ASVAB Mechanical Comprehension Practice Test 719528 Results

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

Review

1 If a 65 lbs. weight is placed 3 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

	48.75 lbs.
	97.5 lbs.
	195 lbs.
	146.25 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:

R_ad_a = R_bd_b

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 R_a, our missing value, and plugging in our variables yields:

R_a = \( \frac{R_bd_b}{d_a} \) = \( \frac{65 lbs. \times 3 ft.}{4 ft.} \) = \( \frac{195 ft⋅lb}{4 ft.} \) = 48.75 lbs.

2 A 360 lb. barrel is rolled up a 9 ft. ramp to a platform that's 1 ft. tall. What effort is required to move the barrel?

53% Answer Correctly

	60 lbs.
	40 lbs.
	36 lbs.
	80 lbs.

Solution

This problem describes an inclined plane and, for an inclined plane, the effort force multiplied by the effort distance equals the resistance force multipied by the resistance distance:

F_ed_e = F_rd_r

Plugging in the variables from this problem yields:

F_e x 9 ft. = 360 lbs. x 1 ft.
F_e = \( \frac{360 ft⋅lb}{9 ft.} \) = 40 lbs.

3 How much resistance could a 145 lb. effort force lift using a block and tackle pulley that has 10 ropes supporting the resistance?

81% Answer Correctly

	2175 lbs.
	1452 lbs.
	1305 lbs.
	1450 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 10. With a MA of 10, a 145 lbs. effort force could lift 145 lbs. x 10 = 1450 lbs. resistance.

The mechanical advantage of a third class lever is always:

37% Answer Correctly

	greater than one
	not equal to one
	equal to one
	less than 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.

What defines the mechanical advantage of a first class lever?

65% Answer Correctly

	output force
	position of the fulcrum
	input force
	output distance

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.