1. Newton’s Laws of Motion

2. Standard and Performance Indicators
Standard 8.P.2 The student will demonstrate an understanding of the effects of forces on the motion and stability of an object.
Performance Indicator 8.P.2A.3 Construct explanations for the relationship between the mass of an object and the concept of inertia (Newton’s First Law of Motion).
8.P.2A.4Analyze and interpret data to support claims that for every force exerted on an object there is an equal force exerted in the opposite direction (Newton’s Third Law of Motion).

3. Newton’s Laws of Motion
I. Law of Inertia
II. F=ma
III. Action-Reaction

4. Newton’s Laws of Motion
1st Law – An object at rest will stay at rest, and an object in motion will stay in motion at constant velocity, unless acted upon by an unbalanced force.
2nd Law – Force equals mass times acceleration.
3rd Law – For every action there is an equal and opposite reaction.

5. Newton’s First Law
“The velocity of an object will remain constant unless a net force acts on it.”
1st Law – An object at rest will stay at rest, and an object in motion will stay in motion at constant velocity, unless acted upon by an unbalanced force.

6. Basically, an object will “keep doing what it was doing” in the same direction at the same speed unless acted on by an unbalanced force. If the object was sitting still, it will remain stationary. If it was moving at a constant velocity, it will keep moving. It takes force to change the motion of an object.

7. RECAP: What is meant by unbalanced force?
If the forces on an object are equal and opposite, they are said to be balanced, and the object experiences no change in motion. If they are not equal and opposite, then the forces are unbalanced and the motion of the object changes.

8. Newton’s First Law is also called the Law of Inertia
Inertia: the tendency of an object to resist changes in its state of motion
The First Law states that all objects have inertia. The more mass an object has, the more inertia it has (and the harder it is to change its motion).
Therefore, inertia depends on mass.

9. The Relationship between Mass & Inertia
The amount of inertia that an object has is dependent on the object’s mass. The more mass an object has the more inertia it has.

13. Mass & Inertia
If an object has a large amount of inertia (due to a large mass)
It will be hard to slow it down or speed it up of it is moving.
It will be hard to make it start moving if it is at rest.
It will be hard to make it change direction.

14. INERTIA DOES NOT DEPEND ON GRAVITY
An object’s inertia is the same on Earth as it is in space.

15. Examples of the effects of inertia might also include:
1. Inertia causes a passenger in a car to continue to move forward even though the car stops.
This is the reason that seat belts are so important for the safety of passengers.
NOTE: Inertia is the reason that it is impossible for vehicles to stop instantaneously.

16. Don’t let this be you. Wear seat belts.
Because of inertia, objects (including you) resist changes in their motion. When the car going 80 km/hour is stopped by the brick wall, your body keeps moving at 80 m/hour.

17. Inertia is the reason that it is harder to
Inertia is the reason that it is harder to start pushing a wheelbarrow full of bricks than to start pushing an empty wheelbarrow. The filled wheelbarrow has more mass and
therefore, more inertia.
3. Inertia is also the reason that it is harder to stop a loaded truck going 55 miles per hour than to stop a car going 55 miles per hour. The truck has more mass resisting the change of its motion and therefore, more inertia.

18. Some Examples from Real Life
A soccer ball is sitting at rest. It takes an unbalanced force of a kick to change its motion.
Two teams are playing tug of war. They are both exerting equal force on the rope in opposite directions. This balanced force results in no change of motion.

19. 1st Law Continued
Unless acted upon by an unbalanced force, this golf ball would sit on the tee forever.

20. 1st Law Continued
Once airborne, unless acted on by an unbalanced force (gravity and air – fluid friction), it would never stop!

21. If objects in motion tend to stay in motion, why don’t moving objects keep moving forever?
Things don’t keep moving forever because there’s almost always an unbalanced force acting upon it.
A book sliding across a table slows down and stops because of the force of friction.
If you throw a ball upwards it will eventually slow down and fall because of the force of gravity.

22. Friction! There are four main types of friction:
What is this unbalanced force that acts on an object in motion?
Friction!
There are four main types of friction:
Sliding friction: ice skating
Rolling friction: bowling
Fluid friction (air or liquid): air or water resistance
Static friction: initial friction when moving an object

23. If objects in motion tend to stay in motion, why don’t moving objects keep moving forever?
Things don’t keep moving forever because there’s almost always an unbalanced force acting upon it.
A book sliding across a table slows down and stops because of the force of friction.
If you throw a ball upwards it will eventually slow down and fall because of the force of gravity.

24. In outer space, away from gravity and any sources of friction, a rocket ship launched with a certain speed and direction would keep going in that same direction and at that same speed forever.

25. QUESTION
Why then, do we observe every day objects in motion slowing down and becoming motionless seemingly without an outside force?

26. Momentum

27. Momentum
Momentum is what Newton called the “quantity of motion” of an object.
What is this quantity of motion?
Today we call it momentum.
Momentum is a characteristic of a moving object that is related to the mass and the velocity of the object.

28. Momentum The momentum of an object: Depends on the object’s mass.
Depends on the object’s velocity.
What is velocity? Velocity includes the speed and the direction. Ex: 10mph north

29. The momentum of a moving object can be determined by multiplying the object’s mass and velocity.
Momentum = Mass x Velocity
Since mass is measured in kilograms and velocity is measured in meters per second, the unit for momentum is kilogram-meters per second (kg x m/s).
Like velocity, acceleration, and force, momentum is described by its direction as well as its quantity.

30. p v m Momentum Formula In symbols: p = mv
Common units of momentum: kg m/s p m v

31. p = m v Momentum Defined p = momentum vector m = mass
v = velocity vector

32. The momentum of an object is in the same direction as its velocity.
The more momentum a moving object has, the harder it is to stop.
The mass of an object affects the amount of momentum the object has.
For example, you can catch a baseball moving at 20 m/s, but you cannot stop a car moving at the same speed. The car has more momentum because it has a greater mass.

33. The velocity of an object also affects the amount of momentum an object has.
For example, an arrow shot from a bow has a large momentum because, although it has a small mass, it travels at a high velocity.

34. Sample Momentum Problems
Which has more momentum: a 3.0 kg sledgehammer swung at 1.5m/s, or a 4.0 kg sledgehammer swung at 0.9 m/s?
Read and Understand
What information are you given?
Mass of smaller sledgehammer =
Velocity of smaller sledgehammer =
Mass of larger sledgehammer =
Velocity of larger sledgehammer =

35. Plan and Solve
What quantities are you trying to calculate?
The momentum of each sledgehammer
What formula contains the given quantities and the unknown quantity?
Momentum = Mass x Velocity
Perform the calculations
Smaller sledgehammer:
Larger sledgehammer:

36. Look Back and Check Does your answer make sense?
The 3.0 kg hammer has more momentum than the 4.0 kg one. This answer makes sense because it is swung at a greater velocity.

37. Choose 2 out of the 4 Momentum Problems to Calculate.
A golf ball travels at 16 m/s while a baseball moves at 7 m/s. The mass of the golf ball is kg and the mass of the baseball is 0.14 kg. Which has a greater momentum?
What is the momentum of a bird with a mass of kg flying at 15 m/s?
Momentum Internet Site

38. 3. Calculate the momentum of a 1200kg car with a velocity of 25m/s.
4. What is the momentum of a child and wagon if the total mass of the child and wagon is 22kg and the velocity is 1.5m/s?

39. How does momentum relate to inertia?
Momentum is similar to inertia. Like inertia, the momentum of an object depends on its mass. Unlike inertia, however, momentum takes into account how fast the object is moving. A wrecking ball that is moving very slowly, for example, has less momentum than a fast moving wrecking ball. With less momentum, the slower-moving wrecking ball would not be able to do as much damage to the wall.

40. Newton’s (2ND)Second Law
Force equals mass times acceleration. F = ma
Acceleration: a measurement of how quickly an object is changing speed and direction (velocity) per unit time.

41. 2nd LAW FORMULA
F = m x a

42. Force equals mass times acceleration.

44. MORE MASS NEEDS MORE FORCE FOR GREATER ACCELERATION

46. Newton’s Third Law “Law of Action and Reaction”
“When one object exerts a force on a second object, the second one exerts a force on the first that is equal in magnitude and opposite in direction.” .

47. For every action, there is an equal and opposite reaction.
3rd Law
For every action, there is an equal and opposite reaction.

48. Even though the forces are equal in magnitude and opposite in direction, they do not cancel each
other. This law addresses two objects, each with only one force exerted on it.
■ Each object is exerting one force on the other object.
■ Each object is experiencing only one force.

50. 3rd Law
The reaction of a rocket is an application of the third law of motion. Various fuels are burned in the engine, producing hot gases.
The hot gases push against the inside tube of the rocket and escape out the bottom of the tube. As the gases move downward, the rocket moves in the opposite direction.

51. 3rd Law- Meaning????
According to Newton, whenever objects A and B interact with each other, they exert forces upon each other. When you sit in your chair, your body exerts a downward force on the chair and the chair exerts an upward force on your body.

52. 3rd Law
There are two forces resulting from this interaction - a force on the chair and a force on your body. These two forces are called action and reaction forces.

53. 3rd Law
Flying gracefully through the air, birds depend on Newton’s third law of motion. As the birds push down on the air with their wings, the air pushes their wings up and gives them lift.

54. The size of the force on the air equals the size of the force on the bird; the direction of the force on the air (downwards) is opposite the direction of the force on the bird (upwards).
Action-reaction force pairs make it possible for birds to fly.

55. Think about it . . .
What happens if you are standing on a skateboard on a slippery floor and push against a wall? You slide in the opposite direction (away from the wall), because you pushed on the wall but the wall pushed back on you with equal and opposite force.
Why does it hurt so much when you stub your toe? When your toe exerts a force on a rock, the rock exerts an equal force back on your toe. The harder you hit your toe against it, the more force the rock exerts back on your toe (and the more your toe hurts).

56. Other Examples of Newton’s 3rd Law of Motion

57. More Examples of the 3rd Law

58. Other examples of Newton’s Third Law
The baseball forces the bat to the left (an action); the bat forces the ball to the right (the reaction).

59. 3rd Law
Consider the motion of a car on the way to school. A car is equipped with wheels which spin backwards. As the wheels spin backwards, they grip the road and push the road backwards.

61. The action and reaction forces are reciprocal on an object.
○ Examples may include:
○ A swimmer swimming forward:
■ The swimmer pushes against the water (action force), the water pushes back on the
swimmer (reaction force) and pushes her forward.
○ A ball is thrown against a wall:
■ The ball puts a force on the wall (action force), and the wall puts a force on the ball
(reaction force) so the ball bounces off.

62. A person is diving off a raft:
■ The person puts a force on the raft (action force) pushing it, and the raft puts a force on the diver (reaction force) pushing them in the opposite direction.
○ A person pushes against a wall (action force), and the wall exerts an equal and opposite force against the person (reaction force).
○ The Space Shuttle engines push out hot gases (action force), and the hot gases put a force on the shuttle engines (reaction force) so the shuttle lifts (there is no sling shot doing it!)