ASVAB Mechanical Comprehension Practice Test 330851 Results

	Your Results	Global Average
Questions	5	5
Correct	0	3.25
Score	0%	65%

Review

1 If the green box weighs 10 lbs. and is 7 ft. from the fulcrum, how far from the fulcrum would a 75 lbs. weight need to be placed to balance the lever?

61% Answer Correctly

	2.8 ft.
	0.47 ft.
	3.73 ft.
	0.93 ft.

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

d_b = \( \frac{R_ad_a}{R_b} \) = \( \frac{10 lbs. \times 7 ft.}{75 lbs.} \) = \( \frac{70 ft⋅lb}{75 lbs.} \) = 0.93 ft.

2 A = 8 ft., the green box weighs 50 lbs., and the blue box weighs 70 lbs. What does distance B need to be for this lever to balance?

65% Answer Correctly

	2.86 ft.
	5.71 ft.
	1.43 ft.
	22.86 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:

f_Ad_A = f_Bd_B

For this problem, the equation becomes:

50 lbs. x 8 ft. = 70 lbs. x d_B

d_B = \( \frac{50 \times 8 ft⋅lb}{70 lbs.} \) = \( \frac{400 ft⋅lb}{70 lbs.} \) = 5.71 ft.

3 If the green arrow in this diagram represents 660 ft⋅lb of work, how far will the box move if it weighs 220 pounds?

72% Answer Correctly

	440 ft.
	3 ft.
	6 ft.
	73 ft.

Solution

The Law of Work states that the work put into a machine is equal to the work received from the machine under ideal conditions. In equation form, that's:

W_in = W_out
F_effort x d_effort = F_resistance x d_resistance

In this problem, the effort work is 660 ft⋅lb and the resistance force is 220 lbs. and we need to calculate the resistance distance:

W_in = F_resistance x d_resistance
660 ft⋅lb = 220 lbs. x d_resistance
d_resistance = \( \frac{660ft⋅lb}{220 lbs.} \) = 3 ft.

4 If you lift a 39 lbs. rock 47 ft. from the ground, how much work have you done?

71% Answer Correctly

	1 ft⋅lb
	1833 ft⋅lb
	-8 ft⋅lb
	3666 ft⋅lb

Solution

Work is force times distance. In this case, the force is the weight of the rock so:
\( W = F \times d \)
\( W = 39 \times 47 \)
\( W = 1833 \)

If the handles of a wheelbarrow are 3 ft. from the wheel axle, what force must you exert to lift the handles if it's carrying a 270 lb. load concentrated at a point 0.5 ft. from the axle?

56% Answer Correctly

	45 lbs
	90 lbs
	0.83 lbs
	810 lbs

Solution

This problem describes a second-class lever and, for a second class lever, 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 3 ft. = 270 lbs x 0.5 ft
F_e = 135 ft-lb. / 3 ft
F_e= 45 lbs