ASVAB Mechanical Comprehension Practice Test 295120 Results

	Your Results	Global Average
Questions	5	5
Correct	0	3.10
Score	0%	62%

Review

Force of friction due to kinetic friction is __________ the force of friction due to static friction.

40% Answer Correctly

	the same as
	lower than
	higher than
	opposite

Solution

The formula for force of friction (F_f) is the same whether kinetic or static friction applies: F_{f =}μF_N. To distinguish between kinetic and static friction, μ_k and μ_s are often used in place of μ.

2 What is the efficiency of a machine has work input of 180 ft⋅lb and work output of 117 ft⋅lb?

67% Answer Correctly

	260%
	65%
	35%
	32%

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:
\( Efficiency = \frac{Work_{out}}{Work_{in}} \times 100 \) \( = \frac{117 ft⋅lb}{180 ft⋅lb} \times 100 \) \( = 65% \) %

Hydraulics is the transmission of force through the use of which of the following?

73% Answer Correctly

	gear systems
	liquids
	air pressure
	torque

Solution

Hydraulics is the transmission of force through the use of liquids. Liquids are especially suited for transferring force in complex machines because they compress very little and can occupy very small spaces. Hydraulic pressure is calculated by dividing force by the area over which it is applied: P = F/A where F is force in pounds, A is area in square inches, and the resulting pressure is in pounds per square inch (psi).

4 If the green box is 7 ft. from the fulcrum and a certain force applied 6 ft. from the fulcrum at the blue arrow balances the lever, what is the mechanical advantage?

61% Answer Correctly

	0.29
	0.94
	0.86
	1.29

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 = \( \frac{E_a}{R_a} \) = \( \frac{6 ft.}{7 ft.} \) = 0.86

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.

A watt is the unit for which of the following?

71% Answer Correctly

	power
	work
	mechanical advantage
	energy

Solution

Power is the rate at which work is done, P = w/t, or work per unit time. The watt (W) is the unit for power and is equal to 1 joule (or newton-meter) per second. Horsepower (hp) is another familiar unit of power used primarily for rating internal combustion engines. 1 hp equals 746 watts.