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Mechanical Comprehension Study Guide 1 for McGraw-Hill's ASVAB (page 3)

By — McGraw-Hill Professional
Updated on Jun 26, 2011

Compound Machines

A compound machine is one in which two or more simple machines work together. For example, a screwdriver (wheel and axle) driving a screw is a compound machine. To find the mechanical advantage of a compound machine, multiply the MA of the simple machines together.

Example

The axe shown is a compound machine. The handle is a lever, and the head is two inclined planes. Find the combined MA by multiplying the individual MA of each simple machine.

Structural Support

Some Mechanical Comprehension questions ask about the load carried by support structures such as beams or bridges—or sometimes by people. You will be given a diagram showing support structures and asked which one is strongest or weakest, or which support in the diagram is bearing the lesser or greater part of the load. To answer this kind of question, keep this in mind: When a load of any kind is supported by two support beams, posts, or people, the load is perfectly balanced if it is exactly centered. In that case, each beam, post, or person is bearing exactly half the load. However, if the load is not centered, then the beam, post, or person nearer to the load is bearing the greater part of the weight.

Example

The figure shows two people carrying a load. The load is centered. Each person is bearing half the load, or 75 lb.

Example

The figure shows two people carrying a load. Which person is bearing the greater part of the load? Common sense tells you that it's the woman, because she is closer to the load.

Questions about structural support may also ask you to decide which of several support structures is the strongest (or weakest). For this kind of question, you can often use your common sense. If a diagram shows several different structures, look for the one with the most brackets or other support elements. Also look for the one that leaves the least horizontal area (for example, the front part of a flat bookshelf) unsupported. A bracket that runs the whole width of the shelf provides better support than one that does not.

Example

Which of these four shelves can bear the most weight?

Choice D can bear the most weight. Of the four shelves, it is the strongest because it has the largest brackets, and because it has the most brackets.

Shape can also make a difference in the strength of a support structure. For example, a structure in which the support beams form rectangular shapes is not as strong as one in which the beams form triangular shapes. The reason is that while rectangular supports can easily bend out of shape, a triangle keeps its shape unless it falls apart entirely. That's why triangular shapes are used in support structures such as shelf brackets. That is also why you often see triangular shapes in the support structures of bridges, towers, and other buildings.

Example

Which bridge is the strongest?

In the diagram, bridge C is the strongest because its framework is made of many triangles.

Properties of Materials

Tools, machines, and structures can be made of many different materials, such as wood, metal, plastic, and the like. Each material has its own properties, or characteristics. Some Mechanical Comprehension questions will ask you about those properties, or ask you to compare the properties of different materials. Most questions of this type can be answered with simple common sense, based on your everyday experience. For some, you may need to recall some of the basic ideas you learned in high school science.

One of the most important properties of materials is the ability to conduct heat. Here is an example of a question about this property.

Example

Three pots are cooking on a stove. Pot A has a wooden handle, pot B has a metal handle, and pot C has a plastic handle. Which handle will feel hottest to the touch?

  1. A
  2. B
  3. C
  4. All three will feel equally hot

To answer this question, think about your everyday experience. For which pot would you need a potholder? You might also remember from a high school science class that metal is a better conductor of heat than either wood or plastic. The correct answer is choice B.

Another important property of some materials is the ability to bend without breaking. This is called flexibility. The springs on a car can bend millions of times without breaking because they are made of spring steel. A ceramic cup, on other hand, cannot flex even once without breaking. Materials that are somewhat flexible include regular steel, copper, and wood. Some flexible materials will spring back to their original shape after bending. Others will stay in the new shape.

Example

Which object shown will resume its original shape after bending?

The spring (choice C) will resume its original shape. Nails, screws, and bolts are not made of spring steel, and while you can bend them, they will not spring back into their original shape.

Another important property of some materials is the ability to be formed into a new shape through repeated blows from a hammer. This property is called malleability. You will probably not need to be able to recall this term, but you should be able to identify substances that have this property. Metals like steel and copper are malleable and can thus be formed into the shapes of tools, containers, and other objects. Glass and ceramics, on the other hand, are not malleable.

Still another important property of some materials is the ability to stretch without breaking. Rubber can stretch in this way. Metal, glass, and plastic can stretch when heated. Wood and paper, on the other hand, will not stretch.

Practice problems for this study guide can be found at:

Mechanical Comprehension Practice Problems for McGraw-Hill's ASVAB

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