ASVAB Mechanical Comprehension Practice Test 2451 Results

	Your Results	Global Average
Questions	5	5
Correct	0	2.98
Score	0%	60%

Review

1 If you lift a 47 lbs. rock 45 ft. from the ground, how much work have you done?

70% Answer Correctly

	0 ft⋅lb
	4230 ft⋅lb
	2115 ft⋅lb
	2 ft⋅lb

Solution

Work is force times distance. In this case, the force is the weight of the rock so:
\( W = F \times d \)
\( W = 47 \times 45 \)
\( W = 2115 \)

2 If you have a gear train with two gears, the first with 36 teeth and the second with 4 teeth, how many revolutions does the second gear make for each revolution of the first gear?

77% Answer Correctly

	9
	4.5
	14
	8

Solution

The gear ratio (V_r) of a gear train is the product of the gear ratios between the pairs of meshed gears. Let N represent the number of teeth for each gear:

V_r = \( \frac{N_1}{N_2} \) \( \frac{N_2}{N_3} \) \( \frac{N_3}{N_4} \) ... \( \frac{N_n}{N_{n+1}} \)

In this problem, we have only two gears so the equation becomes:

V_r = \( \frac{N_1}{N_2} \) = \( \frac{36}{4} \) = 9

A truck is using a rope to pull a car. Tension in the rope is greatest in which of the following places?

50% Answer Correctly

	tension is equal in all parts of the rope
	near the truck
	in the middle
	near the car

Solution

Tension is a force that stretches or elongates something. When a cable or rope is used to pull an object, for example, it stretches internally as it accepts the weight that it's moving. Although tension is often treated as applying equally to all parts of a material, it's greater at the places where the material is under the most stress.

4 A 320 lb. barrel is rolled up a 14 ft. ramp to a platform that's 5 ft. tall. What effort is required to move the barrel?

53% Answer Correctly

	117.3 lbs.
	171.4 lbs.
	342.9 lbs.
	114.3 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 14 ft. = 320 lbs. x 5 ft.
F_e = \( \frac{1600 ft⋅lb}{14 ft.} \) = 114.3 lbs.

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

48% Answer Correctly

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

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.