ASVAB Mechanical Comprehension Practice Test 700594 Results

Your Results Global Average
Questions 5 5
Correct 0 3.28
Score 0% 66%

Review

1 If the handles of a wheelbarrow are 0.5 ft. from the wheel axle and the load is concentrated at a point 1.0 ft. from the axle, how many pounds of load will a 190 lbs. force lift?
47% Answer Correctly
95
None of these is correct
15.4
-41
Solution
This problem describes a second-class lever and, for a second class lever, the effort force multiplied by the effort distance equals the resistance force multipied by the resistance distance: Fede = Frdr. In this problem we're looking for resistance force:
\( F_r = \frac{F_e d_e}{d_r} \)
\( F_r = \frac{190 \times 0.5}{1.0} \)
\( F_r = \frac{95.0}{1.0} \)
\( F_r = 95 \)
2

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

77% Answer Correctly

ice

marble

steel

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 If the green box weighs 45 lbs. and 35 lbs. of force is applied 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?
55% Answer Correctly
2.33 ft.
0 ft.
135 ft.
0.78 ft.
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 da, our missing value, and plugging in our variables yields:

da = \( \frac{R_bd_b}{R_a} \) = \( \frac{35 lbs. \times 3 ft.}{45 lbs.} \) = \( \frac{105 ft⋅lb}{45 lbs.} \) = 2.33 ft.

4

A ramp is an example of which kind of simple machine?

84% Answer Correctly

first-class lever

wedge

inclined plane

none of these
Solution

An inclined plane is a simple machine that reduces the force needed to raise an object to a certain height. Work equals force x distance and, by increasing the distance that the object travels, an inclined plane reduces the force necessary to raise it to a particular height. In this case, the mechanical advantage is to make the task easier. An example of an inclined plane is a ramp.

5 If the green box is 8 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
0.94
0.69
1.88
0.63
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 = \( \frac{E_a}{R_a} \) = \( \frac{5 ft.}{8 ft.} \) = 0.63

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.