ASVAB Mechanical Comprehension Practice Test 35848 Results

	Your Results	Global Average
Questions	5	5
Correct	0	3.16
Score	0%	63%

Review

Friction resists movement in a direction __________ to the movement.

81% Answer Correctly

	perpendicular
	normal
	opposite
	parallel

Solution

Friction resists movement. Kinetic (also called sliding or dynamic) friction resists movement in a direction opposite to the movement. Because it opposes movement, kinetic friction will eventually bring an object to a stop. An example is a rock that's sliding across ice.

2 What is the efficiency of a machine has work input of 155 ft⋅lb and work output of 100 ft⋅lb?

67% Answer Correctly

	260%
	65%
	130%
	16%

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:
\( Efficiency = \frac{Work_{out}}{Work_{in}} \times 100 \) \( = \frac{100 ft⋅lb}{155 ft⋅lb} \times 100 \) \( = 65% \) %

What type of load varies with time or affects a structure that experiences a high degree of movement?

66% Answer Correctly

	concentrated load
	impact load
	static load
	dynamic load

Solution

A concentrated load acts on a relatively small area of a structure, a static uniformly distributed load doesn't create specific stress points or vary with time, a dynamic load varies with time or affects a structure that experiences a high degree of movement, an impact load is sudden and for a relatively short duration and a non-uniformly distributed load creates different stresses at different locations on a structure.

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

59% Answer Correctly

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

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).

5 What is the power output of a 3 hp engine that's 55% efficient?

40% Answer Correctly

	453.8 \( \frac{ft⋅lb}{s} \)
	165 \( \frac{ft⋅lb}{s} \)
	907.5 \( \frac{ft⋅lb}{s} \)
	2722.5 \( \frac{ft⋅lb}{s} \)

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 3 hp x 550 \( \frac{ft⋅lb}{s} \) = 1650 \( \frac{ft⋅lb}{s} \)
\( P_{o} = \frac{E \times P_{i}}{100} = \frac{55 \times 1650 \frac{ft⋅lb}{s}}{100} \) \( = \frac{90750 \frac{ft⋅lb}{s}}{100} \) = 907.5 \( \frac{ft⋅lb}{s} \)