1. Forces and Motion
Chapter 12

2. Chap 12 Pretest What is relative motion?
Relative motion is movement in relation to a frame of reference.

3. Chap 12 Pretest
What is the difference between distance and displacement?
Distance is the length of a path between two points.
Displacement is the direction from the starting point and the length of the straight line from the starting point to the ending point.

4. Chap 12 Pretest How is average speed calculated?
Total distance is divided by total time.
v = d / t

5. Chap 12 Pretest
On a distance-time graph, what does the slope represent?
The slope represents the speed.

6. Chap 12 Pretest
What is velocity?
Velocity is speed with direction.

7. Chap 12 Pretest
A backpack falls out of an open window. The backpack starts from rest and hits the ground 1.0 second later with a velocity of 9.8 m/s. What is the average acceleration of the backpack?
A m/s
B m
C m/s2
D. All of the above

8. Chap 12 Pretest How is acceleration related to velocity?
Acceleration is the change in velocity, that is, any change in speed, direction, or both.

9. Chap 12 Pretest How are mass and weight different?
Mass is the measure of inertia; weight is the measure of the force of gravity acting on an object.

10. Objects in Free Fall
What factors affect a falling object? Perform the following simple activity to begin learning about the forces that act on falling objects.
Stand beside your desk. Hold a sheet of notebook paper level at eye level. Release the sheet of paper and watch it fall. Describe the motion of the paper.
Hold a sheet of notebook paper that has been crumpled into a tight ball at eye level. Release the crumpled paper and watch it fall. Describe the motion of the paper.
How do the motions of the flat sheet of paper and crumpled ball of paper compare? What forces do you think are acting on each sheet of paper?

11. 12.1- Forces A force is a push or a pull that acts on an object.
A force can cause a resting object to move, or it can accelerate a moving object by changing the object’s speed or direction.

12. 12.1 - Forces Force is measured in newtons.
Newtons are abbreviated as N.
One newton is the force that causes a 1kg mass to accelerate at a rate of 1 meter per second each second.
1 N = 1 kg•m/s2
The newton is named after Sir Isaac Newton.

13. 12.1 – Representing & Combining Forces
Forces can be represented by arrows showing the direction and strength of the force.
The net force is the overall force acting on an object after all of the forces are combined.
Force arrows are combined. Forces in the same direction add together, forces in opposite directions subtract from one another.

14. 12.1 – Balanced Forces
Balanced forces are forces that combine to produce a net force of zero.
When the forces on an object are balanced, the net force is zero and there is no change in the object’s motion.
Examples: tug of war, arm wrestling

15. 12.1 – Unbalanced Forces
An unbalanced force is a force that results when the net force acting on an object is not equal to zero.
When an unbalanced force acts on an object, the object accelerates.
Examples: pushing a resting book on table, winners/losers of arm wrestling or tug of war matches

16. Friction
All moving objects are subject to friction, a force that opposes the motion of objects that touch as they move past each other.
Friction acts at the surface where objects are in contact.
There are four main types of friction: static friction, sliding friction, rolling friction, and fluid friction.

17. 12.1 – Friction Types
Static Friction – the friction force that acts on objects that are not moving.
Always acts in the direction opposite of the applied force.
Sliding Friction – a force that opposes the direction of motion of an object as it slides over a surface.
Sliding friction is less than static friction, so less force is needed to keep on object moving that to start it moving.

18. 12.1 – Friction Types
Rolling Friction – the friction force that acts on rolling objects.
Rolling friction is about 100 to 1000 times less than the force of static or sliding friction.
Examples – moving dollies, ball bearings
Fluid Friction – the force that opposes the motion of an object through a fluid.
Fluid friction acting on an object moving through air is known as air resistance.
Examples – stirring cake batter

19. Gravity
Gravity is a force that acts between any two masses. Gravity is an attractive force, pulling objects together.
Earth’s gravity acts downward toward the center of Earth.

20. 12.1 – Falling Objects
Both gravity and air resistance affect the motion of a falling object.
Gravity causes objects to accelerate downward, whereas air resistance acts in the direction opposite to the motion and reduces acceleration.
Terminal velocity is the constant velocity of a falling object when the force of air resistance equals the force of gravity.

21. 12.2 – Newton’s First Law of Motion
According to Newton’s first law of motion, the state of motion of an object does not change as long as the net force acting on the object is zero.
Unless an unbalanced force acts, an object at rest remains at rest, and an object in motion remains in motion with the same speed and direction

22. 12.2 – Newton’s First Law of Motion
Newton’s first law of motion is sometimes called the law of inertia.
Inertia is a tendency of an object to resist change in its motion.

23. 12.2 – Newton’s Second Law of Motion
According to Newton’s second law of motion, the acceleration of an object is equal to the net force acting on it divided by the object’s mass.
Mass is the measure of inertia of an object and depends on the amount of matter the object contains.
Acceleration = net force / mass, or a = F/m

24. 12.2 – Practice Problems
A boy pushes forward a cart of groceries with a total mass of 40.0 kg. What is the acceleration of the cart if the net force on the cart is 60.0 N?
a = F/m = 60 N / 40kg=1.5 m/s2
What is the upward acceleration of a helicopter with a mass of 5,000 kg if a force of 10,000 N acts on it in an upward direction?
a= F/m = 10,000 N / 5,000 kg = 2 m/s2

25. 12.2 – Practice Problems
An automobile with a mass of 1,200 kg accelerates at a rate of 3.0 m/s2 in the forward direction. What is the net force acting on the automobile?
a = F/m, or F = ma = 1,200kg*3.0 m/s2 =3,600 N
A 25 N force accelerates a boy in a wheelchair at 0.5 m/s2. What is the mass of the boy and the wheelchair?
a= F/m, m = F/a = 25 N / 0.5 m/s2 = 50 kg

26. 12.2 – Weight and Mass Weight and mass are not the same thing.
Weight is the force of gravity acting on an object.
Mass is the measure of inertia of an object and depends on the amount of matter the object contains.
Weight = Mass x Acceleration due to gravity
W = mg

27. 12.2 – Weight and Mass Do you weigh more on Earth or the moon? Hints:
Your mass is the same in both locations.
The acceleration due to gravity on Earth is 9.8 m/s2, and the moon’s acceleration due to gravity is 1/6th that on Earth.
Answer:
You weigh only 1/6th as much on the moon as on Earth!

28. 12.3 – Newton’s Third Law of Motion
According to Newton’s third law of motion, whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first object.
These two forces are called action and reaction forces.

29. 12.3 – Action and Reaction Forces
Action-Reaction forces are equal in size and opposite in direction.
Action-Reaction forces may or may not result in motion.
Action-Reaction forces do not act on the same object so they do not cancel or have a net force of zero.
Only when equal and opposite forces act on the same object do they result in a net force of zero.
Examples:
Bumper cars (motion occurs)
Pushing against a wall (no motion occurs)

30. Momentum
Momentum is the product of an object’s mass and its velocity.
Momentum = Mass x Velocity
Momentum is measured in units of kilogram-meters per second.
An object has a large momentum if the product of its mass and velocity is large.
An object with large momentum is hard to stop.
The momentum for any object at rest is zero.

31. Momentum
Which has more momentum, a kilogram golf ball with a speed of 60.0 meters per second, or a 7.0-kilogram bowling ball with a speed of 6.0 meters per second?
Momentum golf ball = kg x 60.0 m/s =2.8kg*m/s
Momentum bowling ball = 7.0 kg x 6.0 m/s = 42 kg*m/s

32. 12.3 – Conservation of Momentum
According to the law of conservation of momentum, if no net force acts on a system, then the total momentum of the system does not change.
In a closed system, the loss of momentum of one object equals the gain in momentum of another object – momentum is conserved.

33. Momentum vs Time graph At what time did the ball have zero momentum?
At t = 0 s; the ball has zero momentum before it is released.
At what time did the ball have the greatest momentum?
At t = 2.5s; about 6.5kg*m/s
What is the ball’s speed after 1.25 seconds?
Momentum = mass x velocity
Momentum = 3.25 kg*m/s
Mass = 0.25 kg
Velocity = momentum/mass
Velocity = 3.25 kg*m/s / 0.25 kg
Speed = 13 m/s