ASVAB Mechanical Comprehension Practice Test 771288 Results

	Your Results	Global Average
Questions	5	5
Correct	0	3.39
Score	0%	68%

Review

1 If the green box weighs 50 lbs. and 65 lbs. of force is applied 9 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?

55% Answer Correctly

	450 ft.
	5 ft.
	11.7 ft.
	46.8 ft.

Solution

To balance this lever the torques at the green box and the blue arrow 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 green box and b the blue arrow, R is resistance (weight/force) and d is the distance from the fulcrum.

Solving for d_a, our missing value, and plugging in our variables yields:

d_a = \( \frac{R_bd_b}{R_a} \) = \( \frac{65 lbs. \times 9 ft.}{50 lbs.} \) = \( \frac{585 ft⋅lb}{50 lbs.} \) = 11.7 ft.

2 If the green box is 5 ft. from the fulcrum and a certain force applied 3 ft. from the fulcrum at the blue arrow balances the lever, what is the mechanical advantage?

61% Answer Correctly

	0.9
	0.54
	-8.4
	0.6

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 = \( \frac{E_a}{R_a} \) = \( \frac{3 ft.}{5 ft.} \) = 0.6

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.

Which of the following is not a type of bridge?

74% Answer Correctly

	arch
	block
	cable
	truss

Solution

The six basic bridge forms are beam, truss, arch, cantilever, cable, and suspension.

4 What is the efficiency of a machine has work input of 245 ft⋅lb and work output of 208 ft⋅lb?

67% Answer Correctly

	42%
	170%
	85%
	1%

Solution

Due to friction, a machine will never be able to utilize 100% of its work input. A certain percentage of that input will be lost in overcoming friction within the machine. Effeciency is a measure of how much of a machine's work input can be turned into useful work output and is calculated by dividing work output by work input and multiplying the result by 100:
\( Efficiency = \frac{Work_{out}}{Work_{in}} \times 100 \) \( = \frac{208 ft⋅lb}{245 ft⋅lb} \times 100 \) \( = 85% \) %

Potential energy is energy that has the potential to be converted into what?

80% Answer Correctly

	work
	power
	kinetic energy
	heat

Solution

Potential energy is the energy of an object by virtue of its position relative to other objects. It is energy that has the potential to be converted into kinetic energy.