ASVAB Mechanical Comprehension Practice Test 568040 Results

Your Results Global Average
Questions 5 5
Correct 0 2.78
Score 0% 56%

Review

1 If the green box weighs 75 lbs. and is 3 ft. from the fulcrum, how far from the fulcrum would a 70 lbs. weight need to be placed to balance the lever?
61% Answer Correctly
6.43 ft.
9.64 ft.
1.61 ft.
3.21 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 db, our missing value, and plugging in our variables yields:

db = \( \frac{R_ad_a}{R_b} \) = \( \frac{75 lbs. \times 3 ft.}{70 lbs.} \) = \( \frac{225 ft⋅lb}{70 lbs.} \) = 3.21 ft.

2

Which of the following is not true of a  first-class lever?

51% Answer Correctly

increases distance

changes the direction of force

increases force

decreases distance
Solution

A first-class lever is used to increase force or distance while changing the direction of the force. The lever pivots on a fulcrum and, when a force is applied to the lever at one side of the fulcrum, the other end moves in the opposite direction. The position of the fulcrum also defines the mechanical advantage of the lever. If the fulcrum is closer to the force being applied, the load can be moved a greater distance at the expense of requiring a greater input force. If the fulcrum is closer to the load, less force is required but the force must be applied over a longer distance. An example of a first-class lever is a seesaw / teeter-totter.

3

The measure of how much of the power put into a machine is turned into movement or force is called:

56% Answer Correctly

power

efficiency

force multiplication

mechanical advantage
Solution

The efficiency of a machine describes how much of the power put into the machine is turned into movement or force. A 100% efficient machine would turn all of the input power into output movement or force. However, no machine is 100% efficient due to friction, heat, wear and other imperfections that consume input power without delivering any output.

4 A mass of air has a pressure of 15.0 psi and a volume of 75 ft.3. If the air is compressed to a new volume of 70 ft.3, what is the new pressure?
56% Answer Correctly
16.1 psi
18.1 psi
19.1 psi
23.1 psi
Solution

According to Boyle's Law, pressure and volume are inversely proportional:

\( \frac{P_1}{P_2} \) = \( \frac{V_2}{V_1} \)

In this problem, V2 = 70 ft.3, V1 = 75 ft.3 and P1 = 15.0 psi. Solving for P2:

P2 = \( \frac{P_1}{\frac{V_2}{V_1}} \) = \( \frac{15.0 psi}{\frac{70 ft.^3}{75 ft.^3}} \) = 16.1 psi

5 If the force applied at the blue arrow over 4 ft. moves the green box 0.67 ft., what is the mechanical advantage of this lever?
56% Answer Correctly
6.6
2
6
12
Solution

Mechanical advantage (MA) can be calculated knowing only the distance the effort (blue arrow) moves and the distance the resistance (green box) moves. The equation is:

MA = \( \frac{E_d}{R_d} \)

where Ed is the effort distance and Rd is the resistance distance. For this problem, the equation becomes:

MA = \( \frac{4 ft.}{0.67 ft.} \) = 6

You might be wondering how having an effort distance of 6 times the resistance distance is an advantage. Remember the principle of moments. For a lever in equilibrium the effort torque equals the resistance torque. Because torque is force x distance, if the effort distance is 6 times the resistance distance, the effort force must be \( \frac{1}{6} \) the resistance force. You're trading moving 6 times the distance for only having to use \( \frac{1}{6} \) the force.