Velocity-time graph

0 1 2 3 40

5

10

15

20

25

Gradient gives

acceleration

Velocity/

Time/s

Velocity-time graph

0 1 2 3 405

1015202530354045

Velocity/

Time/s

Velocity-time graph

0 1 2 3 405

10152025303540

Velocity/

Time/s

Velocity-time graph

0 1 2 3 405

10152025303540

Velocity/

Time/s

Example 8:Describe the motion of the object.

0 1 2 3 4 5 6 7 80

102030405060708090

100Velocity/

Time/s

Example 8: From to , the object is moving from rest

with increasing velocity. The object has uniform acceleration of .

From to , the object is moving with constant velocity of .

From to , the object is moving with decreasing velocity. The object has uniform acceleration of until it comes to a stop.

Need a break?

Velocity-time graphA car moves at constant velocity of for a period of 30 s. What is the displacement of the car?

Displacement = velocity time= 25 30= 750 m 0 10 20 30 40

05

10152025303540

Velocity/

Time/s

Area under v-t graph is

displacement

Velocity-time graph Gradient of v-t graph gives acceleration. Area under v-t graph gives

displacement.

Example 9:The graph shows the velocity of the object over a period of time. Calculate the average velocity of the object from 0 s to 8 s.

0 1 2 3 4 5 6 7 80

102030405060708090

100Velocity/

Time/s

Acceleration due to gravity Video on golf ball and bowling ball Question: What does this tell us about how

objects fall when released from the same height? The change in velocity is the same hence they

take the same time to reach the ground regardless of their masses. (Ignoring air resistance)

Not in syllabus: Find out about Galileo Galilei’s falling bodies experiment

Acceleration due to gravity Acceleration due to gravity, g is a constant. The value of g is 9.81 For O level, we take it as 10 . g is constant for object close to the Earth. Not in syllabus: Is g really constant

everywhere on the surface of the Earth? Where is g minimum or maximum on the surface of the Earth?

Example 10:An object is released from rest at the top of a building. If the acceleration due to gravity is 10 and the object takes 5 s to reach the ground (neglecting air resistance),(a) What is the speed of the object just

before it hits the ground?(b) What is the height of the building?

Example 11

0 1 2 3 4 5 6 7 8

-6

-4

-2

0

2

4

6

8

10

12Velocity/

Time/s

The graph shows the velocity of an object over a period of 8 s.

Example 12A cyclist is travelling along a path and its displacement-time graph is as shown. Deduce and sketch its velocity-time graph.

0 10 20 30

Displacement/m

Time/s

0 10 20 30

Displacement/m

Time/s

Velocity/m s-1

Time/s0 10 20 30

10 minute quiz

Exit card

What you have learnt (a) state what is meant by speed and velocity(b) calculate average speed using

distance travelled / time taken(c) state what is meant by uniform acceleration and

calculate the value of an acceleration using change in velocity / time taken

(d) interpret given examples of non-uniform acceleration(e) plot and interpret a displacement-time graph and a velocity-time graph

What you have learnt (f) deduce from the shape of a displacement-time graph when a body is:

i. at restii. moving with uniform velocityiii. moving with non-uniform velocity

(g) deduce from the shape of a velocity-time graph when a body is:

iv. at restv. moving with uniform velocityvi. moving with uniform accelerationvii. moving with non-uniform acceleration

What you have learnt (h) Calculate the area under a velocity-time graph to determine the displacement travelled for motion with uniform velocity or uniform acceleration

(i) state that the acceleration of free fall for a body near to the Earth is constant and is approximately 10

Homework 4 practice questions (Worksheet 2) 2 not in syllabus questions (optional)

By 5th April, next Friday.

References & links Falling bodies experiment http://

www.youtube.com/watch?v=Z789eth4lFU

“How steep is your slope?” song. http://www.youtube.com/watch?v=ds_7fPIRFPM

Falling feather and ball bearing http://www.youtube.com/watch?v=_XJcZ-KoL9o