# Understanding the Forces of the Universe for the General Science Subtest of the ASVAB

You will want to have a basic understanding of forces for the ASVAB. By applying *force* (a push or pull), you can open the door or close it, speed it up (slam it) or slow it down (catch it before it slams), or make it change direction (push it shut when the wind blows it open).

In physics, applying force allows changes in the velocity (the speed and direction) of an object. A change in velocity is known as acceleration. Here’s the mathematical formula to determine force:

Force = Mass x Acceleration

Martial artists use this concept all the time. Although a larger fighter may have more size (mass), a smaller fighter can usually speed up more quickly (have more acceleration), possibly resulting in both fighters’ applying the same amount of force. This concept is why 110-pound martial artists can break boards and bricks just as well as 200-pound martial artists.

## The basics of action and reaction

Sir Isaac Newton sure was one of the sharpest crayons in the box. His third law of motion states that for every action (force) in nature, there’s an equal and opposite reaction. In other words, if object A exerts a force on object B, then object B also exerts an equal and opposite force on object A. Notice that the forces are exerted on different objects.

As you sit in your chair, your body exerts a downward force on the chair, and the chair exerts an upward force on your body. There are two forces resulting from this interaction: a force on the chair and a force on your body. These two forces are called *action* and *reaction forces.*

This force can also be used to describe how a motorboat moves through the water. As the propellers turn, they push the water behind the boat (action). The water reacts by pushing the boat forward (reaction).

## Equilibrium: Finding a balance

Forces are *vector* quantities. That means they have both a magnitude (size) and a direction associated with them. Forces applied in the same direction as other forces increase the total force, and forces that move in opposite directions reduce the total force. In general, an object can be acted on by several different forces at any one time.

A very basic concept when dealing with forces is the idea of equilibrium or balance. When two or more forces interact so that their combination cancels the other(s) out, a state of equilibrium occurs. In this state, the velocity of an object doesn’t change. The forces are considered to be balanced if the rightward forces are balanced by the leftward forces and the upward forces are balanced by the downward forces.

If an object is at rest and is in a state of equilibrium, then it’s at *static equilibrium. Static* means being stationary or at rest. For example, a glass of water sitting on a table is at static equilibrium. The table exerts an upward force on the glass to counteract the force of gravity.

## Under pressure: Spreading out the force

*Pressure* is a measurement of force over an area. Pressure is usually measured in pounds per square inch (psi). The formula for deriving pressure is

If 50 pounds of force is exerted on 10 square inches of surface, the amount of pressure is 5 pounds per square inch (5 = 50 / 10).

Consider this: If you’re sleeping in bed, the amount of pressure being exerted per square inch is much less than when you’re standing on your feet. The surface area of the bottoms of your feet (supporting all that weight) is much less than the surface area of all your body parts that touch the mattress.

Ever wonder how a person can lie on a bed of nails? The answer involves elementary physics. His or her body rests evenly on hundreds of nails; therefore, no individual nail exerts a great amount of pressure against the skin. Have you ever seen someone stand on a bed of nails? It’s unlikely because more pressure is on the feet, and the nails would puncture the feet.

A *barometer* is a gauge that measures atmospheric pressure. Normal atmospheric pressure is 14.7 psi. A change in air pressure means the weather is about to change.