Force and Pressure Notes

A force is applied when one object pushes or pulls another. A force always involves an interaction between at least two objects. Force has both magnitude and direction, and when two forces act along the same line, the net force is their sum if they are in the same direction and their difference if opposite.

A force can (1) make a stationary object move, (2) stop a moving object, (3) change the speed of a moving object, (4) change the direction of motion, and (5) change the shape or size of an object.

The state of motion of an object is described by its speed and direction. A force is needed to change this state of motion.

Contact forces act only when objects touch. Muscular force is applied by the action of muscles (lifting, pushing a cart). Friction is the force between two surfaces in contact that opposes relative motion.

Non-contact forces act even without touching. Magnetic force acts between magnets and on magnetic materials. Electrostatic force is exerted by a charged body on another charged or uncharged body. Gravitational force is the pull of the Earth (and of every object) — it gives objects their weight and makes things fall.

The pull of gravity acts on every object at all times, which is why a ball thrown up comes back down.

Pressure is the force acting on a unit area of a surface. The same force produces a larger pressure when it acts on a smaller area — which is why a sharp knife (small edge area) cuts more easily and why school bags have wide straps (larger area, less pressure on the shoulder).

Liquids and gases exert pressure on the walls of their container and on any object placed in them. Pressure in a liquid increases with depth, which is why dam walls are built thicker at the bottom.

The air around us has weight and exerts atmospheric pressure in all directions. It is large — a rubber sucker sticks to a smooth surface because air pressure outside pushes it in after the air beneath is pushed out.