Newton's 1st Law

Newton's 1st Law

Objectives

1. Be able to explain and describe Newton's 1st Law of Motion.

2. Understand the relationship between mass and inertia.

3. Know balanced and unbalanced forces.

4. Be able to predict the resulting force in a free-body diagram.

Who was Sir Isaac Newton?

Sir Isaac Newton was an English mathematician, physicist, astronomer, theologian, as well as writer that is commonly identified as one of the most influential scientists of all time. Newton shaped the way we understand motion along with the universe as a whole.

Newton's First Law of Motion: Inertia

Inertia is an objects tendency to maintain its state of motion. So if it is at rest, it will stay at rest and if in motion, it will stay in that constant state of motion. This occurs unless acted upon by an outside force.

Newton's First Law of Motion

Inertia is an object’s tendency to maintain its state of motion, or resist change in velocity.

3 parts to Newton's First Law

¹An object at rest will stay at rest

²An object in motion will stay in motion.

³This occurs unless acted upon by an outside force.

¹An object at rest will stay at rest

For a long time scientist thought that the natural state of things was to be at rest (stopped), and that constant motion requires constant force.

This was wrong!

An object stops because an outside force stops it. An object starts to move because an outside force starts it.

¹An object at rest will stay at rest

A force is required to change an object's state of motion or rest.

²An object in motion will stay in motion.

No force is required to get an object to move if it is already moving.

³This occurs unless acted upon by an outside force.

In this case, the outside force is friction. If there were no friction, the ball would roll forever.

Inertia makes your body want to keep going forward when in the car and you hit the brakes. Because of inertia, your body leans forward.

Inertia and Mass are Directly Related

Inertia is directly related to the mass of an object. An 80 kg human has a whole lot less inertia than a 3000 kg elephant.

The elephant takes a lot more force to start or stop moving.

Inertia comes from mass. The greater the mass of an object, the greater the inertia!

Also, the greater the inertia, the greater the force that's needed to change its motion.

For Example:

It requires a much greater force to move the bowling ball than the balloon because the bowling ball has more mass, and more inertia.

Open Ended

Can you think of a scenario where the greater the inertia the object has, the greater the force that's required to change its motion?

Unbalanced vs. Balanced Forces

Changes in motion come from unbalanced forces. Unbalanced forces are when the net force IS NOT exactly zero.

No change in motion can happen without an unbalanced force!!!

A NET force is the total of all forces acting on an object.

Unbalanced vs. Balanced Forces

In the top image, there is a balanced net force. Each side is pulling the rope with same amount of force. No motion occurs.

Unbalanced vs. Balanced Forces

In the bottom image, the team on the left is using 400 N and the side on the right is only using 300 N. With 100 N more of force, the left side team is winning the tug of war.

Force is a vector

This means that not only the amount of force matters, but the direction matters. These free-body diagrams help to illustrate and predict the resulting force.

Forces have strength AND direction!

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