DYNAMICS The study of why objects move or don’t move. What is the natural state of motion of an object? Aristotle Observed: • All objects eventually come to a stop. • Therefore he concluded the natural state of motion for an object, is for the object to be at rest. • He Incorrectly concluded that the only way an object stays in motion is if it has a force acting on it. • But, what is a force? Force – A push or a pull caused by the interaction of two objects. • • • • F = Force Units: Newtons (N) or pounds (lb) 1lb = 4.45N A force is a vector quantity – It has magnitude (value) and direction. Weight is a Force • It is caused by the interaction between the earth and an object. • It has a value and a direction (down). • Objects have the same mass everywhere, they only have weight when they interact with a planet, like earth. • A 150lb person has a weight of 668N. Copernicus Observed: • The motion of the stars and planets. • He concluded that the earth was in constant motion. • This idea was highly controversial. • People preferred to think the earth was the center of the universe. • As a result he was persecuted and had to work in secret. Galileo devised experiments to discover the natural state of motion. • His experiments represent the birth of modern day science. • He also suffered persecution for his efforts. Galileo’s Experiments: A ball rolling down a ramp, rolls to the same height on a ramp across from it. He worked very hard to eliminate friction, but of course he never could completely. ho hf ho=hf He changed the angle of the second ramp and observed the ball rolled further before reaching hf. ho hf ho=hf He than imagined what would happen if the second ramp was flat. Hmmm? What would happen? (remember to neglect friction) ho hf The ball is trying to roll back to a place where ho = h f The ball would roll forever with a constant velocity!!!!!! Galileo’s experiments demolished the idea that a force is necessary to keep an object moving. The natural state of motion for an object is to be moving with a constant velocity, which could be zero. So an object at rest is just a specific example of an object with a constant velocity. Sir Isaac Newton was the next major player on the scene • He was born on Christmas day, the year Galileo died. • His work influenced today’s world at a monumental level. • He developed three laws that describe the motion of everything. • He very well may have been one of the most influential human beings to ever live. • And you thought he was just some dumb guy who got hit on the head with an apple. 1642-1727 Inertia • The natural tendency of an object to resist a change in motion. • Mass is a measure of inertia. Newtons 1st Law: An object with constant velocity (which could be zero) continues with constant velocity (same speed and direction) unless an unbalanced force acts on it. Newtons 2nd Law: The sum of the forces on an object is equal to the mass times the acceleration of the object. F ma The sum of Newtons 3rd Law: For every force (action), there is an equal and opposite force (reaction).