1. Forces and Motion
Unit 19

2. What words can you think of to
describe the motion of an
object?

3. VELOCITY
The word scientists use to describe how
fast an object is moving in a certain
direction.

4. Velocity is measured as:
a change in distance per
change in time
change in distance
change in time
What are some things you can think of that are
measured like this?

5. What are some examples of
objects that move at high
velocities?
220 mph
1600 mph

6. Extreme Velocities
17,000 mph 40,000 mph

7. What are some examples of low
velocities?
50 yds/hr (.03 mph)
1.2 mph

8. What units would you use
to measure the velocity of
an object moving in this
classroom?
meters per second
m
s
Meters per second tells us how many meters
something can move in one second.

9. 11 hwy, 0 city
highway, 0 city
Converting Velocities
m/s into km/hr
“What kind of mileage does this get?”
m 1000 m/km
s 3600 seconds/hr
Just take the m/s multiply by 1000 and divide by 3600

10. NSF North Mississippi GK-8

11. What else can we know about an
object’s motion besides velocity?
•What if the object speeds up?
• What if the object slows down?
What if the object’s velocity changes?

12. Scientists have a special way to
measure a change in velocity:
Acceleration
Acceleration is a change in
velocity.

13. Visualizing Acceleration
• Here you can see how the ball
travels a further and further
distance each second.
• That is because its velocity is
continuing to increase.

14. • If you’re speeding up, you have
positive acceleration.
• If you’re slowing down, you have
negative acceleration.

15. Example:
You’re driving along in your car and
suddenly have to slam on your brakes for
a red light.
Negative or positive acceleration?
High or low acceleration?

16. Answers
• Negative Acceleration
• High Rate

17. Example:
You’re an astronaut waiting in your shuttle
for takeoff. Suddenly the rockets fire and
you’re on your way.
High or low positive or negative acceleration?

18. Or you are a drag racer

19. Answer
High positive acceleration because
you’re speeding up quickly.

20. Example:
You’re driving down the highway when
your car runs out of gas and you slowly
roll to a stop on the side of the road.

21. Answer
Low negative acceleration because
you’re slowing down slowly.

22. Example:
• You are at cruising speed on the highway.
You no longer feel any pressure against
the seat or are lurching forward.
• What is your acceleration?

23. Answer
Zero, because you are at constant velocity
That velocity can be fast, slow, or not
moving at all.

24. Challenge Question:
You throw a basketball up in
the air. It rises to a certain
height and then falls back
to earth.
Describe its acceleration
Hint: There are multiple steps

25. Answer
First, high positive acceleration because it speeds up
quickly as it leaves your hand.

26. Answer
Then, as it rises, low negative acceleration, because it
begins to slow down slowly because of gravity.

27. Answer
At the top of the arch, it has zero acceleration, because it
briefly stops moving upward and has constant velocity.

28. Answer
As it begins to fall again, low positive acceleration,
because it speeds up slowly.

29. Answer
Finally, when you catch it, high negative acceleration
because it slows down very quickly in your hand.

30. How Do I Graph the Motion of a
Particle?
30
Car 1 Car 2 Car 3
25
20
15
10
5
0
0 1 2 3 4 5 6 7

31. 30
Car 1 Car 2 Car 3
25
20
15
10
5
0
0 1 2 3 4 5 6 7
Car 1 is moving with a velocity of 1 m / sec
There is zero acceleration

32. 30
Car 1 Car 2 Car 3
25
20
15
10
5
0
0 1 2 3 4 5 6 7
Car 2 has constant velocity throughout at 1 m /sec
Its acceleration remains zero

33. 30
Car 1 Car 2 Car 3
25
20
15
10
5
0
0 1 2 3 4 5 6 7
Car 3 has a changing velocity throughout, finishing at 7 m / sec
Acceleration is 1 m / sec2

34. Quick Review
• Velocity: speed of motion in a direction
• Acceleration: change in velocity
What causes an object’s
velocity to change?

35. • If you’re riding your bike and want to
speed up, what do you do?
• If you’re driving a car and
want to slow down,
what do you do?
• If you’re playing soccer and
want to move the ball,
what do you do?

36. Nothing will change speed unless
acted upon by a force.
If it’s sitting still, it will keep
sitting still.
If it’s moving fast, it will
keep moving fast.

37. Only a force can change the
velocity of an object.
• That force can act to accelerate it
NSF North Mississippi GK-8

38. Only a force can change the
velocity of an object.
• That force can act to decelerate it

39. Then why do things slow down?
• Flying balls slow down and
eventually stop because of
the force of gravity.
• Cars and bikes eventually
slow down and stop because
of the force of friction.

40. Then why do things slow down?
• In space with no friction and no
gravity, objects will keep going at the same
speed forever without a force to slow them
down or speed them up.