ASVAB Mechanical Comprehension Practice Test 638002 Results

	Your Results	Global Average
Questions	5	5
Correct	0	3.00
Score	0%	60%

Review

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

51% Answer Correctly

	changes the direction of force
	increases distance
	decreases 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.

A block and tackle with four pulleys would have a mechanical advantage of:

79% Answer Correctly

	4
	2
	1
	0

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 The green box weighs 60 lbs. and a 50 lbs. weight is placed 1 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

	60 ft.
	0.42 ft.
	0.83 ft.
	0.28 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 = \( \frac{R_bd_b}{R_a} \) = \( \frac{50 lbs. \times 1 ft.}{60 lbs.} \) = \( \frac{50 ft⋅lb}{60 lbs.} \) = 0.83 ft.

According to Boyle's law, for a fixed amount of gas kept at a fixed temperature, which of the following are inversely proportional?

63% Answer Correctly

	density, volume
	pressure, density
	volume, mass
	pressure, volume

Solution

Boyle's law states that "for a fixed amount of an ideal gas kept at a fixed temperature, pressure and volume are inversely proportional".

5 If this lever is in equilibrium with an effort force of 11.25 ft. lb. at the blue arrow and a resistance force of 9 ft. lb. at the green box, what is its mechanical advantage?

48% Answer Correctly

	0.72
	0.8
	0.27
	7.8

Solution

Mechanical advantage (MA) is the ratio by which effort force relates to resistance force. If both forces are known, calculating MA is simply a matter of dividing resistance force by effort force:

MA = \( \frac{F_r}{F_e} \) = \( \frac{9 ft.}{11.25 ft.} \) = 0.8

In this case, the mechanical advantage is less than one meaning that each unit of effort force results in just 0.8 units of resistance force. However, a third class lever like this isn't designed to multiply force like a first class lever. A third class lever is designed to multiply distance and speed at the resistance by sacrificing force at the resistance. Different lever styles have different purposes and multiply forces in different ways.