ASVAB Mechanical Comprehension Practice Test 273711 Results

	Your Results	Global Average
Questions	5	5
Correct	0	2.91
Score	0%	58%

Review

1 If you lift a 3 lbs. rock 36 ft. from the ground, how much work have you done?

71% Answer Correctly

	0 ft⋅lb
	12 ft⋅lb
	108 ft⋅lb
	-33 ft⋅lb

Solution

Work is force times distance. In this case, the force is the weight of the rock so:
\( W = F \times d \)
\( W = 3 \times 36 \)
\( W = 108 \)

2 If the green box weighs 30 lbs. and is 9 ft. from the fulcrum, how far from the fulcrum would a 55 lbs. force need to be applied to balance the lever?

58% Answer Correctly

	4.91 ft.
	1.64 ft.
	19.64 ft.
	270 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:

R_ad_a = R_bd_b

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

d_b = \( \frac{R_ad_a}{R_b} \) = \( \frac{30 lbs. \times 9 ft.}{55 lbs.} \) = \( \frac{270 ft⋅lb}{55 lbs.} \) = 4.91 ft.

Sam can do 50 ft. lb. of work in 2 minutes and 5 seconds. What would Sam have to do to increase his power output?

64% Answer Correctly

	do the work in 2 minutes
	do 25 ft. lb. of work in 2 minutes 5 seconds
	do the work in 3 minutes
	do 100 ft. lb. of work in 4 minutes 12 seconds

Solution

Power is the rate of doing work or \(\frac{W}{t}\). To increase power, increase the work being done in the same amount of time or do the same amount of work in less time.

The mechanical advantage of a third class lever is always:

37% Answer Correctly

	less than one
	greater than one
	equal to 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.

5 If 75 lbs. of force is applied 7 ft. from the fulcrum at the blue arrow and the green box is 6 ft. from the fulcrum, how much would the green box have to weigh to balance the lever?

62% Answer Correctly

	21.88 lbs.
	10 lbs.
	262.5 lbs.
	87.5 lbs.

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:

R_ad_a = R_bd_b

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

R_a = \( \frac{R_bd_b}{d_a} \) = \( \frac{75 lbs. \times 7 ft.}{6 ft.} \) = \( \frac{525 ft⋅lb}{6 ft.} \) = 87.5 lbs.