ASVAB Mechanical Comprehension Practice Test 356422 Results

	Your Results	Global Average
Questions	5	5
Correct	0	2.54
Score	0%	51%

Review

The principle of conservation of mechanical energy states that, as long as no other forces are applied, what will remain constant as an object falls?

45% Answer Correctly

	total mechanical energy
	acceleration
	kinetic energy
	potential energy

Solution

As an object falls, its potential energy is converted into kinetic energy. The principle of conservation of mechanical energy states that, as long as no other forces are applied, total mechanical energy (PE + KE) of the object will remain constant at all points in its descent.

2 If this lever is in equilibrium with an effort force of 8.75 ft. lb. at the blue arrow and a resistance force of 7 ft. lb. at the green box, what is its mechanical advantage?

48% Answer Correctly

	2.8
	0.8
	0.72
	8.8

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 = \( \frac{F_r}{F_e} \) = \( \frac{7 ft.}{8.75 ft.} \) = 0.8

In this case, the mechanical advantage is less than one meaning that each unit of effort force results in just 0.8 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.

Force of friction due to kinetic friction is __________ the force of friction due to static friction.

40% Answer Correctly

	the same as
	opposite
	lower than
	higher than

Solution

The formula for force of friction (F_f) is the same whether kinetic or static friction applies: F_{f =}μF_N. To distinguish between kinetic and static friction, μ_k and μ_s are often used in place of μ.

4 The green box weighs 75 lbs. and a 60 lbs. weight is placed 3 ft. from the fulcrum at the blue arrow. How far from the fulcrum would the green box need to be placed to balance the lever?

57% Answer Correctly

	225 ft.
	1.2 ft.
	2.4 ft.
	7.2 ft.

Solution

To balance this lever the torques on each side of the fulcrum must be equal. Torque is weight x distance from the fulcrum so the equation for equilibrium is:

R_ad_a = R_bd_b

where a represents the left side of the fulcrum and b the right, R is resistance (weight) and d is the distance from the fulcrum.

Solving for d_a, our missing value, and plugging in our variables yields:

d_a = \( \frac{R_bd_b}{R_a} \) = \( \frac{60 lbs. \times 3 ft.}{75 lbs.} \) = \( \frac{180 ft⋅lb}{75 lbs.} \) = 2.4 ft.

5 What is the efficiency of a machine has work input of 105 ft⋅lb and work output of 52 ft⋅lb?

67% Answer Correctly

	200%
	50%
	25%
	100%

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:
\( Efficiency = \frac{Work_{out}}{Work_{in}} \times 100 \) \( = \frac{52 ft⋅lb}{105 ft⋅lb} \times 100 \) \( = 50% \) %