ASVAB Mechanical Comprehension Practice Test 4555 Results

Your Results Global Average
Questions 5 5
Correct 0 3.28
Score 0% 66%

Review

1 If a 10 lbs. weight is placed 9 ft. from the fulcrum at the blue arrow and the green box is 4 ft. from the fulcrum, how much would the green box have to weigh to balance the lever?
61% Answer Correctly
67.5 lbs.
7.5 lbs.
22.5 lbs.
0 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 = \( \frac{R_bd_b}{d_a} \) = \( \frac{10 lbs. \times 9 ft.}{4 ft.} \) = \( \frac{90 ft⋅lb}{4 ft.} \) = 22.5 lbs.

2

Two or more pulleys used together are called:

71% Answer Correctly

wheel and axle

third-class lever

gears

block and tackle
Solution

Two or more pulleys used together constitute a block and tackle which, unlike a fixed pulley, does impart mechanical advantage as a function of the number of pulleys that make up the arrangement.  So, for example, a block and tackle with three pulleys would have a mechanical advantage of three.

3

The science that deals with motion and the forces that produce motion is called which of the following?

57% Answer Correctly

mechanics

aeronautics

physics

engineering
Solution

Mechanics deals with motion and the forces that produce motion.

4 If the green box is 8 ft. from the fulcrum and a certain force applied 7 ft. from the fulcrum at the blue arrow balances the lever, what is the mechanical advantage?
61% Answer Correctly
-3.13
0.88
1.75
0.29
Solution

Because this lever is in equilibrium, we know that the effort force at the blue arrow is equal to the resistance weight of the green box. For a lever that's in equilibrium, one method of calculating mechanical advantage (MA) is to divide the length of the effort arm (Ea) by the length of the resistance arm (Ra):

MA = \( \frac{E_a}{R_a} \) = \( \frac{7 ft.}{8 ft.} \) = 0.88

When a lever is in equilibrium, the torque from the effort and the resistance are equal. The equation for equilibrium is Rada = Rbdb where a and b are the two points at which effort/resistance is being applied to the lever.

In this problem, Ra and Rb are such that the lever is in equilibrium meaning that some multiple of the weight of the green box is being applied at the blue arrow. For a lever, this multiple is a function of the ratio of the distances of the box and the arrow from the fulcrum. That's why, for a lever in equilibrium, only the distances from the fulcrum are necessary to calculate mechanical advantage.

If the lever were not in equilibrium, you would first have to calculate the forces and distances necessary to put it in equilibrium and then divide Ea by Ra to get the mechanical advantage.

5 How much resistance could a 235 lb. effort force lift using a block and tackle pulley that has 4 ropes supporting the resistance?
81% Answer Correctly
2820 lbs.
1034 lbs.
940 lbs.
1410 lbs.
Solution

The mechanical advantage (MA) of a block and tackle pulley is equal to the number of times the effort force changes direction. An easy way to count how many times the effort force changes direction is to count the number of ropes that support the resistance which, in this problem, is 4. With a MA of 4, a 235 lbs. effort force could lift 235 lbs. x 4 = 940 lbs. resistance.