History of Laws of Motion

1. History of Laws of Motion

2. Definitions • Force- push or pull, the units of force are Newtons (N) • Friction- force between objects moving across one another. • Friction always opposes motion • It slows objects down by converting its mechanical (moving) energy into heat or sound energy • Other forces include a gravitation force and a force normal (the force of two objects pushing on each other).

3. Aristotle ~350 B.C. • He believed the natural state for all objects was at rest somewhere. • Objects will naturally move to where they are supposed to be. • For example rock-bottom of a hill, smoke-in the sky. • He believed all other motion was caused by a force, if the force stopped the object would eventually stop.

4. Aristotle was INCORRECT!!! • This is a common misconception, many students still think along the lines of Aristotle. • Objects do NOT naturally want to be at rest. • A constant force is NOT required to keep an object moving. • These theories are believed because people cannot imagine a truly frictionless surface. • On Earth, a completely frictionless surface is impossible

5. Galileo ~1600 • Through several experiments Galileo argued that if friction could be removed completely, a moving object would move forever. • He also argued that objects naturally resist CHANGE in motion • He called this inertia. • Unfortunately, Galileo could not continue his work in this area because of his support of Copernicus’ solar-centric theory.

6. Inertia • Inertia is an object’s resistance to change in motion. • NOT just an object’s resistance to stopping… • but also to slowing down, speeding up and turning. • Inertia is measured in mass. • The more mass, the more an object will resist a change in motion.

7. Sir Isaac Newton 1642-1727 • Quite possibly the most brilliant man ever to live. • By age 24 he had developed his laws of motion • He is considered to be the father of classical physics • He also invented calculus

8. Newton’s 1st 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. • basically an object is going to keep on doing what it is already doing unless something comes along and pushes it.

9. Newton’s 1st 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. • basically an object is going to keep on doing what it is already doing unless something comes along and pushes it.

10. Examples • How can a magician pull a table cloth out from under the plates? • The plates have inertia, they will not move unless a force is applied to them. • Why do you need to wear a seat belt? • Your body has inertia and it will be moving as fast as the car. Your body will continue moving in a straight line at the same speed until an outside force stops it.

11. A question • If Newton’s law is true, then why do you need to keep your foot on the gas pedal to make a car stay at the same speed? • You need to overcome the frictional forces (between the tires and the road and air resistance) • Your forward force will have to equal the backwards force so you have no net force.

12. Net force • is not a computer hacker organization • Net force – sum of all forces acting on an object. • All objects on Earth always have forces working on them. • The force of gravity is always pulling you to the center of the Earth. • If you are moving there is also a frictional force against you.