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Law 1: Inertia

Random Inertia and Force Questions

First part of the law: Objects in motion remain in motion in a straight line (unless acted upon by a net force).

These logs are massive. They have a lot of inertia and "want" to keep moving straight.

Inertia is proportional to mass.

Big masses resist changing their motion more than smaller masses.

2nd part of the law: Objects at rest remain at rest unless acted on by a net force.

The person in the stretcher is at rest. His mass does not "want" to move. Therefore he drops to the ground.

When the truck accelerates, his body "wants" to stay put.

Inertia is proportional to mass. Big masses resist changing their motion more than smaller masses. Bigger masses don't "like" a new change in motion. Bigger masses are stubborn.

Inertia is the resistance to changes in motion. Inertia is not a force.

Trains have a lot of inertia!

The large train resists changing its motion.

Since the train is so huge, it is difficult to change its speed. In fact, a large net force is required to change its speed or direction.

A baby carriage has very little inertia. It has very little resistance to changes in motion.

Since the baby carriage is so small, it is very easy to change its speed or direction. A small net force is required to change its speed or direction.

A lot of inertia!

Since the asteroid is so huge, it is difficult to stop it once it is moving. It resists a change in motion. It is difficult to change its velocity. In fact, a large net force is required to change its velocity.

Very little inertia

Since the ping pong ball is so small, it is very easy to stop it and accelerate it in another direction. It does not resist a change in motion. A small net force is required to change its velocity.

Galileo's Ramp Experiments influenced Newton

Galileo's inertia experiment (around 1604) involved rolling balls down sloped ramps at different angles. He observed that a ball would roll up to the same height on another ramp. He then theorized that if there were no friction, the ball would keep rolling forever on a flat surface. Isaac Newton based his ideas on the work done by Galileo.

Isaac Newton

Isaac Newton was a key figure in science known for his discoveries about motion and gravity. Born in England in 1642, he studied at Cambridge University and became interested in new ideas, like the planets moving around the sun.

Newton discovered that white light is made up of all colors, and he developed important theories in physics. With inspiration from Galileo, he formulated the three laws of motion:

Law #1 = Inertia: objects at rest stay at rest; objects in motion stay in motion.

Law #2: Force equals mass times acceleration, F = MA

Law #3: For every action, there is an equal and opposite reaction.

He also explained gravity as the force that keeps planets in orbit around the sun.

Newton taught at Cambridge, became president of the Royal Society, and was knighted in 1705. He died in 1727.

Cool movies!

Other Types of Inertia...

Gyroscopic Inertia = "A body that is set spinning has a tendency to keep spinning in its original orientation."

For example, a top or gyroscope will not fall over when it is spinning.

Rotational Inertia = the property of an object that resists any change in its state of rotation. If at rest, it tends to remain at rest; if rotating, it tends to remain rotating and will continue to do so unless interrupted. For example, think of a metal ring and a solid disk experiment. The metal ring has its mass at the perimeter and therefore has more rotational inertia than the solid disk. This is why a ring resists rolling from a state of rest.

Observe. Which item has the most inertia? The green ring has the most inertia because it resists moving from rest. And, because it has the most inertia, it will certainly be the last one to stop moving along the floor.