# Mrs. Kiley 8th Grade Science

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Newton’s 1st Law Mrs. Kiley 8th Grade Science

Newton’s First Law of Motion is also called the Law of Inertia.
A restatement of Galileo’s ideas about forces not needed to keep an object moving.

Newton’s 1st Law states that every object continues in a state of rest, or of uniform speed in a straight line, unless acted on by a nonzero net force. Also commonly stated: objects at rest tend to stay at rest, objects in motion tend to stay in motion.

Objects at Rest Things tend to keep on doing what they’re already doing. Dishes on a table are in a state of rest – they will remain at rest unless a force acts on them. Example: pulling a tablecloth out from under a table full of dishes.

Objects in Motion Hockey Puck example:
If you slid a hockey puck along the surface of a street, how far would it go? Compare that to an ice rink. How far would it go? Why the difference?

In the absence of forces, a moving object tends to move in a straight line indefinitely.
It is a VERY common misconception that forces must continue to be present for an object to continue moving. Ancient people thought this and many people today still believe this.

Forces are needed to overcome any friction that maybe present and set objects in motion initially.
Once the object is moving, it will move in a straight line by virtue of its own inertia.

Mass as a Measure of Inertia
Kick an empty can – what happens? Kick a can filled with sand – what happens? Kick a can filled with steel nails – what happens? The nail filled can has much more inertia than the other two cans.

The amount of inertia an object has depends on its mass.
The more mass an object has, the greater its inertia and the more force it takes to change its state of motion. Pushing a refrigerator…

Mass is Not Volume Mass and volume are not the same thing.
Volume is a measure of space and measured in units such as cubic centimeters and liters. Mass is measured in the unit of kilograms. For instance, equal size bags of cotton and nails have equal volumes but very different masses.

Which object has greater inertia?

Mass is Not Weight Mass and weight are often confused, but are not the same thing. Mass is a measure of the amount of material in an object and depends only on the number of and kind of atoms that compose it. Weight is a measure of the gravitational force acting on the object. Weight depends on the object’s location.

Mass is Inertia Mass is the quantity of matter in an object.
Mass is a measure of the inertia that an object exhibits in response to any effort to make it start, stop, or otherwise change its state of motion. Weight is the force of gravity on an object.

Summary Newton’s 1st Law is often called the Law of Inertia.
Inertia is an object’s tendency to resist a change in motion. Objects at rest tend to stay at rest; objects in motion tend to stay in motion. Mass is not the same thing as volume or weight.