ASVAB Mechanical Comprehension Practice Test 38355 Results

	Your Results	Global Average
Questions	5	5
Correct	0	3.13
Score	0%	63%

Review

Which of the following is not true of a first-class lever?

51% Answer Correctly

	changes the direction of force
	decreases distance
	increases distance
	increases force

Solution

A first-class lever is used to increase force or distance while changing the direction of the force. The lever pivots on a fulcrum and, when a force is applied to the lever at one side of the fulcrum, the other end moves in the opposite direction. The position of the fulcrum also defines the mechanical advantage of the lever. If the fulcrum is closer to the force being applied, the load can be moved a greater distance at the expense of requiring a greater input force. If the fulcrum is closer to the load, less force is required but the force must be applied over a longer distance. An example of a first-class lever is a seesaw / teeter-totter.

2 If a 45 lbs. weight is placed 5 ft. from the fulcrum at the blue arrow and the green box is 4 ft. from the fulcrum, how much would the green box have to weigh to balance the lever?

61% Answer Correctly

	14.06 lbs.
	168.75 lbs.
	112.5 lbs.
	56.25 lbs.

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

R_a = \( \frac{R_bd_b}{d_a} \) = \( \frac{45 lbs. \times 5 ft.}{4 ft.} \) = \( \frac{225 ft⋅lb}{4 ft.} \) = 56.25 lbs.

Connected gears of different numbers of teeth are used together to change which of the following charasteristics of the input force?

65% Answer Correctly

	force
	rotational direction
	torque
	energy

Solution

Connected gears of different numbers of teeth are used together to change the rotational speed and torque of the input force. If the smaller gear drives the larger gear, the speed of rotation will be reduced and the torque will increase. If the larger gear drives the smaller gear, the speed of rotation will increase and the torque will be reduced.

4 If input effort is 200 ft⋅lb, what output effort will be produced by a machine with a mechanical advantage of 7?

79% Answer Correctly

	1400 ft⋅lb
	2800ft⋅lb
	5600ft⋅lb
	0ft⋅lb

Solution

Mechanical advantage is the ratio of output force to input force and tells us by how many times a machine multiplies input effort. So, a machine with a mechanical advantage of 7 will multiply an input effort of 200 ft⋅lb by 7 to produce an output effort of 1400 ft⋅lb.

A shovel is an example of which class of lever?

56% Answer Correctly

	third
	a shovel is not a lever
	second
	first

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.