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Newton's Laws of Motion

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Background Sir Isaac Newton ( ) an English scientist and mathematician famous for his discovery of the law of gravity also discovered the three laws of motion. He published them in his book Philosophiae Naturalis Principia Mathematica (mathematic principles of natural philosophy) in Today these laws are known as Newton’s Laws of Motion and describe the motion of all objects on the scale we experience in our everyday lives.

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“If I have ever made any valuable discoveries, it has been owing more to patient attention, than to any other talent.” -Sir Isaac Newton

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**Newton’s First Law of Motion**

An object at rest remains at rest, and an object in motion remains in motion at constant speed and in a straight line unless acted on by an unbalanced force. A golf ball will remain at rest on a tee until it is acted upon by unbalanced forces of a moving club.

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**Breakin’ it Down into parts…**

Part 1: Objects at rest. An object that is not moving is said to be at rest. Newton’s 1st Law says that objects will stay at rest unless acted by an unbalanced force. Objects will not start to move until a push or pull is exerted on them.

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**Part 2: Objects in Motion **

Objects will continue to move with the same velocity unless an unbalanced force acts on them. Think about bumper cars! Your bumper cars stops when it hits another car. But, you continue to move forward until the force from your seat belt stops you.

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Think about this…. When you ride a bus, why do you fall forward when the bus stops moving? When the bus is moving, both you and the bus are in motion. When the bus stops moving, no unbalanced force acts on your body, so your body continues to move forward.

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**Newton’s Second Law of Motion**

The acceleration of an object depends of the mass of the object and the amount of force applied.

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**Part 1: Acceleration Depends on Mass **

The acceleration of an object decreases as its mass increases, and that its acceleration increases as its mass decreases. Suppose you are pushing an empty cart. You have to exert only a small force on the cart to accelerate it. But, the same amount of force will not accelerate the full cart as much as the empty cart.

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**Part 2: Acceleration Depends on Force **

An object’s acceleration increases as the force on the object increases, and an object’s acceleration decreases as the force on the object decreases. The acceleration of an object is in the same direction as the force applied. Suppose you give the cart a hard push, the cart will start moving faster than if you gave it only a soft push.

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**a = F F = ma m The Mathematics….**

What is the acceleration of the apple and the watermelon? M = 1.02 F = 10 N M = kg F = 1 N

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*** 1 N = 1 kgm/s2 a = F m a = 1 kgm/s2 = 9.8 m/s2 0.102 kg a = F m**

F = 10 N M = kg F = 1 N a = F m a = 1 kgm/s2 = 9.8 m/s2 0.102 kg a = F m a = 10 kgm/s2 = 9.8 m/s2 1.02 kg * 1 N = 1 kgm/s2

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**What is the acceleration…**

Of a 3 kg mass if a force of 14.4 N is used to move the mass? a = F m a = 14.4 kgm/s2 = 4.8 m/s2 3 kg

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**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.

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All forces act in pairs! If a force is exerted, another force occurs that is equal in size and opposite in direction. The way the force pairs interact affects the motion of objects.

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Action and Reaction… When a force is exerted, there is always a reaction force.

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**For every action, there is an equal and opposite reaction.**

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**Other examples of Newton’s Third Law**

The baseball forces the bat back (an action); the bat forces the ball forward (the reaction).

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**Consider the motion of a car on the way to school**

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.

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