2. Chapter Six: Laws of Motion
6.1 Newton’s First Law
6.2 Newton’s Second Law
6.3 Newton’s Third Law and Momentum

3. 6.1 Force changes motion
A force is a push or pull, or any action that is able to change motion.

4. 6.1 Law of inertia
Newton’s first law says that objects continue the motion they already have unless they are acted on by a net force.
If the net force is zero, an object at rest will stay at rest.
If an object is acted upon by unbalanced forces, its motion will change.

5. According to these vectors, in what direction is the net force?
Newton’s first law is often written in terms of the net force:
“An object at rest will stay at rest and an object in motion will continue in motion at constant velocity UNLESS there is a net force.”
According to these vectors, in what direction is the net force?

6. 6.1 Force changes motion
Forces can be used to increase or decrease the speed of an object, or to change the direction an object is moving.

7. 6.1 Law of inertia
Inertia is the property of an object that resists changes in motion.
Objects with more mass have more inertia and are more resistant to changes in their motion.
Which ball has more inertia?

9. Solving Problems
A car drives along the highway at constant velocity. Find the car’s weight and the friction force if the engine produces a force of 2,000 newtons between the tires and the road and the normal force on the car is 12,000 N.

10. Looking for: Given: Relationships: Solving Problems
…weight of car in newtons, force due to friction
Given:
…ForceN = 12,000N (up);
…ForceE = 2,000N (forward)
Relationships:
Newton’s 1st Law:
net force = zero at constant velocity; so ForceN = ForceW and ForceE = ForceF

11. Solving Problems Solution Draw a free body diagram. FW = -12,000N
There is no net force upward, so the weight of the car is an equal downward force of −12,000 N.
The forward engine force balances the friction force so the friction force is −2,000 N.
FF = -200 N
FE = 200 N
FN = 12,000N

12. 6.2 Newton’s second law
Newton’s first law tells us that motion cannot change without a net force.
According to Newton’s second law, the amount of acceleration depends on both the force and the mass.

13. 6.2 The newton
The S.I. unit of force (newton) is defined by the second law.
A newton is the amount of force needed to accelerate a 1 kg object by 1m/s.

15. 6.2 Newton’s second law
There are three main ideas related to Newton’s Second Law:
Acceleration is the result of unbalanced forces.
A larger force makes a proportionally larger acceleration.
Acceleration is inversely proportional to mass.

16. 6.2 Newton’s second law
Unbalanced forces cause changes in speed, direction, or both.

18. 6.2 Acceleration and force
The second law says that acceleration is proportional to force.
If force is increased or decreased, acceleration will be increased or decreased by the same factor.

19. 6.2 Acceleration and direction
Another important factor of the second law is that the acceleration is always in the same direction as the net force.

20. 6.2 Acceleration and mass
The greater the mass, the smaller the acceleration for a given force.
This means acceleration is inversely proportional to mass.

21. 6.2 Acceleration, force and mass
The acceleration caused by a force is proportional to force and inversely proportional to mass.

22. The stronger the force on an object, the greater its acceleration.
Force is directly proportional to acceleration.
If twice the force is applied, the acceleration is twice as great.

23. The greater the mass, the smaller the acceleration for a given force.
Mass is inversely related to force.
An object with twice the mass will have half the acceleration if the same force is applied.

24. 6.2 Applying the second law
Keep the following important ideas in mind:
1. The net force is what causes acceleration.
2. If there is no acceleration, the net force must be zero.
3. If there is acceleration, there must also be a net force.
4. The force unit of newtons is based on kilograms, meters, and seconds.

25. Solving Problems
A car has a mass of 1,000 kilograms. If a net force of 2,000 N is exerted on the car, what is its acceleration?
Looking for:
…car’s acceleration
Given
…mass = 1,000 kg; net force = 2,000 N
Relationships:
a = F / m
Solution:
2, 000 N ÷ 1,000 kg = 2 N/kg = 2 m/s2

26. 6.3 Newton’s Third Law
Newton’s Third Law (action- reaction) applies when a force is placed on any object, such as a basketball.

27. 6.3 The Third Law: Action/Reaction
Newton’s Third Law states that every action force creates a reaction force that is equal in strength and opposite in direction.
There can never be a single force, alone, without its action-reaction partner.

28. 6.3 Action and reaction
When sorting out action and reaction forces it is helpful to examine or draw diagrams.
Here the action force is on the ________________, and the reaction force is on the _______________.

30. A woman with a weight of 500 newtons is sitting on a chair.
Solving Problems
A woman with a weight of 500 newtons is sitting on a chair.
Describe one action-reaction pair of forces in this situation.

31. Solving Problems Fc = 500 N Looking for: Given Relationships:
…pair of action-reaction forces
Given
…girl’s forceW = -500 N (down)
Relationships:
Action-reaction forces are equal and opposite and act on different objects.
Solution
Draw a free body diagram
The downward force of 500 N exerted by the woman on the chair is an action.
Therefore, the chair acting on the woman provides an upward force of 500 N and is the reaction.
Fw = -500 N

32. 6.3 Collisions
Newton’s third law tells us that any time two objects hit each other, they exert equal and opposite forces on each other.
The effect of the force is not always the same.

33. 6.3 Momentum Momentum is the mass of a object times its velocity.
The units for momentum are kilogram-meter per second (kg·m/s).

34. 6.3 Momentum
The law of conservation of momentum states that as long as the interacting objects are not influenced by outside forces (like friction) the total amount of momentum is constant or does not change.

35. 6.3 Momentum
The result of a skateboarder throwing a 1-kg ball at a speed of -20 m/sec is that he and the skateboard with a total mass of 40 kg move backward at a speed of +0.5 m/sec (if you ignore friction).
We use positive and negative numbers to show opposite directions.

36. 6.3 Collisions
When a large truck hits a small car, the forces are equal.
The small car experiences a much greater change in velocity much more rapidly than the big truck.
Which vehicle ends up with more damage?

37. Solving Problems
If an astronaut in space were to release a 2-kilogram wrench at a speed of 10 m/s, the astronaut would move backward at what speed?
The astronaut’s mass is 100 kilograms.

38. Solving Problems Looking for: Given Relationships: Solution
… the velocity of the astronaut (backward)
Given
…velocity1 = 10 m/s; mass1= 2 kg;
...mass2 = 100 kg;
Relationships:
m1v1 = m2v2
Solution
Draw a free body diagram.