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This text is meant to accompany class discussions. It is not everything there is to know about uniform circular motion. It is meant as a  prep for class. More detailed notes and examples are given in the class notes, presentations, and demonstrations (click here.)

Click for the questions that go with this reading
1st Law

Newton's 1st law says:

A body in motion stays in motion; a body at rest stays at rest, until acted upon by an unbalanced force.

The word motion means constant velocity -no change in speed.

Now that your know what "motion" looks like in context of the first part of the law, what does an unbalanced force look like? In the picture below, two balls are rolling and a picture is taken every second to show its position. The ball on top is experiencing a unbalanced force pointing from horizontally left to right. The ball on the bottom is experiencing balanced forces. Note that balanced force does NOT mean "no forces." It means the force pushing to the right equal the forces pushing to the left.


An unbalanced force causes a body to change velocity. This means it speeds up, slows down or changes direction. If you were to watch a movie and try to identify when your "see" this law in action you would look for two characteristics.

  1. When a object is hard to push or resists being pushed (neglecting outside forces) then the body wants to stay at rest.
  2. When a body is moving and after everything else stops it continues to move, then is this is the first law.

Out of this law comes the concept of inertia. Inertia is the described as the tendency to resist a change in motion. Inertia is a property of an object like color and size are properties of an object. For a resting object, mass makes a good comparison as to how much inertia one body has compared to another. The heavier object is harder to move so it resists a change in motion because it has more inertia. For a moving object, the inertia is replaced by the term of momentum, More about momentum later. The first law is often referred to as the Galileo's law of inertia.

What does an object with inertia "look like?" It looks like anything with mass that is moving at a constant velocity. Sometimes its constant velocity is zero. The more inertia an object has the greater is resistance to a change in velocity. This means the greater is resistance to slowing down, speeding up or changing direction until an unbalanced force changes the velocity.

Example #1

The brain and skull have inertia. They brain is surrounded by spinal fluid in the skull. The brain and skull are both moving at a constant velocity when you are moving at a constant velocity. When the skull changes its velocity due to a sudden impact (outside force) the brain continues to move forward until it hits the skull. This is a concussion.

Example #2

This YouTube video cannot be embedded. But you can see it by clicking here.

This reporter has mass. Therefore he has inertia, Re resists changing velocity. In this case his velocity is constantly zero because he is standing still. When he get hit by the snow tube a force knocks his feet out from underneath him. But he hardly moves horizontally. Watch his waist. It lands in nearly the same place on snow as where he when he was originally standing. His motion velocity is maintained until this outside force tries to move it. Even then he is resisting changing his velocity.

Example #3

When traveling down around town, your car will stop and start as it travels through the stoplights. If you have a helium filled balloon in the car and the car's windows rolled up, then the balloon will behave oddly. It will move to the front of the car as the car accelerates and to the rear of the car as the car slows down. This can be explained in terms of inertia.

The air and the helium filled balloon both have inertia. The air you breathe has more inertia than the helium because a box of air weighs more than a box of helium. When the car accelerates, the air resists the acceleration and piles up in the rear of the car. This creates a higher density in the rear of the car than in the front of the car. The lighter balloon is pushed to the front of the car. In the animation above, the increased density of air is shown as the air getting bluer. When the car stops, the air does not want to stop moving so it piles up in the front of the car. This increased density pushes the helium filled balloon to the rear of the car.

Example #4

A very light car slides, without friction, into a very heavy truck as shown below. how does this demonstrate the 1st law?


by Tony Wayne ...(If you are a teacher, please feel free to use these resources in your teaching.)