ASVAB Mechanical Comprehension Practice Test 73179 Results

	Your Results	Global Average
Questions	5	5
Correct	0	3.82
Score	0%	76%

Review

Two or more pulleys used together are called:

71% Answer Correctly

	wheel and axle
	gears
	third-class lever
	block and tackle

Solution

Two or more pulleys used together constitute a block and tackle which, unlike a fixed pulley, does impart mechanical advantage as a function of the number of pulleys that make up the arrangement. So, for example, a block and tackle with three pulleys would have a mechanical advantage of three.

The steering wheel of a car is an example of which type of simple machine?

89% Answer Correctly

	block and tackle
	fixed pulley
	first-class lever
	wheel and axle

Solution

A wheel and axle uses two different diameter wheels mounted to a connecting axle. Force is applied to the larger wheel and large movements of this wheel result in small movements in the smaller wheel. Because a larger movement distance is being translated to a smaller distance, force is increased with a mechanical advantage equal to the ratio of the diameters of the wheels. An example of a wheel and axle is the steering wheel of a car.

3 If you have a gear train with two gears, the first with 24 teeth and the second with 4 teeth, how many revolutions does the second gear make for each revolution of the first gear?

77% Answer Correctly

	6
	7
	6.6
	18

Solution

The gear ratio (V_r) of a gear train is the product of the gear ratios between the pairs of meshed gears. Let N represent the number of teeth for each gear:

V_r = \( \frac{N_1}{N_2} \) \( \frac{N_2}{N_3} \) \( \frac{N_3}{N_4} \) ... \( \frac{N_n}{N_{n+1}} \)

In this problem, we have only two gears so the equation becomes:

V_r = \( \frac{N_1}{N_2} \) = \( \frac{24}{4} \) = 6

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

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.

5 How much resistance could a 90 lb. effort force lift using a block and tackle pulley that has 10 ropes supporting the resistance?

81% Answer Correctly

	1800 lbs.
	900 lbs.
	2700 lbs.
	903 lbs.

Solution

The mechanical advantage (MA) of a block and tackle pulley is equal to the number of times the effort force changes direction. An easy way to count how many times the effort force changes direction is to count the number of ropes that support the resistance which, in this problem, is 10. With a MA of 10, a 90 lbs. effort force could lift 90 lbs. x 10 = 900 lbs. resistance.