# The Law of Inertia.

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The Law of Inertia

Who was Isaac Newton? 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 Mathematic Principles of Natural Philosophy in Today these laws are known as Newton’s Laws of Motion and describe the motion of all objects we experience in our everyday lives. 2

Newton’s First Law States:
“An object at rest will remain at rest, and an object in motion will remain in motion unless acted upon by an outside force.”

What does this mean? Basically, an object will “keep doing what it’s doing” unless it is 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. 4

What is Force? Force is any push or pull which causes something to move or change its speed or direction Forces can be balanced or unbalanced

What is a Balanced Force?
Balanced forces are equal in size and opposite in direction They result in no change in velocity

What is an Unbalanced Force?
Unbalanced forces are not equal and opposite in direction. They result in a change in motion of the object.

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. 8

Newton’s First Law is also called the Law of Inertia
Inertia is 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). Mass is the measure of the amount of matter in an object; It is measured in Kilograms. 9

More Examples from Real Life
A powerful locomotive begins to pull a long line of boxcars that were sitting at rest. Since the boxcars are so massive, they have a great deal of inertia and it takes a large force to change their motion. Once they are moving, it takes a large force to stop them. On your way to school, a bug flies into your windshield. Since the bug is so small, it has very little inertia and exerts a very small force on your car (so small that you don’t even feel it). 10

Newton’s First Law Don’t let this be you. Wear seat belts.
Because of inertia, objects (including you) resist changes in their motion. When the car going 60 m/hour is stopped by the brick wall, your body keeps moving at 60 m/hour.

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. 12

What is Friction? Friction is a force that opposes motion between 2 surfaces. The amount of friction depends on the: types of surfaces force between the surfaces

What is Air Resistance? Air can create friction and cause a moving object to slow down. This is called air resistance. Air resistance is the force of air exerted on a falling object. The air pushes up as gravity pulls down. It is dependent upon the shape and surface area of the object. In the absence of air resistance, all objects will fall at the same rate due to gravity.

What is Gravity? Gravity is an attraction force between all masses. According to Newton, every object in the universe exerts a gravitational attraction to all other objects in the universe. The amount of gravitational force depends upon the mass of the objects and the distance between the objects.

What is Gravity? The greater the mass….
the greater the force of gravity…. the greater the inertia the object is said to have.

Who experiences more gravity - the astronaut or the lumberjack?
Which exerts more gravity on the astronaut- the Earth or the moon? The Earth! less distance = more gravity more mass = more gravity

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. 18

Comprehension Check Ben and Jerry are arguing in the cafeteria. Ben says that if he throws ice cream with a greater speed, it will have a greater inertia. Jerry argues that inertia doesn’t depend on an objects speed, but upon its mass. Why do you agree with and why? Imagine that you are in space, far away from Earth. If you were to throw a rock, will the rock gradually stop or continue in motion in the same direction and speed? Explain why. Lori Lostalot is being chased through the woods by a grizzly bear. The large mass of the bear is intimidating, but Lori believes she can use the bear’s mass to her advantage. Instead of running in a straight line, she makes a zigzag pattern through the woods. Explain why her plan will work and help her to escape from the bear.