ASVAB Mechanical Comprehension Subtest: Kinds of Forces - dummies

ASVAB Mechanical Comprehension Subtest: Kinds of Forces

By Rod Powers

To prepare yourself for the Mechanical Comprehension subtest of the ASVAB, you will need to know some basics about the kinds of forces. Here are some of the forces that act on objects:

  • Friction: Resistance to the motion of two objects or surfaces that touch

  • Gravity: The physical property that draws objects toward the center of Earth (and other objects that have mass)

  • Magnetism: The property of attracting iron or steel

  • Recoil: The property of kicking back when released

  • Static electricity: The production of stationary electrical charges, often the result of friction

Friction

When one surface (such as a floor) resists the movement of another surface (the bottom of a piano), the result is frictional resistance.

In order to perform work — that is, to get an object to move in the direction you’re pushing or pulling — sometimes you have to overcome friction by applying more force.

For example, when you’re moving a piano across a smooth, vinyl floor, little friction is produced, so the amount of force required to push the piano comes from the piano’s weight and the very minor friction produced by the smooth floor. But when you’re moving a piano across a carpeted floor, more friction is produced, so you have to push harder to move the same piano the same distance.

Rolling friction (like the friction that occurs when you roll a wheel along the pavement) is always less than sliding friction (which occurs when you shove a piano along the floor). If you put wheels on a piano, it’s much easier to push!

You can decrease friction by using a lubricant. Oil, grease, and similar materials reduce friction between two surfaces. So theoretically, if you oil the bottom of a piano, it’s easier to move! (Oiling the bottom of your piano isn’t recommended — for reasons involving the appearance of your floor and piano.)

Gravity

Sir Isaac Newton invented gravity in 1687 when he failed to pay attention while sitting under a tree and got bonked on the noggin by an apple. Before that, gravity didn’t exist, and everyone just floated around. Okay, that’s a joke. Isaac Newton didn’t invent gravity. But the famous mathematician was the first to study gravity seriously, and he came up with the theory (now a scientific law) of how gravity works.

Newton’s law of universal gravitation states that every object in the universe attracts every other object in the universe. Earth produces gravity, and so do the sun, other planets, your car, your house, and your body. The amount (force) of the attraction depends on the following:

  • Mass: The force of gravity depends on the mass of (amount of matter in) the object. If you’re sitting in front of your television, you may be surprised to know that the television set is attracting you. However, because the mass of the TV is so small compared to the mass of Earth, you don’t notice the physical “pull” toward the television set.

    Note that the force of gravity acting on an object is equal to the weight of the object. Of course, other planets have lesser or greater masses than Earth, so the weight of objects on those planets will be different.

  • Distance: Newton’s law also says that the greater the distance is between two objects, the less the objects will attract each other. In other words, the farther away an object is from Earth (or any large body), the less it will weigh. If you stand at the top of a high mountain, you will weigh less than you will at sea level. Don’t get too excited about this weight-loss technique. Gravitational pull isn’t the next big diet craze. The difference is incredibly small. Sorry!

    For an object to really lose weight, it must be far away from Earth (or any other large body). When an object is far enough away from these bodies that it experiences practically no gravitational pull from them, it is said to experience weightlessness — just like the astronauts you see on TV.

Gravity pulls objects downward toward the center of Earth, so the old saying “what goes up must come down” is appropriate when discussing gravity. If you fire a bullet straight up into the air, it will travel (overcoming the force of gravity) until it reaches its furthest or highest point, and then it will fall.

Tension

Tension force is the force transmitted through a rope, string, or wire when force is applied to both ends. The force is the amount of tension directed along the rope, string, or wire and pulls equally on the objects at both ends. Tension force is usually measured in either pounds-force or newtons (N); 4.45 newtons equal 1 pound-force.

An example of tension force.
An example of tension force.

Elastic recoil

Liquids and gases don’t have a specific shape, but solid matter does. Solids are perfectly happy with the way they look and resist changes in shape. If you exert a force on a solid shape, it responds by exerting a force in the opposite direction. This force is called elastic recoil.

Take a look below. The cat is standing on a board suspended on two blocks. While the board bends, the cat can feel the force of the board trying to regain its original shape. If the cat steps off the board, the board will spring back to its normal state.

The concept of elastic recoil.
The concept of elastic recoil.