History of Laws of Motion

Aristotle ~ 350 B.C.

He believed that the natural state for all objects was at rest.

He believed all motion was caused by a force. If the force stopped, the object would eventually stop.

Aristotle was WRONG!!!

Many people still think along the lines of Aristotle.

Objects do not “want” to be at rest. If we can remove friction, objects will not

stop.On Earth, a completely frictionless surface

is impossible.

Galileo ~ 1600

Galileo argued that a moving object would move forever, without friction.

He also argued that objects naturally resist CHANGE in motion.

He called this inertia.

Inertia

Inertia — an object’s resistance to change in motion.

NOT just an object’s resistance to stopping, but also to slowing down and speeding up.

Inertia is measured by its mass.The more mass, the more an object will

resist a change in motion.

Sir Isaac Newton 1642-1727

Developed and explained three laws to explain how objects move.

He also explained other concepts including gravity and calculus.

Newton’s First Law of Motion

An object at rest tends to stay at rest, and an object in motion tends to stay in motion in a straight line at a constant speed unless it is acted on by a net outside force.

An object is going to keep on doing what it is already doing unless something comes along and pushes it.

Examples

How can a magician pull a tablecloth out from under a stack of plates?

The plates have inertia; they will not move unless a force is applied to them.

Why do you need to wear a seatbelt?You have inertia. When moving, you will

continue moving at the same speed until an outside force stops you.