ASVAB Mechanical Comprehension Practice Test 88396 Results

Your Results Global Average
Questions 5 5
Correct 0 3.04
Score 0% 61%

Review

1 What is the power output of a 2 hp engine that's 95% efficient?
40% Answer Correctly
1045 \( \frac{ft⋅lb}{s} \)
2090 \( \frac{ft⋅lb}{s} \)
190 \( \frac{ft⋅lb}{s} \)
0 \( \frac{ft⋅lb}{s} \)
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 \( \frac{ft⋅lb}{s} \), Pi becomes 2 hp x 550 \( \frac{ft⋅lb}{s} \) = 1100 \( \frac{ft⋅lb}{s} \)
\( P_{o} = \frac{E \times P_{i}}{100} = \frac{95 \times 1100 \frac{ft⋅lb}{s}}{100} \) \( = \frac{104500 \frac{ft⋅lb}{s}}{100} \) = 1045 \( \frac{ft⋅lb}{s} \)
2

Which of the following surfaces would have the lowest coefficient of friction?

85% Answer Correctly

ice

leather

tile

concrete
Solution

Coefficient of friction (μ) represents how much two materials resist sliding across each other.  Smooth surfaces like ice have low coefficients of friction while rough surfaces like concrete have high μ.

3

Which class of lever offers no mechanical advantage?

45% Answer Correctly

first

none of these, all levers offer mechanical advantage

third

second
Solution

A third-class lever is used to increase distance traveled by an object in the same direction as the force applied. The fulcrum is at one end of the lever, the object at the other, and the force is applied between them. This lever does not impart a mechanical advantage as the effort force must be greater than the load but does impart extra speed to the load. Examples of third-class levers are shovels and tweezers.

4 If the green box is 2 ft. from the fulcrum and a certain force applied 5 ft. from the fulcrum at the blue arrow balances the lever, what is the mechanical advantage?
61% Answer Correctly
2.25
1.25
5.5
2.5
Solution

Because this lever is in equilibrium, we know that the effort force at the blue arrow is equal to the resistance weight of the green box. For a lever that's in equilibrium, one method of calculating mechanical advantage (MA) is to divide the length of the effort arm (Ea) by the length of the resistance arm (Ra):

MA = \( \frac{E_a}{R_a} \) = \( \frac{5 ft.}{2 ft.} \) = 2.5

When a lever is in equilibrium, the torque from the effort and the resistance are equal. The equation for equilibrium is Rada = Rbdb where a and b are the two points at which effort/resistance is being applied to the lever.

In this problem, Ra and Rb are such that the lever is in equilibrium meaning that some multiple of the weight of the green box is being applied at the blue arrow. For a lever, this multiple is a function of the ratio of the distances of the box and the arrow from the fulcrum. That's why, for a lever in equilibrium, only the distances from the fulcrum are necessary to calculate mechanical advantage.

If the lever were not in equilibrium, you would first have to calculate the forces and distances necessary to put it in equilibrium and then divide Ea by Ra to get the mechanical advantage.

5

A a seesaw / teeter-totter is an example of which of the following?

69% Answer Correctly

first-class lever

second-class lever

third-class lever

inclined plane
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.