ASVAB Mechanical Comprehension Practice Test 701915 Results

Your Results Global Average
Questions 5 5
Correct 0 2.89
Score 0% 58%

Review

1 A = 9 ft., the green box weighs 30 lbs., and the blue box weighs 65 lbs. What does distance B need to be for this lever to balance?
65% Answer Correctly
4.15 ft.
0 ft.
1.38 ft.
1.04 ft.
Solution
In order for this lever to balance, the torque acting on side A must equal the torque acting on side B. Torque is weight x distance from the fulcrum which means that the following must be true for the lever to balance:

fAdA = fBdB

For this problem, the equation becomes:

30 lbs. x 9 ft. = 65 lbs. x dB

dB = \( \frac{30 \times 9 ft⋅lb}{65 lbs.} \) = \( \frac{270 ft⋅lb}{65 lbs.} \) = 4.15 ft.

2 If the radius of the axle is 3 and the radius of the wheel is 8, what is the mechanical advantage of this wheel and axle configuration?
41% Answer Correctly
-5
0.38
2.67
5
Solution

The mechanical advantage of a wheel and axle is the input radius divided by the output radius:

MA = \( \frac{r_i}{r_o} \)

In this case, the input radius (where the effort force is being applied) is 3 and the output radius (where the resistance is being applied) is 8 for a mechanical advantage of \( \frac{3}{8} \) = 0.38

3

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

59% Answer Correctly

when an attempt is made to move the box

at all times

if the coefficient of friction is greater than one

only if normal force is present
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).

4

Which of the following is not a characteristic of a ceramic?

61% Answer Correctly

chemically stable

high melting point

low corrosive action

low density
Solution

Ceramics are mixtures of metallic and nonmetallic elements that withstand exteme thermal, chemical, and pressure environments. They have a high melting point, low corrosive action, and are chemically stable. Examples include rock, sand, clay, glass, brick, and porcelain.

5 If the green box is 4 ft. from the fulcrum and a certain force applied 9 ft. from the fulcrum at the blue arrow balances the lever, what is the mechanical advantage?
61% Answer Correctly
2.03
2.25
3.38
-5.75
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 = \( \frac{E_a}{R_a} \) = \( \frac{9 ft.}{4 ft.} \) = 2.25

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.