ASVAB Mechanical Comprehension Practice Test 872849 Results

Your Results Global Average
Questions 5 5
Correct 0 2.96
Score 0% 59%

Review

1 If the green box weighs 60 lbs. and 40 lbs. of force is applied 9 ft. from the fulcrum at the blue arrow, how far from the fulcrum would the green box need to be placed to balance the lever?
55% Answer Correctly
18 ft.
2 ft.
0 ft.
6 ft.
Solution

To balance this lever the torques at the green box and the blue arrow must be equal. Torque is weight x distance from the fulcrum so the equation for equilibrium is:

Rada = Rbdb

where a represents the green box and b the blue arrow, R is resistance (weight/force) and d is the distance from the fulcrum.

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

da = \( \frac{R_bd_b}{R_a} \) = \( \frac{40 lbs. \times 9 ft.}{60 lbs.} \) = \( \frac{360 ft⋅lb}{60 lbs.} \) = 6 ft.

2

The mechanical advantage of a third class lever is always:

37% Answer Correctly

equal to one

greater than one

less than one

not equal to one
Solution

A third class lever is designed to multiply distance and speed at the expense of effort force. Because the effort force is greater than the resistance, the mechanical advantage of a third class lever is always less than one.

An example of a third class lever is a broom. The fulcrum is at your hand on the end of the broom, the effort force is your other hand in the middle, and the resistance is at the bottom bristles. The effort force of your hand in the middle multiplies the distance and speed of the bristles at the bottom but at the expense of producing a brushing force that's less than the force you're applying with your hand.

3 A = 9 ft., the green box weighs 5 lbs., and the blue box weighs 10 lbs. What does distance B need to be for this lever to balance?
65% Answer Correctly
90 ft.
4.5 ft.
1.13 ft.
13.5 ft.
Solution
In order for this lever to balance, the torque acting on side A must equal the torque acting on side B. Torque is weight x distance from the fulcrum which means that the following must be true for the lever to balance:

fAdA = fBdB

For this problem, the equation becomes:

5 lbs. x 9 ft. = 10 lbs. x dB

dB = \( \frac{5 \times 9 ft⋅lb}{10 lbs.} \) = \( \frac{45 ft⋅lb}{10 lbs.} \) = 4.5 ft.

4 If you lift a 29 lbs. rock 37 ft. from the ground, how much work have you done?
72% Answer Correctly
1 ft⋅lb
2146 ft⋅lb
None of these is correct
1073 ft⋅lb
Solution
Work is force times distance. In this case, the force is the weight of the rock so:
\( W = F \times d \)
\( W = 29 \times 37 \)
\( W = 1073 \)
5

What defines the mechanical advantage of a first class lever?

65% Answer Correctly

input force

output force 

output distance

position of the fulcrum
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.