ASVAB Mechanical Comprehension Practice Test 583812 Results

	Your Results	Global Average
Questions	5	5
Correct	0	2.99
Score	0%	60%

Review

1 If the force applied at the blue arrow over 7 ft. moves the green box 0.88 ft., what is the mechanical advantage of this lever?

56% Answer Correctly

	8
	4
	24
	8.8

Solution

Mechanical advantage (MA) can be calculated knowing only the distance the effort (blue arrow) moves and the distance the resistance (green box) moves. The equation is:

MA = \( \frac{E_d}{R_d} \)

where E_d is the effort distance and R_d is the resistance distance. For this problem, the equation becomes:

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

You might be wondering how having an effort distance of 8 times the resistance distance is an advantage. Remember the principle of moments. For a lever in equilibrium the effort torque equals the resistance torque. Because torque is force x distance, if the effort distance is 8 times the resistance distance, the effort force must be \( \frac{1}{8} \) the resistance force. You're trading moving 8 times the distance for only having to use \( \frac{1}{8} \) the force.

2 If A = 9 ft., B = 2 ft., C = 4 ft., the green box weighs 35 lbs. and the blue box weighs 45 lbs., what does the orange box have to weigh for this lever to balance?

44% Answer Correctly

	315 lbs.
	112.5 lbs.
	0 lbs.
	56.25 lbs.

Solution

In order for this lever to balance, the torque acting on each side of the fulrum must be equal. So, the torque produced by A must equal the torque produced by B and C. 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 + f_Cd_C

For this problem, this equation becomes:

35 lbs. x 9 ft. = 45 lbs. x 2 ft. + f_C x 4 ft.

315 ft. lbs. = 90 ft. lbs. + f_C x 4 ft.

f_C = \( \frac{315 ft. lbs. - 90 ft. lbs.}{4 ft.} \) = \( \frac{225 ft. lbs.}{4 ft.} \) = 56.25 lbs.

Which of the following surfaces would have the lowest coefficient of friction?

85% Answer Correctly

	leather
	tile
	concrete
	ice

Solution

Coefficient of friction (μ) represents how much two materials resist sliding across each other. Smooth surfaces like ice have low coefficients of friction while rough surfaces like concrete have high μ.

4 If 45 lbs. of force is applied 6 ft. from the fulcrum at the blue arrow and the green box is 2 ft. from the fulcrum, how much would the green box have to weigh to balance the lever?

62% Answer Correctly

	135 lbs.
	45 lbs.
	67.5 lbs.
	7 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 = \( \frac{R_bd_b}{d_a} \) = \( \frac{45 lbs. \times 6 ft.}{2 ft.} \) = \( \frac{270 ft⋅lb}{2 ft.} \) = 135 lbs.

Which of the following statements about this pulley configuration is false?

48% Answer Correctly

	Changes the direction of and multiplies the effort force
	Only multiplies the effort force
	Mechanical advantage is the number of ropes that support the resistance
	This is a block and tackle pulley configuration

Solution

A block and tackle is a combination of one or more fixed pulleys and one or more movable pulleys where the fixed pulleys change the direction of the effort force and the movable pulleys multiply it. The mechanical advantage is equal to the number of times the effort force changes direction and can be increased by adding more pulley wheels to the system. An easy way to find the mechanical advantage of a block and tackle pulley system is to count the number of ropes that support the resistance.