ASVAB Mechanical Comprehension 2020
((Looking at kinds of forces)) Here are some of the forces that act on objects.
1- Friction: Resistance to the motion of two objects or surfaces that touch 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.
Using the mathematics of mechanics Mechanical principles are based on mathematical principles. Therefore, a screw making a complete revolution turns 360 degrees, because a mathematical principle states that 360 degrees are in a circle, a complete revolution. If you have to know the surface area of a floor to determine the pounds per square inch that a ton of tile would put on the floor, that's a mathematical principle, too (Area = Length X Width)) Suppose you run across this question:
A 3-inch-diameter flanged pipe with six holes is being fitted to a base with six holes. What's the maximum number of degrees the pipe must be rotated in order to line up the holes? (A) 120 degrees (B) 180 degrees (C) 60 degrees (D) 360 degrees This isn't really a Mechanical Comprehension question at all — it's a math question. The only part that requires mechanical knowledge is knowing that the holes are spaced equally distant from one another on a flanged pipe. The answer is , Choice (C). 360 degrees divided by 6 = 60 degrees.
Multiplying your effort: Wheels and axles The wheel-and-axle machine multiplies the effort you use, producing a greater force. When you steer a car by using a steering wheel (which is a wheel-and-axle device), a little effort exerted on the steering wheel turns the wheels of the car in the direction you desire. Turning your car wheels would be a lot more complicated if you didn't have the steering wheel. In true wheel-and-axle machines, the wheel and the axle are fixed together and turn at the same time. This arrangement multiplies the amount of force you can exert by a considerable amount. The relationship between the radius of the wheel and the radius of the area to which force is being applied determines the mechanical advantage you receive by using this piece of equipment. (Remember, the radius of a circle equals half the diameter; a straight line extending from the center of the circle to the edge is the radius of a circle.) A hand drill may apply 200 pounds of force for your 10 pounds of effort. (A hand drill uses a gear to convert the direction of the force.) See Figure
A hand drill increases mechanical advantage.
Block and tackle systems When used in a block and tackle arrangement (see Figure ) pulleys make lifting heavy objects easier. In block and tackle systems, pulleys can also be used to change the direction of your pull. If you tie a 200-pound crate to one end of a rope, run the rope through a pulley, and grab the other end of the rope, you can pull down on the rope to lift the crate up. Without a pulley, you could pull down on the crate all day, and it wouldn't go up. In this case, using a simple pulley, the force of your pull must equal the weight of the object being lifted. The regular pulley doesn't multiply your force, but it makes the process of lifting easier.
A pulley used in a block and tackle system. Using a block and tackle allows you to distribute your force more effectively. Instead of hoisting that entire 200-pound crate in one try, you can pull on a rope to lift it a few inches, pull on the rope some more to lift it a few more inches, and so on. This makes the work easier to perform.
Force= Mass x Acceleration (F= M x A_ so how does acceleration relate both mass and force? Remember you can rewrite formula (A= F/M).
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.
1. The moisture that forms on the inside of a window on a cold day is called (A) condensation. (B) distillation. (C) evaporation. (D) tarnation.
A. Distillation is the process of extracting or refining a substance using both boiling and condensation. Evaporation is the process of removing moisture from the surface of a liquid — the water molecules escape the surface and assume gas form. Tarnation is an interjection used to express anger. The correct answer is Choice (A), condensation.
8. The shock absorber on a car is a very large spring. If Abra's car hits a pothole with 600 pounds of force and the shock absorber compresses 3 inches, what is its spring constant in pounds per inch? (A) 200 (B) 600 (C) 20 (D) 1,800
A. The formula for force on a spring is , with F representing force, k representing the spring constant, and x representing the displacement of the spring. The spring is compressed so the displacement, 3 inches, is negative. Just plug in the values from the question and solve:
4. A block and tackle is used to lift a truck engine with a weight of nearly 7,406 newtons. The input force required to lift this weight using the block and tackle is 308.6 newtons. What is the mechanical advantage of the block and tackle? (A) 23.99 (B) 15 (C) 25 (D) 24.75
A. To find the mechanical advantage of the block and tackle, use the following formula:
17. When two or more forces act to balance each other out, the condition is called (A) equilibrium. (B) static recoil. (C) gravitational balance. (D) concurrent forces.
A. When two or more forces interact so their combination cancels the other(s) out, there's a state of equilibrium. In this state, the velocity (speed and direction) of an object doesn't change. Choice (A) is the correct answer.
Using levers to your advantage
All levers work by using a fulcrum (point of support) to reduce resistance and multiply the effect of effort. Resistance is exerted at one end of the lever (the resistance arm) and effort is exerted at the other (the effort arm). The effort arm moves the resistance arm. To determine how much a lever reduces the amount of effort needed to do work, use the following formula:
10. Daisy's single-speed bicycle has a front gear with 48 teeth and a rear gear with 12 teeth. If Daisy pedaled at 80 revolutions per minute (rpm), how fast would her rear wheel rotate? (A) 280 rpm (B) 320 rpm (C) 70 rpm (D) 128 rpm
B. Each full turn of the pedals turns the rear wheel revolutions. If the pedals were to turn at 80 rpm, the rear wheel would rotate at ((4 X 80 RPM = 320 rpm.
12. Four gears are connected in a series. If Gear #1 is turning clockwise, Gear #4 will turn (A) clockwise. (B) counterclockwise. (C) more quickly than Gear #1. (D) more slowly than Gear #1.
B. Gears connected in series turn in opposite directions of each other. Gears 1 and 3 rotate clockwise, and Gears 2 and 4 rotate counterclockwise. The size of gears is unknown, so there isn't enough information to determine whether Choice (C) or Choice (D) is correct. The correct answer is Choice (B).
6. Two people are carrying a 100-pound crate on a 2-x-8-x-12-foot board. To distribute the load evenly between the two people, the crate should be placed (A) 2 feet from the end of the board. (B) in the middle of the board. (C) 3 feet from the end of the board. (D) The load can't be evenly distributed.
B. If the weight is placed closer to one person or the other, that person would carry more of the load, so the weight should be placed in the middle. Choice (B) is the correct answer.
3. While throwing a football, Dan exerts a forward force of 50 newtons on the ball and pushes it forward a distance of 1.2 meters. How much work does he do on the football? (A) 45 joules (B) 60 joules (C) 50 joules (D) 50.5 joules
B. Work is the amount of energy transferred by a force. To calculate how much work is done, use this formula (note that a newton-meter equals a joule):
16. The sideways force one feels when a car turns sharply is often called (A) thrust force. (B) angle force. (C) centrifugal force. (D) positive force.
C. Although commonly referred to as centrifugal force, this property isn't actually a force at all but rather a property of inertia, one of Newton's laws of motion. As the car turns, your body is trying to continue traveling in a straight line. The correct answer is Choice (C).
18. Two balls of the same density, one large and one small, roll toward each other at the same speed. When they collide, what will happen to the larger ball? (A) It will be propelled backward. (B) It will jump over the smaller ball. (C) It will continue forward. (D) It will stop.
C. Because both balls are moving at the same speed, the ball with more mass (the larger one in this case, because both balls have the same density) will have more momentum. Upon impact, the heavier ball will slow down but continue in the same direction and knock the smaller, lighter ball backward.
5. Which of the tanks will overfill? Look at image; (A) Tank A (B) Tanks A and B (C) Tank C (D) Tanks B and C
C. Tank C has an inflow greater than its outflow, so it will eventually overfill.
2. If a 200-pound barrel must be lifted 4 feet to the bed of a box truck, an inclined plane will reduce the amount of effort required to move the barrel by half if the inclined plane is (A) 2 feet long. (B) 6 feet long. (C) 8 feet long. (D) 9 feet long.
C. The formula used for determining how an inclined plane reduces effort is Length of Ramp / Height of Ramp = Weight of the Object / Force, or . The amount of force needed to lift the object is equivalent to the object's weight, but the question wants to reduce that amount of force to half, so half of the object's weight is 100. Now do the math: ; . The correct answer is Choice (C).
9. In the following figure, a 600-pound weight is placed on a 10-pound board that has been evenly balanced between two scales. How much does the left scale measure if the weight is closer to the left than to the right? (A) 400 (B) 300 (C) 405 (D) 410
C. Two-thirds of the weight of the block (400 pounds) is supported by the left scale because the block is closer to the left than to the right. The board is evenly placed, so each scale supports of its weight (5 pounds). So the total weight being supported by the left scale is::: 400 Pounds + 5 Pounds = 405 pounds.
Under pressure: Spreading out the force
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. 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? That would be a lot more painful — and because there are fewer nails exerting pressure on a smaller surface area, they'd most likely 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.
11. Not including friction, a stationary single pulley gives a mechanical advantage of (A) 2. (B) 4. (C) 3. (D) 1.
D. A stationary single pulley allows you to change the direction of force but doesn't result in an increased mechanical advantage. The correct answer is Choice (D).
7. Wheel A has a diameter of 9 feet. Wheel B has a diameter of 12 feet. If both wheels revolve at the same rate, Wheel B will cover a linear distance of 24 feet (A) at the same speed as Wheel A. (B) more slowly than Wheel A. (C) in half the time of Wheel A. (D) more quickly than Wheel A.
D. Because Wheel A has a smaller circumference, it covers a shorter linear distance than Wheel B when turning at the same rate. Thus, Wheel B covers the distance of 24 feet faster than Wheel A. Choice (C) is inaccurate because the diameter of Wheel A isn't exactly half the diameter of Wheel B. If both wheels revolve at the same rate, then Wheel A turns 25 percent more slowly than Wheel B, because Wheel A's diameter is three-quarters of Wheel B's. Choice (D) is the correct answer.
A block and tackle system can also be used to reduce effort by magnifying force. To help understand how this works, look at Figure
Example 1 shows a 100-pound box secured to the ceiling by a single line. The weight supported by the line is equal to the weight of the box. In Example 2, the box is secured to the ceiling by using two lines. Each line is supporting one-half the weight of the box. In Example 3, a single line is threaded through a pulley. Although the line (as a whole) is supporting the entire weight of the box, each section of the line is supporting only one-half of the box's weight, just as in Example 2. In Example 4, a man is using this principle to lift the 100-pound box by applying only 50 pounds of force. In short, this block and tackle system provides the man with a mechanical advantage of 2. In receiving a mechanical advantage of force, the man must pull the rope farther than if he weren't using a pulley. In this example, the man would have to pull 2 feet of rope to raise the box 1 foot.
Working out the difference between potential and kinetic energy. Energy is the capacity to do work. Every object in the universe has energy, and it's either potential or kinetic. Potential energy is stored energy — energy that's not doing anything at the moment but that's in the object by virtue of its position in a field. If a book is resting in your hands right now, the book itself is holding potential energy. If you raise the book over your head, you're increasing its potential energy (thanks to the Earth's gravitational pull). When you accidentally drop it, all its potential energy becomes kinetic energy, or energy in motion. When the book hits the ground, its energy becomes potential again.
For the gravitational force the formula is P.E. = mgh, where m is the mass in kilograms, g is the acceleration due to gravity (9.8 m / s2 at the surface of the earth) and h is the height in meters. Notice that gravitational potential energy has the same units as kinetic energy, kg m2 / s2.
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.
The mechanical advantage of using a lever can be stated as Mechanical Advantage = Effort Arm / Resistance Arm.
If the effort arm is 6 inches and the resistance arm is 3 inches, the mechanical advantage is 2. If the effort arm is 6 feet and the resistance arm is 3 feet, the mechanical advantage is still 2.
Additional pulleys can be added to a block and tackle arrangement to further increase the mechanical advantage. Figure shows a couple of examples Two examples of a block and tackle arrangement.
In Example 1, three sections of rope produce a mechanical advantage of 3. Lifting a weight with this pulley arrangement requires only of the effort required to lift the weight directly. However, in order to lift the crate 1 foot, you have to pull 3 feet of rope. Example 2 illustrates a block and tackle system with six sections of rope. Using this arrangement provides you with a mechanical advantage of 6, but you have to pull the rope 6 feet for every foot you want to raise the box.
Understanding the Forces of the Universe Mathematical formula Force= Mass x Acceleration.
In physics, applying force allows changes in the velocity (the speed and direction) of an object. An increase in velocity is known as acceleration. Here's the mathematical formula to determine force: 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.)))
Magnifying your force with liquid: Hydraulic jacks A hydraulic jack uses a nearly incompressible liquid, such as oil, to exert force in order to move an object (see Figure A hydraulic jack.). As the handle moves, it applies pressure to the oil. Because the oil doesn't compress, the oil transmits whatever force is applied to it to the work cylinder with no (or little) loss in efficiency. The mechanical advantage is the ratio between the diameters of the two cylinders.
In the figure, the small cylinder has a diameter of 1 inch and the large cylinder has a diameter of 4 inches. This difference in diameter results in a mechanical advantage of 4. If the rocks weigh a total of 100 pounds, only 25 pounds of force has to be applied to the piston in the small cylinder in order to lift the load. However, although the force required is reduced by a factor of 4, the smaller piston has to move 4 feet for every foot the piston in the larger cylinder moves.
Relying on Machines to Help You Work
Machines give you the ability to magnify and change the direction of forces. When a machine multiplies the force you use, it gives you a mechanical advantage. 45 N Divided by 3 N = 15 N 45 N / 3 N= 15 N
3- Gravity: What goes up must come down 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 farthest or highest point, and then it will fall. The physical property that draws objects toward the center of Earth (and other objects that have mass)
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.
Power is the rate of work. If Mary Lou is able to lift more 50-pound sacks of potatoes onto the truck bed in 10 minutes than Joe is, Mary Lou is more powerful than Joe. Mathematically speaking,
Power = Work / Time In this formula, work is usually measured in foot-pounds, time is measured in minutes, and power is measured in foot-pounds per minute. However, the unit of measure for power is commonly put in terms of horsepower (hp).
Turning direction: Unless the driving belt is reversed (twisted), pulleys connected in series rotate in the same direction. ((Figure)) illustrates this concept with two sets of pulleys. In the first set of pulleys, all the pulleys turn in the same direction (counterclockwise) as the driving pulley. However, in the second set of pulleys, the driving pulley and the lower pulley are rotating counterclockwise, but the right-hand pulley is rotating in a clockwise direction because the belt is twisted.
Pulleys rotate in the same direction unless the belt is reversed.
13. James applies force at one end of a hydraulic jack. The area at the other end of the jack is five times the area where James is applying the force. How much larger is the exerted force than what James is applying? (A) twice as large (B) half as large (C) one-fifth as large (D) five times as large
See image for answer
14. Mr. Roth's children — Jake, Paul, and Jill — weigh 80, 60, and 50 pounds, respectively. They all sit on the same side of a seesaw together. Jake sits 3 feet from the fulcrum, Paul sits 5 feet from the fulcrum, and Jill sits 6 feet from the fulcrum. How far from the fulcrum must Mr. Roth sit on the other side to balance the seesaw if he weighs 200 pounds? (A) 4.2 feet (B) 5.5 feet (C) 5 feet (D) 4 feet
See image for answer
15. A rope is pulling a 320-pound box up an incline that's 16 feet long. If 80 pounds of force is used to move the box up the incline, how tall is the incline? (A) 6 feet (B) 10 feet (C) 4 feet (D) 8 feet
See image for answer
Elastic recoil: The trampoline of physics 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.
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.
CENTRIFUGAL FORCE: FALSE GRAVITY An object traveling in a circle appears to experience a gravitational force. This isn't really gravity, but instead it's a concept known as centrifugal force. The amount of force depends on the mass of the object, the speed of rotation, and the distance from the center: 1- The more massive the object, the greater the force. 2- The greater the speed of the object, the greater the force. 3-The greater the distance from the center, the greater the force.
The centrifugal force, or effect, on an object is actually a fictitious outward force on an object moving along a curved path, which can be equal to the centripetal force on an object. Centripetal force points toward the center of an object's circular path, perpendicular to the direction of motion of an object. If you're riding on a merry-go-round on the playground (whee!), you have to exert a constant force to keep from flying off. This feeling of being pushed outward isn't due to something actually pushing you in that direction but to your body's inertia trying to keep you moving in a straight line. Because one of Newton's laws states that moving objects tend to want to travel in one direction, as the merry-go-round turns, your body wants to keep traveling in one direction (tangent to the circle, if you like math), so you feel you're being pushed outward.
Guessing with a mechanical mind Like most of the other subtests on the ASVAB, you can and should guess on the Mechanical Comprehension subtest when you don't know the answer (unless you're nearing the end of the CAT-ASVAB; too many incorrect answers at the end of a subtest give the impression you're just guessing and can bring on a penalty). Check out these tips to help you narrow the field: The amount of force needed to move an object (not including friction resistance) is never greater than the weight of the object. Any answer that includes a force that's greater than the weight of the object being moved is probably wrong.
The correct answer is a mechanical answer. For example, if the question asks, "What's the purpose of lubricating oil in an engine?" the correct answer won't be "to make the parts look shiny." The answer may be "to reduce friction between moving parts." Any change in a mechanical operation almost always has pluses and minuses associated with it. So when a question proposes a change, the correct answer is probably the one that specifies the good, the bad, and the ugly. For instance, suppose the question says, "Enlarging the wheel on a hand drill will ...?" The correct answer is the one that says something like "increase the mechanical advantage and decrease the amount of effort needed to operate the drill."
Ramping up the inclined plane
The inclined plane, also called a ramp, is another very simple machine that makes moving an object from one point to another easier. The ramp spreads your work out over a longer distance, so less force is needed to do the work. For instance, suppose you have to lift a 50-pound barrel to a truck bed that's 3 feet off the ground. You'd have to use 50 pounds of force for 3 feet to move the barrel. But if you put a 6-foot ramp in place and pushed the barrel up the ramp, you'd only use half as much force to get the barrel in the truck (assuming there was no friction) because the mechanical advantage of such a ramp is 2.
Understanding how gears work The motion of gears with an even number of gears aligned in a series. Machines often use gears to transmit motion from one place to another. An additional advantage of using gears is that they can be used to change direction, increase or decrease speed, or increase or decrease force. Gears arranged in a series turn in the opposite direction of each other. If you have an even number of gears connected in a series, the first and last gear turn in opposite directions. If you have an odd number of gears aligned in a series, the first and last gear spin in the same direction. Look at (Figure). Gear 1 is rotating counterclockwise, which causes Gear 2 to turn clockwise, resulting in Gear 3 spinning counterclockwise, with Gear 4 turning clockwise.
The ratio of teeth between two gears affects rotational speed.
Overcoming resistance
The resistance that the work overcomes isn't the same thing as the weight of the object. In other words, if you try to move a 1,200-pound piano, you'll probably notice a measurable difference between the amount of work it takes to shove it along the floor and the amount of work it takes to carry it up the stairs. When you move the piano across the floor, you're really working (pushing) against the frictional resistance (the force that's produced when two surfaces rub together) of the piano rather than its full weight. Under these circumstances, the frictional resistance of the piano offers less resistance than its full weight. There are times when an object's full weight is less than its frictional resistance.
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).
IN THIS CHAPTER Using the forces of physics Figuring out the principles of work Manipulating machines to help you work
To do well on this subtest, you also have to bone up on your mathematical skills. The Mechanical Comprehension subtest often asks you to make calculations based on formulas to explain mechanical principles. Don't panic; the formulas are easy to understand, but you do have to use math to come up with a final answer. In this chapter, you get the mathematical formulas for commonly asked questions on the ASVAB, so pay especially close attention to these little beauties. (If the information probably isn't on the ASVAB, I don't burden you with it here.)
When gear shafts aren't parallel to one another, bevel gears can be used to connect gears that have shafts at different angles. The principles of gear rotation remain the same. (The Figure) shows an example of bevel gears designed to connect shafts having a 90-degree angle to the other.
Two bevel gears meet at a right angle.
Working Your Way to a Better Test Score When you take the Mechanical Comprehension subtest, you may not know the correct answer to a question, or you may not know the mechanical principle involved. You may know the mechanical principle but not remember the formula you need to come up with the right answer. Never fear — you can still stumble through this test without totally crashing and burning.
Using your observations and common sense Questions on this subtest often include illustrations. The ASVAB test-makers expect you to look at the illustrated device and guess how it operates. When you run across these types of questions, make sure you understand the illustration. Often, parts of the device are labeled. Make certain you read and understand these labels before you try to answer a question about the illustration.
The advantage of using a ramp can be expressed as in the image
Wedges are a form of inclined plane and can multiply your effort in much the same way as a ramp can. Screws are also inclined planes, only in spiral form. Screw jacks, which you can use to lift your house up to build a new foundation, are a combination of a lever and an inclined plane.
