1. 3

2. Unit 1: Energy and Motion
Table of Contents 3
Unit 1: Energy and Motion
Chapter 3: Forces
3.1: Newton’s Second Law
3.2: Gravity
3.3: The Third Law of Motion

3. Force, Mass, and Acceleration
Newton’s Second Law
3.1
Force, Mass, and Acceleration
Newton’s first law of motion states that the motion of an object changes only if an unbalanced force acts on the object.
Newton’s second law of motion describes how the forces exerted on an object, its mass, and its acceleration are related.

4. Force and Acceleration
Newton’s Second Law
3.1
Force and Acceleration
What’s different about throwing a ball horizontally as hard as you can and tossing it gently?
When you throw hard, you exert a much greater force on the ball.

5. Force and Acceleration
Newton’s Second Law
3.1
Force and Acceleration
The hard-thrown ball has a greater change in velocity, and the change occurs over a shorter period of time.

6. Force and Acceleration
Newton’s Second Law
3.1
Force and Acceleration
Recall that acceleration is the change in velocity divided by the time it takes for the change to occur.
So, a hard-thrown ball has a greater acceleration than a gently thrown ball.

7. Newton’s Second Law
3.1
Mass and Acceleration
If you throw a softball and a baseball as hard as you can, why don’t they have the same speed?
The difference is due to their masses.

8. Newton’s Second Law
3.1
Mass and Acceleration
If it takes the same amount of time to throw both balls, the softball would have less acceleration.
The acceleration of an object depends on its mass as well as the force exerted on it.
The heavier the object the less acceleration
Force, mass, and acceleration are related.

9. Newton’s Second Law 3.1 Newton’s second law of motion –
F= MA Force equals the mass x acceleration
Force- Newtons (N) Mass- Kg
Acceleration- M/S²
Sample Problems:

10. Calculating Net Force with the Second Law
Newton’s Second Law
3.1
Calculating Net Force with the Second Law
Newton’s second law also can be used to calculate the net force if mass and acceleration are known.
To do this, the equation for Newton’s second law must be solved for the net force, F.

11. Calculating Net Force with the Second Law
Newton’s Second Law
3.1
Calculating Net Force with the Second Law
To solve for the net force, multiply both sides of the equation by the mass:
The mass, m, on the left side cancels, giving the equation:

12. Friction 3.1 Suppose you give a skateboard a push with your hand.
Newton’s Second Law
3.1
Friction
Suppose you give a skateboard a push with your hand.
According to Newton’s first law of motion, if the net force acting on a moving object is zero, it will continue to move in a straight line with constant speed.
Does the skateboard keep moving with constant speed after it leaves your hand?

13. Newton’s Second Law
3.1
Friction
Friction- force that opposes the motion of two surfaces that are touching each other.
Friction depends on
the roughness of surfaces
force pressing the surfaces together.

14. Newton’s Second Law
3.1
Static Friction
Suppose you have filled a cardboard box with books and want to move it.
It’s too heavy to lift, so you start pushing on it, but it doesn’t budge.
If the box doesn’t move, then it has zero acceleration.

15. Newton’s Second Law
3.1
Static Friction
Static friction frictional force that prevents two surfaces from moving.

16. Newton’s Second Law
3.1
Sliding Friction
If you stop pushing, the box quickly comes to a stop.
This is because as the box slides across the floor, another forcesliding frictionopposes the motion of the box.
Sliding friction opposes the motion of two surfaces sliding past each other.

17. Newton’s Second Law
3.1
Rolling Friction
As a wheel rolls over a surface, the wheel digs into the surface, causing both the wheel and the surface to be deformed.

18. Newton’s Second Law
3.1
Rolling Friction
Static friction acts over the deformed area where the wheel and surface are in contact, producing a frictional force called rolling fiction.
Rolling friction- is the frictional force between a rolling object and the surface it rolls on.

19. Newton’s Second Law
3.1
Air Resistance
air resistance opposes the motion of objects that move through the air.
Air resistance causes objects to fall with different accelerations and different speeds.

20. Newton’s Second Law
3.1
Air Resistance
Air resistance acts in the opposite direction to the motion of an object through air.
If the object is falling downward, air resistance acts upward on the object.

21. Newton’s Second Law
3.1
Air Resistance
The amount of air resistance on an object depends on the speed, size, and shape of the object.
Air resistance, not the object’s mass, is why feathers, leaves, and pieces of paper fall more slowly than pennies, acorns, and apples.

22. Newton’s Second Law
3.1
Terminal Velocity
As an object falls, the downward force of gravity causes the object to accelerate.
However, as an object falls faster, the upward force of air resistance increases.
This causes the net force on a sky diver to decrease as the sky diver falls.

23. Newton’s Second Law
3.1
Terminal Velocity
Finally, the upward air resistance force becomes large enough to balance the downward force of gravity.
This means the net force on the object is zero.
Then the acceleration of the object is also zero, and the object falls with a constant speed called the terminal velocity.

24. Newton’s Second Law
3.1
Terminal Velocity
The terminal velocity is the highest speed a falling object will reach.
The terminal velocity depends on the size, shape, and mass of a falling object.

25. Section Check
3.1
Question 1
Newton’s second law of motion states that _________ of an object is in the same direction as the net force on the object.
A. acceleration
B. momentum
C. speed
D. velocity

26. Section Check
3.1
Answer
The answer is A. Acceleration can be calculated by dividing the net force in newtons by the mass in kilograms.

27. Question 2 3.1 The unit of force is __________. A. joule B. lux
Section Check
3.1
Question 2
The unit of force is __________.
A. joule
B. lux
C. newton
D. watt

28. Section Check
3.1
Answer
The answer is C. One newton = 1 kg · m/s2

29. Question 3 Answer 3.1 What causes friction?
Section Check
3.1
Question 3
What causes friction?
Answer
Friction results from the sticking together of two surfaces that are in contact.

30. Gravity
3.2
What is gravity?
Gravity is an attractive force between any two objects that depends on the masses of the objects and the distance between them.

31. The Law of Universal Gravitation
Gravity
3.2
The Law of Universal Gravitation
Isaac Newton formulated the law of universal gravitation, which he published in 1687.

32. The Law of Universal Gravitation
Gravity
3.2
The Law of Universal Gravitation
G = Universal gravitational constant
M1 and M2 = mass of objects
d = distance between objects
F= force of gravity between objects
The law of universal gravitation enables the force of gravity to be calculated between any two objects if their masses and the distance between them is known.

33. Gravity
3.2
The Range of Gravity
According to the law of universal gravitation, the gravitational force between two masses decreases rapidly as the distance between the masses increases.

34. Gravity
3.2
The Range of Gravity
No matter how far apart two objects are, the gravitational force between them never completely goes to zero.

35. Gravity
3.2
Finding Other Planets
In the 1840s the most distant planet known was Uranus.
The motion of Uranus calculated from the law of universal gravitation disagreed slightly with its observed motion.
Some astronomers suggested that there must be an undiscovered planet affecting the motion of Uranus.

36. Gravity
3.2
Finding Other Planets
Using the law of universal gravitation and Newton’s laws of motion, two astronomers independently calculated the orbit of this planet.
As a result of these calculations, the planet Neptune was found in 1846.

37. Earth’s Gravitational Acceleration
Gravity
3.2
Earth’s Gravitational Acceleration
Earth’s gravity acceleration- 9.8 m/s2.
Represented by the symbol – g

38. Gravity
3.2
Weight
Weight- gravitational force exerted on an object

39. Weight and Mass 3.2 Weight and mass are not the same.
Gravity
3.2
Weight and Mass
Weight and mass are not the same.
Weight is a force and mass is a measure of the amount of matter an object contains.

40. Gravity
3.2
Weight and Mass
The weight of an object usually is the gravitational force between the object and Earth.
The weight of an object can change, depending on the gravitational force on the object.

41. Gravity
3.2
Weight and Mass
The table shows how various weights on Earth would be different on the Moon and some of the planets.

42. Gravity
3.2
Projectile Motion
If you’ve tossed a ball to someone, you’ve probably noticed that thrown objects don’t always travel in straight lines. They curve downward.

43. Horizontal and Vertical Motions
Gravity
3.2
Horizontal and Vertical Motions
When you throw a ball, the force exerted by your hand pushes the ball forward.
This force gives the ball horizontal motion.
No force accelerates it forward, so its horizontal velocity is constant, if you ignore air resistance.

44. Horizontal and Vertical Motions
Gravity
3.2
Horizontal and Vertical Motions
However, when you let go of the ball, gravity can pull it downward, giving it vertical motion.
The ball has constant horizontal velocity but increasing vertical velocity.

45. Horizontal and Vertical Motions
Gravity
3.2
Horizontal and Vertical Motions
Gravity exerts an unbalanced force on the ball, changing the direction of its path from only forward to forward and downward.
The result of these two motions is that the ball appears to travel in a curve.

46. Click image to view movie
Gravity
3.2
Horizontal and Vertical Distance
If you were to throw a ball as hard as you could from shoulder height in a perfectly horizontal direction, would it take longer to reach the ground than if you dropped a ball from the same height?
Click image to view movie

47. Horizontal and Vertical Distance
Gravity
3.2
Horizontal and Vertical Distance
Surprisingly, it wouldn’t.
Both balls travel the same vertical distance in the same amount of time.

48. Gravity
3.2
Centripetal Force
centripetal acceleration Acceleration toward the center of a curved or circular path is called.

49. Gravity
3.2
Centripetal Force
centripetal force The net force exerted toward the center of a curved path is called a.
Force pushes objects to the outside

50. Centripetal Force and Traction
Gravity
3.2
Centripetal Force and Traction
When a car rounds a curve on a highway, a centripetal force must be acting on the car to keep it moving in a curved path.
This centripetal force is the frictional force, or the traction, between the tires and the road surface.

51. Centripetal Force and Traction
Gravity
3.2
Centripetal Force and Traction
Anything that moves in a circle is doing so because a centripetal force is accelerating it toward the center.

52. Gravity Can Be a Centripetal Force
3.2
Gravity Can Be a Centripetal Force
Imagine whirling an object tied to a string above your head.
The string exerts a centripetal force on the object that keeps it moving in a circular path.

53. Gravity Can Be a Centripetal Force
3.2
Gravity Can Be a Centripetal Force
Earth’s gravity exerts a centripetal force on the Moon that keeps it moving in a nearly circular orbit.

54. Section Check
3.2
Question 1
Gravity is an attractive force between any two objects and depends on __________.
Answer
Gravity is an attractive force between any two
objects and depends on the masses of the objects
and the distance between them.

55. Question 2 3.2 Which is NOT one of the four basic forces? A. gravity
Section Check
3.2
Question 2
Which is NOT one of the four basic forces?
A. gravity
B. net
C. strong nuclear
D. weak nuclear

56. Section Check
3.2
Answer
The answer is B. The fourth basic force is the electromagnetic force, which causes electricity, magnetism, and chemical interactions between atoms and molecules.

57. Section Check
3.2
Question 3
Which of the following equations represents the law of universal gravitation?
A. F = G(m1m2/d2)
B. G = F(m1m2/d2)
C. F = G(m1 - m2/d2)
D. F = G(d2/m1m2)

58. Section Check
3.2
Answer
The answer is A. In the equation, G is the universal gravitational constant and d is the distance between the two masses, m1 and m2.

59. Newton’s Third Law 3.3 Newton’s third law of motion
The Third Law of Motion
3.3
Newton’s Third Law
Newton’s third law of motion
For every force… there is an equal and opposite force

60. The Third Law of Motion
3.3
Action and Reaction
When you jump on a trampoline, for example, you exert a downward force on the trampoline.
Simultaneously, the trampoline exerts an equal force upward, sending you high into the air.

61. Action and Reaction Forces Don’t Cancel
The Third Law of Motion
3.3
Action and Reaction Forces Don’t Cancel
According to the third law of motion, objects are experiencing unbalanced forces
Thus, even though the forces are equal, they are not balanced because they act on different objects.

62. Action and Reaction Forces Don’t Cancel
The Third Law of Motion
3.3
Action and Reaction Forces Don’t Cancel
For example, a swimmer “acts” on the water, the “reaction” of the water pushes the swimmer forward.
Thus, a net force, or unbalanced force, acts on the swimmer so a change in his or her motion occurs.

63. The Third Law of Motion
3.3
Rocket Propulsion
In a rocket engine, burning fuel produces hot gases. The rocket engine exerts a force on these gases and causes them to escape out the back of the rocket.
Newton’s third law, the gases exert a force on the rocket and push it forward.

64. The Third Law of Motion
3.3
Momentum
momentum that is related to how much force is needed to change its motion.
momentum of an object is the product of its mass and velocity.

65. Momentum 3.3 Momentum = symbol p The unit for momentum is kg · m/s.
The Third Law of Motion
3.3
Momentum
Momentum = symbol p
The unit for momentum is kg · m/s.

66. Force and Changing Momentum
The Third Law of Motion
3.3
Force and Changing Momentum
Recall that acceleration is the difference between the initial and final velocity, divided by the time.
Also, from Newton’s second law, the net force on an object equals its mass times its acceleration.

67. Force and Changing Momentum
The Third Law of Motion
3.3
Force and Changing Momentum
Calculating force from momentum
mvf is the final momentum
mvi is the initial momentum.

68. Law of Conservation of Momentum
The Third Law of Motion
3.3
Law of Conservation of Momentum
The momentum of an object doesn’t change unless its mass, velocity, or both change.
Momentum, however, can be transferred from one object to another.
The law of conservation of momentum-
if a group of objects exerts forces only on each other, their total momentum doesn’t change.

69. The Third Law of Motion
3.3
When Objects Collide
A collision depend on the momentum of each object.
When the first puck hits the second puck from behind, it gives the second puck momentum in the same direction.

70. The Third Law of Motion
3.3
When Objects Collide
If the pucks are speeding toward each other with the same speed, the total momentum is zero.

71. Question 1 Answer 3.3 According to Newton’s third law of motion,
Section Check
3.3
Question 1
According to Newton’s third law of motion,
what happens when one object exerts a force
on a second object?
Answer
According to Newton’s law, the second object
exerts a force on the first that is equal in
strength and opposite in direction.

72. Section Check
3.3
Question 2
The momentum of an object is the product of its __________ and __________.
A. mass, acceleration
B. mass, velocity
C. mass, weight
D. net force, velocity

73. Answer 3.3 The correct answer is B. An object’s momentum
Section Check
3.3
Answer
The correct answer is B. An object’s momentum
is the product of its mass and velocity, and is
given the symbol p.

74. Section Check
3.3
Question 3
When two objects collide, what happens to their momentum?

75. Section Check
3.3
Answer
According to the law of conservation of momentum, if the objects in a collision exert forces only on each other, their total momentum doesn’t change, even when momentum is transferred from one object to another.

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