ASVAB Mechanical Comprehension Practice Test 867878 Results

Your Results Global Average
Questions 5 5
Correct 0 2.92
Score 0% 58%

Review

1 What is the efficiency of a machine has work input of 155 ft⋅lb and work output of 147 ft⋅lb?
67% Answer Correctly
47%
23%
5%
95%
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:
\( Efficiency = \frac{Work_{out}}{Work_{in}} \times 100 \) \( = \frac{147 ft⋅lb}{155 ft⋅lb} \times 100 \) \( = 95% \) %
2

Which of these will have the most impact on the kinetic energy of an object?

54% Answer Correctly

its speed

its weight

its mass

its direction
Solution

Kinetic energy is the energy of movement and is a function of the mass of an object and its speed: \(KE = {1 \over 2}mv^2\) where m is mass in kilograms, v is speed in meters per second, and KE is in joules. The most impactful quantity to kinetic energy is velocity as an increase in mass increases KE linearly while an increase in speed increases KE exponentially.

3

Which class of lever is used to increase force on an object in the same direction as the force is applied?

52% Answer Correctly

first

third

all of these

second
Solution

A second-class lever is used to increase force on an object in the same direction as the force is applied. This lever requires a smaller force to lift a larger load but the force must be applied over a greater distance. The fulcrum is placed at one end of the lever and mechanical advantage increases as the object being lifted is moved closer to the fulcrum or the length of the lever is increased. An example of a second-class lever is a wheelbarrow.

4 The green box weighs 20 lbs. and a 75 lbs. weight is placed 1 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
15 ft.
0.94 ft.
20 ft.
3.75 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:

Rada = Rbdb

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 da, our missing value, and plugging in our variables yields:

da = \( \frac{R_bd_b}{R_a} \) = \( \frac{75 lbs. \times 1 ft.}{20 lbs.} \) = \( \frac{75 ft⋅lb}{20 lbs.} \) = 3.75 ft.

5 If 65 lbs. of force is applied 6 ft. from the fulcrum at the blue arrow and the green box is 4 ft. from the fulcrum, how much would the green box have to weigh to balance the lever?
62% Answer Correctly
390 lbs.
97.5 lbs.
195 lbs.
0 lbs.
Solution

To balance this lever the torques at the green box and the blue arrow must be equal. Torque is weight x distance from the fulcrum so the equation for equilibrium is:

Rada = Rbdb

where a represents the green box and b the blue arrow, R is resistance (weight/force) and d is the distance from the fulcrum.

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

Ra = \( \frac{R_bd_b}{d_a} \) = \( \frac{65 lbs. \times 6 ft.}{4 ft.} \) = \( \frac{390 ft⋅lb}{4 ft.} \) = 97.5 lbs.