1. Chapter 10 Motion

2. Measuring Motion
Motion—when an object changes its position relative to a reference point
Distance—how far an object has moved
Displacement—distance and direction of an object’s change of position from a starting point

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

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

5. 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.

6. Measuring Motion Speed—distance an object travels per unit of time
Rate—any change over time
Calculation for speed: speed = distance/time
Speed that doesn’t change over time—constant speed
Speed is usually not constant; usually an object has changing speed.
Average speed—speed of motion when speed is changing: speed = total distance/total travel time
Instantaneous speed—speed at any given point in time

7. Speed v d t
Speed
rate of motion
distance traveled per unit time

8. Speed & Velocity Instantaneous Speed Average Speed
speed at a given instant
Average Speed

9. Measuring Motion
A distance-time graph displays motion of an object over time.
Plot distance on a(n) vertical axis.
Plot time on a(n) horizontal axis.
Velocity—speed and direction of an object’s motion
Motion of Earth’s crust—so slow we don’t notice

10. 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!

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

12. Acceleration Acceleration—change in velocity’s rate
Positive acceleration—speed is increasing.
Negative acceleration—speed is decreasing.
When an object changes speed or direction, it is accelerating.

13. Acceleration Calculating acceleration
Acceleration = change in velocity/time
Change in velocity = final velocity – initial velocity
Unit for acceleration—meters per second squared
Positive acceleration—positive number with a positive slope on a velocity-time graph
Negative acceleration—negative number with a negative slope on a velocity-time graph

14. Acceleration Amusement park acceleration—Roller coasters
Changes in speed cause acceleration.
Changes in direction cause acceleration.

15. t a Acceleration Acceleration a: acceleration vf: final velocity
vf - vi t
Acceleration
Acceleration
the rate of change of velocity
change in speed or direction
a: acceleration
vf: final velocity
vi: initial velocity
t: time

16. Motion and Force Force—a push or pull that one body applies to another
A force can cause an object’s motion to change.
When two or more forces combine at the same time, they create a net force.
Balanced forces are equal in size and opposite in direction.
Unbalanced forces are unequal in size and / or are not in the same direction.

17. F = ma F m a Force F m F: force (N) m: mass (kg) a: accel (m/s2)
1 N = 1 kg ·m/s2

18. Force
What forces are being exerted on the football?
Fkick
Fgrav

19. Force
Balanced Forces
forces acting on an object that are opposite in direction and equal in size
no change in velocity

20. Force Fnet Ffriction Fpull N W Net Force
unbalanced forces that are not opposite and equal
velocity changes (object accelerates)
Fnet
Ffriction
Fpull N W

21. Motion and Force Inertia and Mass
Inertia—an object’s resistance to any change in motion
Objects with greater mass have greater inertia.
Newton’s first law of motion —an object moving at a constant velocity keeps moving at that velocity unless a net force acts on it; an object at rest will stay at rest unless a net force acts on it.

22. Motion and Force Auto crashes—the law of inertia at work
A passenger not wearing a seat belt keeps moving forward at the car’s speed even after the car stops.
A passenger wearing a seat belt slows down as the car slows down and stops.

23. a Calculations F m F = ? F = ma m = 40 kg F = (40 kg)(4 m/s2)
What force would be required to accelerate a 40 kg mass by 4 m/s2?
GIVEN:
F = ?
m = 40 kg
a = 4 m/s2
WORK:
F = ma
F = (40 kg)(4 m/s2)
F = 160 N m F a

24. a Calculations F m m = 4.0 kg a = F ÷ m F = 30 N a = (30 N) ÷ (4.0 kg)
A 4.0 kg shotput is thrown with 30 N of force. What is its acceleration?
GIVEN:
m = 4.0 kg
F = 30 N
a = ?
WORK:
a = F ÷ m
a = (30 N) ÷ (4.0 kg)
a = 7.5 m/s2 m F a

25. a Calculations F m F(W) = 557 N m = F ÷ a m = ?
Mrs. J. weighs 557 N. What is her mass?
GIVEN:
F(W) = 557 N
m = ?
a(g) = 9.8 m/s2
WORK:
m = F ÷ a
m = (557 N) ÷ (9.8 m/s2)
m = 56.8 kg m F a

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

27. t a Calculations a = (vf - vi) ÷ t t = 3 s a = (32m/s - 10m/s) ÷ (3s)
A roller coaster starts down a hill at 10 m/s. Three seconds later, its speed is 32 m/s. What is the roller coaster’s acceleration?
GIVEN:
vi = 10 m/s
t = 3 s
vf = 32 m/s
a = ?
WORK:
a = (vf - vi) ÷ t
a = (32m/s - 10m/s) ÷ (3s)
a = 22 m/s ÷ 3 s
a = 7.3 m/s2 a
vf - vi t

28. t d v Calculations v = 330 m/s t = d ÷ v d = 1km = 1000m
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:
v = 330 m/s
d = 1km = 1000m
t = ?
WORK:
t = d ÷ v
t = (1000 m) ÷ (330 m/s)
t = 3.03 s v d t

29. t a Calculations t = ? t = (vf - vi) ÷ a t = (0m/s-30m/s)÷(-3m/s2)
How long will it take a car traveling 30 m/s to come to a stop if its acceleration is m/s2?
GIVEN:
t = ?
vi = 30 m/s
vf = 0 m/s
a = -3 m/s2
WORK:
t = (vf - vi) ÷ a
t = (0m/s-30m/s)÷(-3m/s2)
t = -30 m/s ÷ -3m/s2
t = 10 s a
vf - vi t

30. Graphing Motion speed faster speed constant speed no motion slope =
Distance-Time Graph A B
speed
slope =
steeper slope =
straight line =
flat line =
faster speed
constant speed
no motion

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

32. Graphing Motion
Distance-Time Graph
Acceleration is indicated by a curve on a Distance-Time graph.
Changing slope = changing velocity

33. Graphing Motion acceleration slope = straight line = flat line =
Speed-Time Graph
acceleration
+ve = speeds up
-ve = slows down
slope =
straight line =
flat line =
constant accel.
no accel. (constant velocity)

34. Graphing Motion Speed-Time Graph
Specify the time period when the object was...
slowing down
5 to 10 seconds
speeding up
0 to 3 seconds
moving at a constant speed
3 to 5 seconds
not moving
0 & 10 seconds