Mechanical Comprehension Review for Armed Services Vocational Aptitude Battery (ASVAB) Study Guide (page 3)
This article will help you prepare for the Mechanical Comprehension subtest of the ASVAB. It presents the most commonly tested concepts: basic and compound mechanical machines and devices, mechanical motion, fluid dynamics, properties of materials, and structural support.
Every day, often without even thinking about it, you use mechanical devices. These could be simple machines such as levers and pulleys, or more complex compound machines such as linkages and gears. The ability to understand and use mechanical concepts is important both for many tasks required in the armed services and in everyday life.
The Mechanical Comprehension subtest of ASVAB may cover topics you are familiar with, as well as some that are new. Understanding the concepts in this chapter will benefit you both for the exam and in your career in the armed services. After an introduction to the Mechanical Comprehension subtest, this chapter summarizes definitions and the most commonly tested mechanical concepts. It also suggests how you can add to your knowledge of mechanical concepts and related scientific and mathematical knowledge. At the end of the chapter, you get an opportunity to review what you've learned by answering sample Mechanical Comprehension questions like those found on the ASVAB.
What Mechanical Comprehension Questions are Like
The Mechanical Comprehension subtest covers a wide range of topics. It consists of 25 multiple-choice questions, which you will have 19 minutes to answer. Most questions require previous knowledge of the topic, while some questions will themselves provide all of the information you need to figure out the answer.
Some questions require you to identify various mechanical machines or devices. Some of the mechanical devices that may appear on the exam—and are covered in this chapter—include gears, pulleys, levers, fasteners, springs, gauges, hinges, and linkages.
Other questions require knowledge of mechanical motion such as velocity, acceleration, direction, and friction for both solid bodies and fluids. These questions test concepts such as the motion and acceleration of automobiles or the buoyancy and pressure of fluids.
The Mechanical Comprehension subtest also covers the properties of materials and the concept of structural support. The material properties include weight, strength, density, thermal properties, and center of gravity. Structural support includes concepts such as weight distribution.
A typical mechanical comprehension question will look something like this:
- What is the main function of a pulley?
- to increase the strength of a construction crane
- to override the power of an electric motor
- to add energy to a system
- to change the direction of a pulling force
The correct answer is d, to change the direction of a pulling force. Pulleys are used to change not the strength of a force but its direction.
Review of Mechanical Comprehension Concepts
As aforementioned, some of the mechanical concepts most likely to appear on the ASVAB include basic and compound machines, mechanical motion, the behavior of fluids, the properties of materials, and structural support.
Basic and Compound Machines
Most mechanical machines and devices were invented in a similar manner: people were looking for easier ways to perform their everyday jobs. Some mechanical devices are thousands of years old, such as the lever, the wheel, and many hand tools. Other more complex devices, such as pumps and valves, were invented more recently. Often the idea of a new mechanical device exists, but the technology to actually make it does not. For example, many years before the pump was invented, people probably discussed the need for an easier way to move water from the river to the town on the hill. However, the technologies of the electric motor and metal casting had not yet been discovered, so the modern pump could not be invented.
In general, a mechanical device is a tool that does physical work and is governed by mechanical forces and movements. In other words, you can usually see what a mechanical device does and how it works—as opposed to, say, electrical devices such as light switches or batteries. Some tools are used to directly accomplish a specific task, as when you use a hand saw to cut a piece of wood. Others, such as pulleys and gears, may be used indirectly to accomplish tasks that would be possible without the device but are easier with it. Still others, such as gauges, provide feedback on how well other mechanical devices are working. You see and use mechanical devices many times each day, so there's little reason to be intimidated by an exam question on a mechanical device.
A gear is a toothed wheel or cylinder that meshes with another gear to transmit motion or to change speed or direction. Gears are usually attached to a rotating shaft that is turned by an energy source such as an electric motor or an internal combustion engine. Mechanical devices that use gears include automotive transmissions, carpenter's hand drills, elevator lifting mechanisms, bicycles, and carnival rides such as Ferris wheels and merry-go-rounds.
Gears are used in different configurations. In an automotive transmission, for instance, two gears may directly touch each other. As one gear spins clockwise, the other rotates counterclockwise. Another possible configuration is to have two gears connected by a loop of chain, as on a bicycle. In this arrangement, the first gear rotates in one direction, causing the chain to move. Since the chain is directly connected to the second gear, the second gear will rotate in the same direction as the first gear.
Often a system will use two gears of different sizes, as on a ten-speed bicycle. This allows changes in speed of the bicycle or machine.
Test questions about gears will always involve rotation, or spinning. The easiest way to approach questions about gears is to use the picture given or to draw one, if it's not already provided. Draw an arrow next to each gear to indicate which direction (clockwise or counterclockwise) it is rotating.
A pulley consists of a wheel with a grooved rim through which a rope or cable is run.
Pulleys are often used to change the direction of a pulling force. For instance, a pulley could be attached to the ceiling of a room. A rope could be run from the floor, up through the pulley, and back down to a box sitting on the floor. The pulley would allow you to pull down on the rope and cause the box to go up.
Another common use for a pulley is to connect an electric motor to a mechanical device such as a pump. One pulley is placed on the shaft of the motor, and a second pulley is placed on the shaft of the pump. A belt connects the two pulleys. When the motor is turned on, the first pulley rotates and causes the belt to rotate, which in turn causes the second pulley to rotate and turn the pump. This arrangement is very similar to the previous example of a bicycle chain and gears.
You may have seen pulleys used in a warehouse to lift heavy loads. Another use for a pulley is on a large construction crane. The cable extends from the object being lifted up to the top of the crane boom, across a pulley, and back down to the electric winch that is used to pull on the cable. In this situation the pulley again causes a change in direction of the pulling force, from the downward force of the winch that pulls the cable to the upward movement of the object being lifted.
The lever is a very old mechanical device. A lever typically consists of a metal or wooden bar that pivots on a fixed point. The point of using a lever is to gain a mechanical advantage. Mechanical advantage results when you use a mechanical device in order to make a task easier; that is, you gain an advantage by using a mechanical device. A lever allows you to complete a task, typically lifting, that would be more difficult or even impossible without the lever.
The most common example of a lever is a playground seesaw. A force—a person's weight—is applied to one side of the lever and causes the weight on the other side—the other person—to be lifted. However, since the pivot point on a seesaw is in the center, each person must weigh the same or the seesaw won't work well. A seesaw is a lever with no mechanical advantage. If you push down on one side with a weight of ten pounds, you can only lift a maximum of ten pounds on the other side. This is no great advantage.
This brings us to the secret of the lever: in order to lift an object that is heavier than the force you want to apply to the other side of the lever, you must locate the pivot point closer to the object you want to lift. If two 50-pound children sit close to the center of the seesaw, one 50-pound child close to the end of the board on the other side will be able to lift them both.
Test questions about levers may require a bit of math—simple multiplication and division. Lever problems rely on one simple concept: the product of the weight to be lifted times the distance from the weight to the pivot point must be equal to the product of the lifting force times the distance from the force to the pivot point. Stated as an equation, w × d1 = f × d2. Here's an example of a test question using this concept:
- Bill has a 15-foot long lever and he wants to lift a 100-pound box. If he locates the pivot point 5 feet from the box, leaving 10 feet between the pivot point and the other end of the lever where he will apply the lifting force, how hard must he press on the lever to lift the box?
To solve this problem, use the lever formula, w × d1 = f × d2. The weight of 100 pounds times 5 feet must equal 10 feet times the force: 100 × 5 = 10 × f. Multiply 100 by 5 to get 500, and then divide by 10 to get 50 pounds of force, which Bill must apply to the lever to raise the box.
For the ASVAB, it is important to know the three types of levers. They are called first, second, and third class levers. First class levers have the fulcrum, or pivot point, in the middle, as in a see-saw. Second class levers have the fulcrum at one end, with the effort (force) at the other end and the load in the middle. An ideal example of a second class lever is a wheelbarrow. Third class levers have the fulcrum at one end and the load at the other end, with the effort (force) in the middle. A pair of bar-b-que tongs is an example of a third class lever.
A mechanical fastener is any mechanical device or process used to connect two or more items together. Typical examples of fastening devices are bolts, screws, nails, and rivets. Processes used to join items together mechanically include gluing and welding. The "hook and loop" is a unique mechanical fastener consisting of two tapes of material with many small plastic hooks and loops that stick together. Children's sneakers often use such fastening tape instead of laces.
A spring is an elastic mechanical device, normally a coil of wire, that returns to its original shape after being compressed or extended. There are many types of springs including the compression coil, spiral coil, flat spiral, extension coil, leaf spring, and torsional spring.
Springs are used for many applications such as car suspensions (compression coil and leaf springs), garage doors (extension coil and torsion springs), wind-up clocks (flat spiral and torsion springs), and some styles of retractable pens (compression coil).
In most questions on the ASVAB, you can assume that springs behave linearly. That is, if an extension spring stretches one inch under a pull of ten pounds, then it will stretch two inches under a pull of twenty pounds. In real life, if you pull too hard on a spring, it will not return to its original shape. This is called exceeding the spring's elastic limit.
If several springs are used for one application, they can be arranged in one of two ways: in series or in parallel. The easiest way to remember the difference is that if the springs are all hooked together, end to end, then you have a series of springs. The other option is for the springs not to be hooked together but to be lined up side by side, parallel to each other. If two springs are arranged in series, they will stretch much farther than the same two springs arranged in parallel under the same pulling force. This is because in series, the total pulling force passes through both springs. If the same springs are arranged in parallel, the pulling force is divided equally with half going through each spring.
The key to solving spring problems is to draw a diagram of the arrangement, if one isn't already provided, and follow the pulling force through the system.
A valve is a mechanical device that controls the flow of liquids, gases, or loose material through piping systems. There are many types of valves including butterfly valves, gate valves, plug valves, ball valves, and check valves.
A valve is basically a gate that can be closed or opened in order to permit a fluid or gas to travel in a particular direction. Exam questions on valves typically require you to follow a piping flow diagram through several sets of valves. The best way to approach these problems is to methodically follow each branch of the piping system from start to finish.
Gauges and Pumps
Gauges and pumps may appear in the Mechanical Comprehension subtest. These devices are discussed in Chapter 10, "Auto and Shop Information."
A linkage is a way of connecting objects in order to transfer energy. Belts and chains are commonly used in conjunction with gears and pulleys for this purpose. Chains are typically made of steel or some other metal, while belts are typically made of fiber-reinforced rubber. An example of the use of a belt is the fan belt on the engine of an automobile, which helps transfer the energy from the engine camshaft to the fan. A bicycle uses a chain to transfer the energy from the pedals to the wheel. Another mechanical linkage is the tie rod that connects the piston and crankshaft in an internal combustion engine.
Motion simply means a change of position. The parameters that describe mechanical motion include velocity, direction, acceleration, and friction.
Velocity means the rate at which an object is moving in such units as miles per hour or feet per minute. Exam questions on velocity might ask you to use velocity and time to determine the distance traveled. For instance, if a car travels at a constant velocity of 60 miles per hour for two hours, how far does it travel? The answer is velocity multiplied by time, or 60 mph times 2 hours for a total of 120 miles. You might also be asked about relative velocity in a question in which two objects travel at different speeds for different lengths of time.
If you want to travel quickly from Kansas City to Denver, your velocity is unimportant if you're not traveling in the right direction. When answering mechanical motion questions, always note the direction of travel of the object or objects, if this information is given. Again, drawing a sketch of the situation usually helps.
Acceleration is the rate of change of velocity or, in other words, how much faster you are going from one minute to the next. This is simpler than it sounds. If you are sitting in your car at a stop sign and then you press hard on the gas pedal, you get pushed back into the seat a bit. If you are traveling along the highway at a constant 50 mph, you don't have this feeling. However, if you hit the gas and accelerate to 65 mph, you are again pushed back into your seat. You have the same sensation when your airplane takes off on the runway. This sensation is the result of acceleration, an increase in how fast an object is traveling. The opposite of acceleration is deceleration, slowing down. Exam questions on acceleration may involve a little simple math.
Friction is the naturally occurring force that acts to hold back an object in motion. If you slide a block of wood across a floor, the friction between the floor and the block causes a drag on the movement of the block. There are two things you should remember if you encounter an exam question about friction:
- Friction always slows down movement.
- All movement experiences frictional force to some degree.
The drag force of friction varies depending on the materials involved. If you've ever tried to drag a piece of furniture from a room with a carpeted floor to another room with a wood floor, you found that the piece of furniture was much easier to drag on the wood floor than on the carpet. The carpet has a higher coefficient of friction than wood. Materials with a high coefficient of friction include such things as sandpaper and brick. Examples of materials with a low coefficient of friction include non-stick cooking surfaces and ice. The differing coefficients of fiction explain why it's more difficult to pull a wooden block across a rough surface such as sandpaper than across a slick surface such as ice.
Fluid Statics and Dynamics
The Mechanical Comprehension subtest includes questions on the behavior of fluids, including questions on pressure, density, and buoyancy.
As a solid object is submerged below the surface of a fluid, the fluid exerts a pressure on it. Have you ever noticed that when diving in a swimming pool you feel more and more pressure on your ears as you go deeper? This is the effect of the pressure of the fluid, water in this case, on your body. The fluid has weight. As you go deeper, more of this weight presses on your body. All fluids behave this way. The deeper a solid object is submerged, the higher the pressure. This behavior of fluids affects the design of machines such as submarines.
- The formula for pressure is:
- pressure = density × depth.
Density is a proportion of weight to volume. If you are comparing two fluids, for example, a gallon of the one with the higher density weighs more than the same volume (a gallon) of another liquid. The density of a solid object or other fluid is usually compared to the density of water, 62.4 pounds per cubic foot. Density controls whether an a solid object or another fluid will sink or float in a given fluid. If a solid object sinks when placed in water, then its density is more than that of water. Conversely, if an object floats, then it is less dense than water. Some liquids, such as mercury, are more dense than water. If mercury and water are combined in a jar, the water will float on top of the mercury. Other fluids, such as gasoline or motor oil, are slightly less dense than water. That is why when an oil tanker has a spill, it leaves an oil slick—the oil is floating on the surface of the water.
Density influences the amount of pressure a fluid exerts on an object. The denser the fluid, the faster the pressure increases on an object as it is submerged. Exam questions on density may include simple mathematical calculations, such as computing pressure by multiplying density times depth. Or they may simply ask you to compare the effects of pressure at different depths and densities.
Buoyancy is the force that acts to push an object submerged in a fluid to the surface. When you force a beach ball under water and then let it go, it springs to the surface. That's the effect of buoyancy.
Here's an example that shows how buoyancy works for an object that is denser than water. Let's say you have a glass that is completely full of water, and the water in the glass weighs one pound. Now put in a eight-pound steel ball, which occupies half of the volume of the glass. When the ball sinks, what happens? Half of the water in the glass, a half-pound worth, spills over the edge of the glass because the ball occupies half the volume of the glass. Now, here's a definition: the uplifting buoyant force acting on this ball is equal to the weight of the water displaced out of the glass by the ball. By definition, therefore, this ball weighs half a pound less when submerged in water than it does just sitting on the table.
The ball weighs less underwater, but it still sinks. Why? Because the ball weighs more than the water it displaces. How, then, is it possible to make a ship that floats in water out of steel, when steel is more dense than water? Simple. Take a thin sheet of steel and form it into a kind of bowl shape. As this thin shell is lowered into the water, it will displace enough water to make it float.
Properties of Materials
Mechanical components and systems can be fabricated using many different materials such as steel, wood, concrete, and plastic. All of these materials react differently to stress, temperature changes, and other external factors. You must understand the properties of materials—weight, strength, density, and thermal properties—in order to answer test questions about them.
The weight of an object is simply a measure of its heaviness.
The loads and stresses placed on a material must be less than the strength of the material in order to prevent failure. A material's strength can be measured in several ways. A concrete building foundation has lots of weight compressing on it and must have high compressive strength. A steel construction girder has a large pulling force acting on it and must therefore have a high tensile strength. The materials selected for a given project depend in part on the loads the structure will have to bear.
Think of a one-gallon bottle full of feathers and another full of steel. Which bottle would be heavier? Both bottles have the same volume, but the one full of steel would obviously weigh more, because steel has a higher density (weight per unit volume) than feathers. Feathers have a low density; it would take a large volume—a big stack of them—to amount to any significant weight. On the other hand, a small volume of steel, which has a fairly high density, is reasonably heavy. Just remember that a material with a higher density will hurt more if you drop it on your toe!
In the English system of units, density is typically measured in pounds per cubic foot or pounds per cubic inch.
The thermal properties of materials—how they respond to changes in termperature—affect their suitability for various applications. Most materials expand slightly as the temperature increases and contract as the temperature decreases. This amount of expansion and contraction varies for each material but is typically very small; you could not see it with your eyes.
The effect of even this small amount of expansion or contraction can be significant on some mechanical systems. For instance, the internal combustion engine of a vehicle generates heat as it operates. All of the parts of the engine must be manufactured so that they fit together properly at both high and low temperatures. Likewise, an airplane experiences very low temperatures when flying at high altitudes, so that the metal of its body contracts a bit. The designers of the airplane must take this effect into account.
The strength of some materials is also affected by changes in temperature. Most materials get weaker as the temperature increases because the bonds between the individual molecules that make up the material get weaker as the molecules move more rapidly. This is why some building materials, such as steel, are coated with insulation during construction. If the building catches fire, the insulation will help maintain the strength of the steel girders.
Choosing Materials for a Given Application
In deciding what materials to use for a given application, weight, strength, density, and thermal properties must all be taken into consideration. For instance, if you wanted to build an airplane wing, you might consider using either steel or aluminum. Steel is stronger than aluminum. However, aluminum has a lower density; that is, an aluminum wing would be lighter than a steel wing of the same size. Therefore, you could use more aluminum to provide adequate strength and still have a lighter total weight.
Other factors, such as cost and how easy the materials are to work with, are also taken into account when selecting materials for a project.
Mechanical systems such as buildings and bridges require proper structural support in so they can hold up heavy loads. An object's center of gravity and its weight distribution affect the design of structural support.
Center of Gravity
The center of gravity of an object is the point at which all of the object's weight appears to act. For instance, you can balance a pencil on your finger by placing your finger under the pencil at the middle of its length. The center of gravity of that pencil is halfway along its length. Likewise, a round ball has its center of gravity at its center. Other objects that are not so symmetrical also have a center of gravity, which can be located through calculations.
Exam questions on center of gravity usually involve symmetrical objects so that the math does not become complicated. Take your time, draw a sketch of the object, and use common sense.
The distribution of weight on a structure such as on a bridge is also important to understand. If there are three trucks uniformly spaced across the length of a bridge that is supported only at its ends, then each support bears an equal amount of the load. However, if the trucks are all located close to one end of the bridge, then the support on that end will be holding up a higher load than the support on the opposite end.
Bridges and buildings have highly variable loads. The worst-case weight distribution must be accounted for—for instance, trucks standing nose to tail for the whole length of the bridge—even if that isn't very likely to happen
As with most Mechanical Comprehension questions, using the picture given, or drawing one if it's not provided, will help you see the location and distribution of the objects.