ASVAB Mechanical Comprehension Practice Test 452864 Results

	Your Results	Global Average
Questions	5	5
Correct	0	2.73
Score	0%	55%

Review

The mechanical advantage of a third class lever is always:

37% Answer Correctly

	greater than one
	less than one
	not equal to one
	equal to one

Solution

A third class lever is designed to multiply distance and speed at the expense of effort force. Because the effort force is greater than the resistance, the mechanical advantage of a third class lever is always less than one.

An example of a third class lever is a broom. The fulcrum is at your hand on the end of the broom, the effort force is your other hand in the middle, and the resistance is at the bottom bristles. The effort force of your hand in the middle multiplies the distance and speed of the bristles at the bottom but at the expense of producing a brushing force that's less than the force you're applying with your hand.

2 What is the efficiency of a machine has work input of 200 ft⋅lb and work output of 50 ft⋅lb?

67% Answer Correctly

	6%
	0%
	25%
	50%

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:
\( Efficiency = \frac{Work_{out}}{Work_{in}} \times 100 \) \( = \frac{50 ft⋅lb}{200 ft⋅lb} \times 100 \) \( = 25% \) %

Connected gears of different numbers of teeth are used together to change which of the following charasteristics of the input force?

65% Answer Correctly

	energy
	torque
	force
	rotational direction

Solution

Connected gears of different numbers of teeth are used together to change the rotational speed and torque of the input force. If the smaller gear drives the larger gear, the speed of rotation will be reduced and the torque will increase. If the larger gear drives the smaller gear, the speed of rotation will increase and the torque will be reduced.

4 What is the power output of a 9 hp engine that's 40% efficient?

40% Answer Correctly

	7920 \( \frac{ft⋅lb}{s} \)
	3960 \( \frac{ft⋅lb}{s} \)
	1980 \( \frac{ft⋅lb}{s} \)
	5940 \( \frac{ft⋅lb}{s} \)

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 9 hp x 550 \( \frac{ft⋅lb}{s} \) = 4950 \( \frac{ft⋅lb}{s} \)
\( P_{o} = \frac{E \times P_{i}}{100} = \frac{40 \times 4950 \frac{ft⋅lb}{s}}{100} \) \( = \frac{198000 \frac{ft⋅lb}{s}}{100} \) = 1980 \( \frac{ft⋅lb}{s} \)

Gear ratio indicates which of the following about two connected gears?

59% Answer Correctly

	power conversion
	work done
	efficiency
	mechanical advantage

Solution

The mechanical advantage (amount of change in speed or torque) of connected gears is proportional to the number of teeth each gear has. Called gear ratio, it's the ratio of the number of teeth on the larger gear to the number of teeth on the smaller gear. For example, a gear with 12 teeth connected to a gear with 9 teeth would have a gear ratio of 4:3.