What are two differences between force and pressure?

Answer 1

A force is an application of energy that tries to make something change its motion. A good example of this is when you put a ball on a level floor and give it a push. The push applies a force to the ball, which then changes from not moving to rolling across the floor. To stop the ball, you would apply a force in the opposite direction and make it come to rest.

Sometimes a force doesn't make something move because it's exactly matched by an opposite force. When you sit on a chair, your weight is a force acting on the chair: the chair pushes back just as hard onto you, so you don't fall through the chair. The two forces (your weight and the chair's pushing back) balance out.The units of force are pounds (in the US) or Newtons (in the Metric system)

Pressure is a force spread out over an area that it acts upon. For example, lets say I was walking along and I stepped onto a one-inch square piece of brick on the sidewalk. I weigh around 175 pounds, so my body would apply a 175 pound force to that piece of brick. The sidewalk would also feel that force transmitted through the piece of brick, spread out over the one square inch of surface the brick has in contact with the sidewalk. The pressure is the force divided by the area it acts on (P = Force/Area), so in this case the pressure applied to the sidewalk is 175 pounds per square inch. If that piece of brick were 5 inches square, its area would be 25 square inches: the pressure applied to the sidewalk would be 175 pounds/25 square inches = 7 pounds per square inch. The same force is acting on the sidewalk: it's just spread out over more brick surface. Pressures can have units of pounds per square inch, or Newtons per square meter (called Pascals).

Liquids also have pressure which depend on their weight, how closely packed their molecules are, and their temperature. This pressure is caused by the molecules of the liquid bouncing agains a surface they're in contact with - like the inside of a jar, or the inner surface of an eye. If a liquid has a pressure of one pound per square inch, every square inch of surface it presses against has one pound of force spread over it.

The total amount of force the pressure exerts on the surface is found by multiplying the pressure by the area of the surface it is pushing on (F = Pressure x Area). The one pound per square inch from before pushing on ten square inches of surface adds up to a total force of ten pounds trying to push whatever the surface is attached to.

So, a force is essentially a push, and pressure is a push spread out over a surface.