Mechanical Comprehension Study Guide 1 for McGraw-Hill's ASVAB
Practice problems for this study guide can be found at:
The ASVAB Mechanical Comprehension test is all about the basic materials and mechanical devices that you see around you every day. The questions deal with things like levers, pipes, water wheels, gears, pulleys, and the like. This is not a test of high school physics, so don't worry if you can't explain things in terms of Newton's laws or use words like inertiaand elastic rebound. You won't have to know whymachines function as they do. You won't have to know a lot of complicated scientific terms. But you will have to know whatsimple machines and materials are and howthey operate. Most questions on the Mechanical Comprehension test can be answered using simple common sense, but you may need to do a few calculations. Many questions include a picture of some kind of simple machine, and you can use the picture to help you answer the question. Spinning gears, heat transfer from one material to another, and barrels rolling up inclined planes are all standard situations that you'll find on the Mechanical Comprehension test.
Whichever ASVAB version you take, you'll have only about a minute to answer each Mechanical Comprehension question, so you'll have to work fast if you want to get a good score. That's why it pays to spend time studying the test topics and tackling plenty of sample Mechanical Comprehension questions.
Mechanical Comprehension Question Topics
The topics covered on the Mechanical Comprehension test will probably be familiar to you if you have ever operated machines or taken them apart to repair them. The test-makers list the topics as follows: This chapter will help you prepare for the Mechanical Comprehension test. It starts by listing the important simple machines and explaining how they operate. Then it covers other important topics, such as properties of materials, mechanical motion, and fluid dynamics. At the end of the chapter, there is a short quiz with questions modeled on those on the actual test. Read carefully through the review materials in this chapter, then use the quiz to find out how well you have mastered this subject area. Go back and reread the review materials for any quiz question you miss.
Some Basic Concepts: Work, Energy, and Force
To do well on Mechanical Comprehension questions, you'll need to understand just a few basic concepts. For starters, you should know that in mechanics, work refers to a specific force applied over a specific distance. For example, your arm does work when it uses force to pick up a book. A lever does work when it uses force to lift a heavy object.
The ability to do work is called energy. Energy comes in several forms:
- Kinetic energy: Energy in a moving object.
- Potential energy: Energy that can be released under certain conditions. For example, potential energy is stored in objects when they are lifted off the ground. It is released when if the objects fall.
- Chemical energy: Energy stored in chemicals, such as in a flashlight battery. Chemical energy is potential until it is released in a chemical reaction.
- Electric energy: Energy in moving electrons in an electric current.
- Nuclear energy: Energy released by reactions in the nucleus of an atom.
- Solar energy: Energy in the heat and light from the sun.
On the ASVAB, you may be asked to tell which kind of energy is present in a given situation. For example, when a child's swing reaches its highest point and pauses momentarily before swinging back down, the swing has only potential energy and no kinetic energy. When it swings through its lowest point, the opposite is true: The swing has no potential energy (because it cannot fall any further) and all kinetic energy. As it swings back up the other side, the kinetic energy is converted back into potential energy.
Similarly, a battery stores electric energy as chemical energy. When the flashlight is turned on, electric energy passes through the filament in the flashlight bulb, where it is converted into heat and light.
Forces are powers that push or pull objects. A force has a magnitude (strength) that you can measure, and it has a direction. Some forces are obvious—when a bat hits a baseball, you can even hear the force being applied. Other forces, like gravity and air pressure, are much less obvious, but they are still real.
One kind of force is gravity. Gravity is an attractive force between objects. All objects create a gravitational attraction for each other. On Earth, gravity causes objects to fall toward the center of the Earth. Falling objects accelerate (fall faster) as they fall. (Acceleration is defined as the change in velocity, or speed in a particular direction.) If you discount air resistance, all objects fall at the same rate—a fact proved centuries ago by the Italian scientist Galileo. In real life, however, air resistance often disguises the fact that objects fall at the same rate.
Air resistance is a kind of friction. Friction is a force that results from the interaction between two surfaces that are touching each other. Friction acts as a resistance to the movement of an object. For example, friction makes it harder to push a heavy crate up a ramp. Friction also helps to keep the crate from sliding back down the ramp. Without friction, a car would slide all over the place because the tires could not get a grip on the pavement. Tables would slide all over the room, and billiard balls would never stop rolling! In machinery, friction can often be a problem. It can prevent machines from running smoothly and efficiently. That's why lubricants are used to reduce friction.
Two other forces are compression and tension. Compression is a force that pushes materials together. Tension is a force that pulls materials apart. Air pressure and water pressure are forms of compression. The force exerted by the cable in a pulley is a kind of tension. In a bridge, some parts are in compression and others in tension. The weight of the structure squeezes (compresses) the top and puts tension on the bottom. Steel reinforcing is strong in tension, so it is placed at the bottom of a bridge, where its tensile strength can support the load.