ASVAB Mechanical Comprehension Practice Test 750559 Results

	Your Results	Global Average
Questions	5	5
Correct	0	2.91
Score	0%	58%

Review

Which of the following statements about this pulley configuration is false?

48% Answer Correctly

	This is a block and tackle pulley configuration
	Only multiplies the effort force
	Mechanical advantage is the number of ropes that support the resistance
	Changes the direction of and multiplies the effort force

Solution

A block and tackle is a combination of one or more fixed pulleys and one or more movable pulleys where the fixed pulleys change the direction of the effort force and the movable pulleys multiply it. The mechanical advantage is equal to the number of times the effort force changes direction and can be increased by adding more pulley wheels to the system. An easy way to find the mechanical advantage of a block and tackle pulley system is to count the number of ropes that support the resistance.

The standard unit of energy is the:

73% Answer Correctly

	Watt
	Horsepower
	Volt
	Joule

Solution

The Joule (J) is the standard unit of energy and has the unit \({kg \times m^2} \over s^2\).

3 If this lever is in equilibrium with an effort force of 20.0 ft. lb. at the blue arrow and a resistance force of 8 ft. lb. at the green box, what is its mechanical advantage?

48% Answer Correctly

	0.44
	0.4
	7.4
	2.4

Solution

Mechanical advantage (MA) is the ratio by which effort force relates to resistance force. If both forces are known, calculating MA is simply a matter of dividing resistance force by effort force:

MA = \( \frac{F_r}{F_e} \) = \( \frac{8 ft.}{20.0 ft.} \) = 0.4

In this case, the mechanical advantage is less than one meaning that each unit of effort force results in just 0.4 units of resistance force. However, a third class lever like this isn't designed to multiply force like a first class lever. A third class lever is designed to multiply distance and speed at the resistance by sacrificing force at the resistance. Different lever styles have different purposes and multiply forces in different ways.

4 If you have a gear train with three gears, the first with 30 teeth, the second with 18 teeth, and the third with 8 teeth, what is its mechanical advantage?

51% Answer Correctly

	6.8
	3.4
	11.3
	3.8

Solution

The mechanical advantage of a gear train is its gear ratio. The gear ratio (V_r) 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 three gears so the equation becomes:

V_r = \( \frac{N_1}{N_2} \) \( \frac{N_2}{N_3} \) = \( \frac{30}{18} \) \( \frac{18}{8} \) = \( \frac{30}{8} \) = 3.8

Two or more pulleys used together are called:

71% Answer Correctly

	wheel and axle
	gears
	block and tackle
	third-class lever

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.