1. Chapter Five: Forces 5.1 Forces 5.2 Friction
5.3 Forces and Equilibrium

2. 5.1 The cause of forces
A force is a push or pull, or an action that has the ability to change motion.
Forces can increase or decrease the speed of a moving object.
Forces can also change the direction in which an object is moving.

3. 5.1 How are forces created? Forces are created in many ways.
For example, your muscles create force when you swing a baseball bat.

4. Four Elemental Forces
All forces in the universe come from only four basic forces.
Electromagnetic forces are important to technology.
Gravity is a universal force.

5. 5.1 Units of force
The pound is a unit of force commonly used in the United States.
For smaller amounts, pounds are divided into ounces (oz.).
There are 16 ounces in 1 pound.

6. 5.1 Newtons
Although we use pounds all the time in our everyday life, scientists prefer to measure forces in newtons.
The newton (N) is a metric unit of force.

7. 5.1 Unit conversions
The newton (N) is a smaller unit of force than the pound (lb).
If one pound of force equals newtons, then a 100 lb person weighs newtons.

8. 5.1 Drawing a force vector
The arrow points in the direction of the force.

9. 5.1 Gravity The force of gravity on an object is called weight.
At Earth’s surface, gravity exerts a force of 9.8 N on every kilogram of mass.

10. 5.1 Weight vs. mass Weight and mass are not the same.
Mass is a fundamental property of matter measured in kilograms (kg).
Weight is a force measured in newtons (N).
Weight depends on mass and gravity.

11. Weight depends on mass and gravity
A 10-kilogram rock has the same mass no matter where it is in the universe. On Earth, the10 kg. rock weighs 98 N.. On the moon, the same rock only weighs 16 N.

12. 5.1 Calculating weight

13. Solving Problems
Calculate the weight of a 60-kilogram person (in newtons) on Earth and on Mars.
Looking for:
…weight of person in newtons on both planets
Given:
…mass = 60 kg; g = 3.7 N/kg on Mars;
…implied g = 9.8 N/kg on Earth
Relationships:
W = m x g
Solution:
60 kg x 9.8 N/kg = 588 N
60 kg x 3.7 N/kg = 222 N
Sig. fig. = 600 N
Sig. fig. = 200 N

14. Chapter 5.3 Learning Goals
Determine the net force acting on an object.
Define equilibrium.
Draw free-body diagrams to represent all forces acting on a body.

15. 5.3 Forces and Equilibrium
The sum of all the forces on an object is called the net force.
The word net means total but also means the direction of the forces has been taken into account.
In what direction will this plane go?

16. 5.3 Adding forces
To figure out if or how an object will move, we look at ALL of the forces acting on it.
Four forces act on a plane:
weight
drag (air friction)
the thrust of the engines, and
the lift force caused by the flow of air over the wings.

18. 5.3 Equilibrium When several forces act on the same object:
The net force is zero, or
The net force is NOT zero.

19. 5.3 Normal forces When the forces are balanced, the net force is zero.
When the net force on an object is zero, we say the object is in equilibrium.

20. 5.3 Equilibrium and normal forces
A normal force is created whenever an object is in contact with a surface.
The normal force has equal strength to the force pressing the object into the surface, which is often the object’s weight.
The normal force is sometimes called the support force.

21. 5.3 The free body diagram
How do you keep track of many forces with different directions?
Draw a free-body diagram that contains the objects, like a book on a table.

23. 5.3 Solving equilibrium problems
For an object to be in equilibrium, all the forces acting on the object must add up to zero.
Is this object in equilibrium?

24. Two chains are used to support a small boat weighing 1,500 newtons.
Solving Problems
Two chains are used to support a small boat weighing 1,500 newtons.
One chain has a tension of 600 newtons.
What is the force exerted by the other chain?

25. Looking for: Given Relationships: Solving Problems …tension on chain 2
…weightboat = 1,500N; tension1 = 600 N
Implied: weight and tension are forces
Relationships:
Net force on boat = zero

26. Upward force of chains = weight of boat 600 N + tension2 = 1,200 N
Solving Problems
Solution:
Draw free body diagram
Upward force of chains = weight of boat
600 N + tension2 = 1,200 N
tension2 = 900 N

27. Satellites and “Weightlessness”
Objects in orbit are said to experience weightlessness. They do have a gravitational force acting on them, though!
The satellite and all its contents are in free fall, so there is no normal force. This is what leads to the experience of weightlessness.

28. 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

29. 6.1 Force changes motion
A force is a push or pull, or any action that is able to change motion.

30. 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
an object in motion will continue at the same velocity.
If an object is acted upon by unbalanced forces, its motion will change.

31. 6.1 Law of inertia An object moving at constant speed does not
require a force to make it keep moving (net force = zero)

32. 6.1 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?

33. 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.

34. 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?

36. 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.

37. 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

38. Solution Solving Problems 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.
Try two different heights
FF = -200 N
FE = 200 N
FN = 12,000N

39. 6.1 Why do all objects fall at the same rate?

40. 6.1 Why do all objects fall at the same rate?
Objects with more mass have more inertia, which resists their change in motion. So the elephant has 1000 times as much inertia as a mouse. So, even though the elephant weighs times as much, it has 1000 times more inertia, too.

41. 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.

42. 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.

44. 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.

45. 6.2 Newton’s second law
Unbalanced forces cause changes in speed, direction, or both.

47. 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.

48. 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.

49. 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.

50. 6.2 Acceleration, force and mass
The acceleration caused by a force is proportional to force and inversely proportional to mass.

51. 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.

52. 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.

53. 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.

54. 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

55. 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.

56. 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.

57. 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 _______________.

59. 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.

60. 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

61. 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.

62. 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).

63. 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.

64. 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.

65. 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?

66. The astronaut’s mass is 100 kilograms.
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.

67. Looking for: Given Relationships: Solution Solving Problems
… 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.

68. Newton’s Law of Universal Gravitation
If the force of gravity is being exerted on objects on Earth, what is the origin of that force?
Newton’s realization was that the force must come from the Earth.
He further realized that this force must be what keeps the Moon in its orbit.

69. Newton’s Law of Universal Gravitation
The gravitational force on you is one-half of a Third Law pair: the Earth exerts a downward force on you, and you exert an upward force on the Earth.

70. Newton’s Law of Universal Gravitation
Therefore, the gravitational force must be proportional to both masses.
By observing planetary orbits, Newton also concluded that the gravitational force must decrease as the inverse of the square of the distance between the masses.
In its final form, the Law of Universal Gravitation reads:
where
(5-4)

71. Newton’s Law of Universal Gravitation
So, if one or both of the masses increase, the gravitational force increases
And, if the distance increases, the force decreases
Closer together – more force Farther apart – less force