ASVAB Mechanical Comprehension Practice Test 421040 Results

	Your Results	Global Average
Questions	5	5
Correct	0	3.07
Score	0%	61%

Review

A watt is the unit for which of the following?

71% Answer Correctly

	mechanical advantage
	power
	work
	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.

2 If the green box weighs 35 lbs. and is 5 ft. from the fulcrum, how much force would need to be applied at the blue arrow to balance the lever if the arrow's distance from the fulcrum is 7 ft.?

62% Answer Correctly

	0 lbs.
	5 lbs.
	75 lbs.
	25 lbs.

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

R_b = \( \frac{R_ad_a}{d_b} \) = \( \frac{35 lbs. \times 5 ft.}{7 ft.} \) = \( \frac{175 ft⋅lb}{7 ft.} \) = 25 lbs.

For any given surface, the coefficient of static friction is ___________ the coefficient of kinetic friction.

54% Answer Correctly

	opposite
	lower than
	equal to
	higher than

Solution

For any given surface, the coefficient of static friction is higher than the coefficient of kinetic friction. More force is required to initally get an object moving than is required to keep it moving. Additionally, static friction only arises in response to an attempt to move an object (overcome the normal force between it and the surface).

4 What is the efficiency of a machine has work input of 180 ft⋅lb and work output of 171 ft⋅lb?

67% Answer Correctly

	47%
	380%
	95%
	190%

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:
\( Efficiency = \frac{Work_{out}}{Work_{in}} \times 100 \) \( = \frac{171 ft⋅lb}{180 ft⋅lb} \times 100 \) \( = 95% \) %

Torque involves a perpendicular force applied to a lever arm that moves around a center of rotation. Increasing the length of the lever arm will do which of the following?

54% Answer Correctly

	decrease applied force
	decrease torque
	increase applied force
	increase torque

Solution

Torque measures force applied during rotation: τ = rF. Torque (τ, the Greek letter tau) = the radius of the lever arm (r) multiplied by the force (F) applied. Radius is measured from the center of rotation or fulcrum to the point at which the perpendicular force is being applied. The resulting unit for torque is newton-meter (N-m) or foot-pound (ft-lb).