ASVAB Mechanical Comprehension Practice Test 774153 Results

Your Results Global Average
Questions 5 5
Correct 0 3.00
Score 0% 60%

Review

1 If a 15 lbs. weight is placed 9 ft. from the fulcrum at the blue arrow and the green box is 8 ft. from the fulcrum, how much would the green box have to weigh to balance the lever?
61% Answer Correctly
0 lbs.
1 lbs.
67.5 lbs.
16.88 lbs.
Solution

To balance this lever the torques on each side of the fulcrum must be equal. Torque is weight x distance from the fulcrum so the equation for equilibrium is:

Rada = Rbdb

where a represents the left side of the fulcrum and b the right, R is resistance (weight) and d is the distance from the fulcrum.

Solving for Ra, our missing value, and plugging in our variables yields:

Ra = \( \frac{R_bd_b}{d_a} \) = \( \frac{15 lbs. \times 9 ft.}{8 ft.} \) = \( \frac{135 ft⋅lb}{8 ft.} \) = 16.88 lbs.

2

A a seesaw / teeter-totter is an example of which of the following?

69% Answer Correctly

second-class lever

inclined plane

third-class lever

first-class lever
Solution

A first-class lever is used to increase force or distance while changing the direction of the force. The lever pivots on a fulcrum and, when a force is applied to the lever at one side of the fulcrum, the other end moves in the opposite direction. The position of the fulcrum also defines the mechanical advantage of the lever. If the fulcrum is closer to the force being applied, the load can be moved a greater distance at the expense of requiring a greater input force. If the fulcrum is closer to the load, less force is required but the force must be applied over a longer distance. An example of a first-class lever is a seesaw / teeter-totter.

3

A wedge is most similar to what other type of simple machine?

70% Answer Correctly

second-class lever

third-class lever

inclined plane

first-class lever
Solution

The wedge is a moving inclined plane that is used to lift, hold, or break apart an object. A wedge converts force applied to its blunt end into force perpendicular to its inclined surface. In contrast to a stationary plane where force is applied to the object being moved, with a wedge the object is stationary and the force is being applied to the plane. Examples of a wedge include knives and chisels.

4

Which of the following will increase the mechanical advantage of a second-class lever?

55% Answer Correctly

move the object being lifted closer to the fulcrum

decrease the length of the lever

move the object being lifted farther away from the fulcrum

move the fulcrum between the force and the object being lifted
Solution

A second-class lever is used to increase force on an object in the same direction as the force is applied. This lever requires a smaller force to lift a larger load but the force must be applied over a greater distance. The fulcrum is placed at one end of the lever and mechanical advantage increases as the object being lifted is moved closer to the fulcrum or the length of the lever is increased. An example of a second-class lever is a wheelbarrow.

5 If A = 9 ft., B = 3 ft., C = 4 ft., the green box weighs 50 lbs. and the blue box weighs 60 lbs., what does the orange box have to weigh for this lever to balance?
44% Answer Correctly
270 lbs.
5 lbs.
67.5 lbs.
33.75 lbs.
Solution
In order for this lever to balance, the torque acting on each side of the fulrum must be equal. So, the torque produced by A must equal the torque produced by B and C. Torque is weight x distance from the fulcrum which means that the following must be true for the lever to balance:

fAdA = fBdB + fCdC

For this problem, this equation becomes:

50 lbs. x 9 ft. = 60 lbs. x 3 ft. + fC x 4 ft.

450 ft. lbs. = 180 ft. lbs. + fC x 4 ft.

fC = \( \frac{450 ft. lbs. - 180 ft. lbs.}{4 ft.} \) = \( \frac{270 ft. lbs.}{4 ft.} \) = 67.5 lbs.