Dynamics

Newton's Laws of Motion

A physical quantity that can change the state of motion of an object
Simply, a push or a pull
Net Force: the vector sum of all forces acting on a body. If every force has an equal opposing force, there is no net force and motion is not changed
unit is newton (N), symbol F

Aristotle (350 BC) thought a body's natural state was at rest--force needed for motion
Galileo (late 1500's) showed no force was needed to continue motion, if friction was absent
Newton (late 1600's) studied motion, formulated 3 laws of motion

As long as no net force is acting on it, a body at rest will remain at rest, a body in motion will continue in straight line motion at a constant speed.
Inertia: the property of a body that opposes a change in motion
Mass is the measure of an object's inertia
Greater the mass, greater the inertia, and harder it is to change motion
On earth, friction and gravity cause moving objects to stop, but in space, 1st law is easily observed
Galileo first to show this law by rolling ball down one ramp and up another

A net force on a body will cause acceleration in the direction of the net force; acceleration will be directly proportional to the net force and inversely proportional to the mass of the body
a = F_net/m or F_net= ma
If there is no net force, there is no change in motion and 1st law applies
If force is in direction of motion, speed will increase; if in opposite direction, it will decrease; if in another direction, direction will be changed (and maybe speed too)
1 newton = force needed to accelerate a mass of 1 kg at 1 m/s²
Weight is a force that creates the acceleration due to gravity (g): F_w=mg

If one body exerts a force on a second body, the second body exerts an equal and opposite force on the first.
For every action force, there is an equal and opposite reaction force
Forces always occur in pairs
Action-reaction forces always act on different objects
They cannot cancel each others action because they act on different objects
Often there are equal and opposite forces canceling each other, but these are not action-reaction pairs; e.g. book lying on table

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