ASVAB Mechanical Comprehension Practice Test 86245 Results

	Your Results	Global Average
Questions	5	5
Correct	0	3.72
Score	0%	74%

Review

1 If you lift a 5 lbs. rock 43 ft. from the ground, how much work have you done?

71% Answer Correctly

	0 ft⋅lb
	215 ft⋅lb
	48 ft⋅lb
	-38 ft⋅lb

Solution

Work is force times distance. In this case, the force is the weight of the rock so:
\( W = F \times d \)
\( W = 5 \times 43 \)
\( W = 215 \)

The mechanical advantage of a block and tackle is equal to which of the following?

69% Answer Correctly

	the number of input forces
	the number of connecting ropes
	the number of loads
	the number of pulleys

Solution

Two or more pulleys used together constitute a block and tackle which, unlike a fixed pulley, does impart mechanical advantage as a function of the number of pulleys that make up the arrangement. So, for example, a block and tackle with three pulleys would have a mechanical advantage of three.

3 If the green box is 2 ft. from the fulcrum and a certain force applied 4 ft. from the fulcrum at the blue arrow balances the lever, what is the mechanical advantage?

61% Answer Correctly

	3
	6
	2
	0.67

Solution

Because this lever is in equilibrium, we know that the effort force at the blue arrow is equal to the resistance weight of the green box. For a lever that's in equilibrium, one method of calculating mechanical advantage (MA) is to divide the length of the effort arm (E_a) by the length of the resistance arm (R_a):

MA = \( \frac{E_a}{R_a} \) = \( \frac{4 ft.}{2 ft.} \) = 2

When a lever is in equilibrium, the torque from the effort and the resistance are equal. The equation for equilibrium is R_ad_a = R_bd_b where a and b are the two points at which effort/resistance is being applied to the lever.

In this problem, R_a and R_b are such that the lever is in equilibrium meaning that some multiple of the weight of the green box is being applied at the blue arrow. For a lever, this multiple is a function of the ratio of the distances of the box and the arrow from the fulcrum. That's why, for a lever in equilibrium, only the distances from the fulcrum are necessary to calculate mechanical advantage.

If the lever were not in equilibrium, you would first have to calculate the forces and distances necessary to put it in equilibrium and then divide E_a by R_a to get the mechanical advantage.

The steering wheel of a car is an example of which type of simple machine?

89% Answer Correctly

	wheel and axle
	fixed pulley
	first-class lever
	block and tackle

Solution

A wheel and axle uses two different diameter wheels mounted to a connecting axle. Force is applied to the larger wheel and large movements of this wheel result in small movements in the smaller wheel. Because a larger movement distance is being translated to a smaller distance, force is increased with a mechanical advantage equal to the ratio of the diameters of the wheels. An example of a wheel and axle is the steering wheel of a car.

5 What is the mechanical advantage of this inclined plane if the length of the ramp is 25 ft. and the height of the green box is 5 ft.?

82% Answer Correctly

	5.5
	5
	2
	15

Solution

The mechanical advantage (MA) of an inclined plane is the effort distance divided by the resistance distance. In this case, the effort distance is the length of the ramp and the resistance distance is the height of the green box:

MA = \( \frac{d_e}{d_r} \) = \( \frac{25 ft.}{5 ft.} \) = 5