ASVAB Mechanical Comprehension Practice Test 512994 Results

	Your Results	Global Average
Questions	5	5
Correct	0	3.25
Score	0%	65%

Review

Which of the following is not a modulus of elasticity?

47% Answer Correctly

	shear modulus
	bulk modulus
	stretch modulus
	stress modulus

Solution

The modulus of elasticity measures how much a material or structure will deflect under stress. Stretch modulus is longitudinal stretch (like stretching raw bread dough), shear modulus is longitudinal deflection (like the horizontal displacement of a stack of magzines when a heavy object is placed upon them), and bulk modulus is compression of volume (like the compression of a loaf of bread under a heavy can at the bottom of a grocery bag).

A box is resting on a smooth floor. Static friction is present:

60% Answer Correctly

	at all times
	if the coefficient of friction is greater than one
	when an attempt is made to move the box
	only if normal force is present

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).

3 What is the efficiency of a machine has work input of 175 ft⋅lb and work output of 87 ft⋅lb?

67% Answer Correctly

	0%
	100%
	2%
	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:
\( Efficiency = \frac{Work_{out}}{Work_{in}} \times 100 \) \( = \frac{87 ft⋅lb}{175 ft⋅lb} \times 100 \) \( = 50% \) %

4 If you have a gear train with two gears, the first with 20 teeth and the second with 8 teeth, how many revolutions does the second gear make for each revolution of the first gear?

77% Answer Correctly

	2.5
	2.8
	1.3
	2.3

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 = \( \frac{N_1}{N_2} \) = \( \frac{20}{8} \) = 2.5

Friction between two or more solid objects that are not moving relative to each other is called:

73% Answer Correctly

	dynamic friction
	kinetic friction
	gravitational friction
	static friction

Solution

Static friction is friction between two or more solid objects that are not moving relative to each other. An example is the friction that prevents a box on a sloped surface from sliding farther down the surface.