Motion and Forces

Newton's first law states that an object at rest stays at rest and an object in motion stays in motion unless acted on by an unbalanced force. This is called inertia.

Newton's third law states that for every action there is an equal and opposite reaction. When you push against a wall, the wall pushes back on you with the same force.

Momentum is mass times velocity. A heavy truck moving slowly can have the same momentum as a light car moving fast. Momentum is conserved in collisions.

Air resistance, also called drag, is a force that pushes against objects moving through the air. A falling feather drifts down slowly because air resistance acts on its wide surface, while a compact stone meets less drag and falls faster.

Pressure is the amount of force pushing on a certain area. Spreading a force over a larger area lowers the pressure, which is why wide snowshoes keep you from sinking while a sharp, thin blade cuts easily.

Terminal velocity is the steady top speed a falling object reaches when air resistance pushing up equals gravity pulling down. Once these forces balance, a skydiver stops speeding up and falls at a constant rate.

Mechanical advantage tells how much a machine multiplies the force you put in. A machine with a mechanical advantage of four lets you lift a load using only one-fourth of the force, but you must move it a greater distance.