ASVAB Mechanical Comprehension Practice Test 103708 Results

	Your Results	Global Average
Questions	5	5
Correct	0	2.88
Score	0%	58%

Review

1 If the green box weighs 35 lbs. and is 9 ft. from the fulcrum, how much weight would need to be placed at the blue arrow to balance the lever if the arrow's distance from the fulcrum is 6 ft.?

63% Answer Correctly

	5 lbs.
	52.5 lbs.
	105 lbs.
	210 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_b, our missing value, and plugging in our variables yields:

R_b = \( \frac{R_ad_a}{d_b} \) = \( \frac{35 lbs. \times 9 ft.}{6 ft.} \) = \( \frac{315 ft⋅lb}{6 ft.} \) = 52.5 lbs.

Lisa lifts a 25 pound box from the floor onto a loading dock 4 ft. off the ground. Sam slides the same box along a ramp to move it up another 4 ft. onto a flatbed truck. Who has done more work?

50% Answer Correctly

	Sam
	Neither have done any work
	They have done an equal amount of work
	Lisa

Solution

Work is force multiplied by distance. Because both Connie and Sam moved the same weight the same distance they have done an equal amount of work. Sam employed the mechnacial advantage of an inclined plane so he exerted less effort to do the work but the amount of work done was still the same.

3 What is the power output of a 5 hp engine that's 95% efficient?

40% Answer Correctly

	2612.5 \( \frac{ft⋅lb}{s} \)
	870.8 \( \frac{ft⋅lb}{s} \)
	5225 \( \frac{ft⋅lb}{s} \)
	7837.5 \( \frac{ft⋅lb}{s} \)

Solution

\( Efficiency = \frac{Power_{out}}{Power_{in}} \times 100 \)
Solving for power out: \( P_{o} = \frac{E \times P_{i}}{100} \)
Knowing that 1 hp = 550 \( \frac{ft⋅lb}{s} \), P_i becomes 5 hp x 550 \( \frac{ft⋅lb}{s} \) = 2750 \( \frac{ft⋅lb}{s} \)
\( P_{o} = \frac{E \times P_{i}}{100} = \frac{95 \times 2750 \frac{ft⋅lb}{s}}{100} \) \( = \frac{261250 \frac{ft⋅lb}{s}}{100} \) = 2612.5 \( \frac{ft⋅lb}{s} \)

A watt is the unit for which of the following?

71% Answer Correctly

	work
	energy
	power
	mechanical advantage

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.

Sam can do 50 ft. lb. of work in 2 minutes and 5 seconds. What would Sam have to do to increase his power output?

64% Answer Correctly

	do 25 ft. lb. of work in 2 minutes 5 seconds
	do the work in 3 minutes
	do 100 ft. lb. of work in 4 minutes 12 seconds
	do the work in 2 minutes

Solution

Power is the rate of doing work or \(\frac{W}{t}\). To increase power, increase the work being done in the same amount of time or do the same amount of work in less time.