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)

The ball is trying to roll back to a place where ho = hf

ho hf

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.

maF The sum of

Newtons 3rd Law:

For every force (action), there is an equal and opposite force (reaction).