ASVAB Mechanical Comprehension Practice Test 428023 Results

	Your Results	Global Average
Questions	5	5
Correct	0	2.97
Score	0%	59%

Review

Which class of lever offers no mechanical advantage?

45% Answer Correctly

	third
	none of these, all levers offer mechanical advantage
	first
	second

Solution

A third-class lever is used to increase distance traveled by an object in the same direction as the force applied. The fulcrum is at one end of the lever, the object at the other, and the force is applied between them. This lever does not impart a mechanical advantage as the effort force must be greater than the load but does impart extra speed to the load. Examples of third-class levers are shovels and tweezers.

2 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

	1
	0.67
	3
	2

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.

3 The green box weighs 10 lbs. and a 15 lbs. weight is placed 3 ft. from the fulcrum at the blue arrow. How far from the fulcrum would the green box need to be placed to balance the lever?

57% Answer Correctly

	9 ft.
	2.25 ft.
	18 ft.
	4.5 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_a, our missing value, and plugging in our variables yields:

d_a = \( \frac{R_bd_b}{R_a} \) = \( \frac{15 lbs. \times 3 ft.}{10 lbs.} \) = \( \frac{45 ft⋅lb}{10 lbs.} \) = 4.5 ft.

A shovel is an example of which class of lever?

56% Answer Correctly

	second
	a shovel is not a lever
	third
	first

Solution

5 If A = 4 ft. and the green box weighs 30 lbs. what is the torque acting on the A side of this lever?

75% Answer Correctly

	120 ft⋅lb
	0 ft⋅lb
	30 ft⋅lb
	480 ft⋅lb

Solution

For a lever, torque is weight x distance from the fulcrum which, in this case, is: 30 ft. x 4 lbs. = 120 ft⋅lb