1 t



Units: meters/sec/sec or m/s²

http://www.arocspeed.com/images/img-acceleration-bolt.jpg

 Try 

these…

You are riding your bike at 9 km/h. Ten minutes later, your speed is 6km/h.

 You

ride your bike around the block at a constant speed of 11 km/h.

 You

ride your bike in a straight line at a constant speed of 10 km/h.



Calculating acceleration: 

A plane passes over point A at a velocity of 240 m/s north. Forty seconds later, it passes over point B at a velocity of 260 m/s north. What is the plane’s average acceleration?

http://www.antonineeducation.co.uk/Physics_AS/Module_2/Topic_1/graph_1.gif

 Y-axis:

velocity (m/s)  X-axis: time (s)  Increasing

velocity with time: segment

0-A  Constant speed: A-B  Decreasing velocity with time: B-C http://www.revisioncentre.co.uk/gcse/maths/travelgrap h2.gif

http://www.scienceclass.net/Notes/images_7th_notes/dist_time_graph.gif

 An

object traveling in a circular motion is always changing direction.

 Because

its velocity is always changing, it is accelerating!

 Centripetal

acceleration

 What

distinguishes the measurement of speed from that of velocity and acceleration?

 What  How

is centripetal acceleration?

do you calculate average speed? Acceleration?