1. Motion and Forces
Describing Motion
•Motion
•Speed & Velocity

2. Newton’s First Law
An object at rest will
remain at rest, and an
object in motion will
continue in motion at a
constant velocity unless
acted upon by a net
force.

3. Motion
Problem:
− Is your desk moving?
We need a reference point...
− nonmoving point from which motion is measured

4. Motion
Motion
− Change in position in relation to a reference point.
Reference point
Motion

5. Relative Motion
If you are sitting in a chair reading this sentence, you
are moving.
You are not moving relative to your desk or your school
building, but you are moving relative to the other
planets in the solar system and the Sun.

6. Motion
• Distance
• Distance describes how far an object has
moved.
• The SI unit of length or distance is the
meter (m). Longer distances are
measured in kilometers (km).
• Shorter distances are measured in
centimeters (cm).

7. Motion
• Displacement is the distance and direction of an
object's change in position from a reference point.
• Suppose a runner jogs to the 50-m mark and then
turns around and runs back to the 20-m mark.
• The runner travels 50 m in the original direction
(north) plus 30 m in the opposite direction (south),
so the total distance she ran is 80 m.

8. Motion
Problem:
• You are a passenger in a car stopped at a stop
sign. Out of the corner of your eye, you notice
a tree on the side of the road begin to move
forward.
• You have mistakenly set yourself as the
reference point.

9. Speed & Velocity
Speed
− rate of motion
− distance traveled per unit time
d
s t
distance
speed =
time

10. Calculating Speed
The SI unit for distance is the meter and the SI unit of
time is the second (s), so in SI, units of speed
Sometimes it is more convenient to express speed in
other units, such as kilometers per hour (km/h).

11. Speed & Velocity
Instantaneous Speed
− speed at a given instant
− Speed odometer reading
Average Speed
− the total distance traveled divided by the total time of travel
total distance
avg. speed =
total time

12. Changing Instantaneous Speed
When something is speeding up or slowing down, its
instantaneous speed is changing.
If an object is moving with constant speed, the
instantaneous speed doesn't change.
To determine direction you need to know the velocity

13. Speed & Velocity
Problem:
− A storm is 10 km away and is moving at a speed of 60
km/h. Should you be worried?

It depends on the
storm’s direction!

14. Speed & Velocity
Velocity
− speed in a given direction
− can change even when the speed is constant!

15. Speed an Acceleration Video

16. Calculations

Your neighbor skates at a speed of 4 m/s. You can skate
100 m in 20 s. Who skates faster?
GIVEN:
WORK:
d = 100 m
t = 20 s
s=?
s=d÷t
d
s t
s = (100 m) ÷ (20 s)
s = 5 m/s
You skate faster!

17. Calculations

Sound travels 330 m/s. If a lightning bolt strikes the
ground 1 km away from you, how long will it take for
you to hear it?
GIVEN:
WORK:
s = 330 m/s
d = 1km = 1000m
t=?
t=d÷s
d
s t
t = (1000 m) ÷ (330 m/s)
t = 3.03 s

18. Graphing Motion
Distance-Time Graph
A
slope = speed
steeper slope =
Straight line =
flat line =
B
faster speed
no motion
constant speed
Single point = instantaneous
speed

19. Graphing Motion
Distance-Time Graph
A
Who started out faster?
− A (steeper slope)
Who had a constant speed?
− A
Describe B from 10-20 min.
− B stopped moving
Find their average speeds.
B
− A = (2400m) ÷ (30min)
80 m/min
− B = (1200m) ÷ (30min)
40 m/min
A=
B=

20. Graphing Motion
Distance-Time Graph
Acceleration is indicated
by a curve on a DistanceTime graph.
400
Distance (m)
300
Changing slope =
changing velocity
200
100
0
0
5
10
Time (s)
15
20

21. Graphing Motion
Specify the time period
when the object was...
slowing down
− 5 to 10 seconds
speeding up
− 0 to 3 seconds
Speed-Time Graph
3
2
Speed (m/s)
moving at a constant
speed
− 3 to 5 seconds
not moving
− 0 & 10 seconds
1
0
0
2
4
6
Time (s)
8
10

22. Speed and Velocity
Check on Learning

23. 1
_____is a unit of speed.
A
m/s
B
s
C
kg
D
hr

24. 2
_____ describes how fast something
is going, whereas, _____describes
how fast something is going and in a
certain direction.
A
speed, velocity
B
rate, speed
C
rate, velocity
D
speed, acceleration

25. 3
The SI unit for speed is
_____.
A
In/hour
B
m/s
C
cm/min
D
none of the answers

26. 4
If a person walked at 2 m/s for 12 s,
he/she would travel a distance of___
A
24 m
B
6m
C
4m
D
none of the answers

27. 5
The _____ of a distance versus time
graph is speed.
A
slope
B
Y-intercept
C
origin
D
None of the answers

28. 6
How far would you travel
moving at 12 m/s for 3.00
minutes?
A
36.0 m
B
2160 m
C
40.0 m
D
36.0 miles

29. 7
The speed at any instant of
time is known as_____
A
average speed
B
velocity
C
given speed
D
Instantaneous speed

30. 8
If a total distance of 750 m is
covered in a time interval of 25 s,
the average speed is _____?
A
3974 mi/hr
B
3 mi/hr
C
30 mi/hr
D
30 m/s

31. 9
How long would it take to
travel 50 km traveling at a
speed of 10 km/hr?
A
3 hours
B
1 hour
C
5 hours
D
50 hours

32. 10 Which of the following is a
measure of velocity?
A
30 s
B
30 South
C
30 m/s
D
30 m/s, South

33. Well done. You’re
finished!

34. Well
done!

35. Try
again!