ASVAB Mechanical Comprehension Practice Test 571530 Results

Your Results Global Average
Questions 5 5
Correct 0 3.34
Score 0% 67%

Review

1

What type of load acts on a relatively small area of a structure?

74% Answer Correctly

non-uniformly distributed load

impact load

concentrated load

dynamic load
Solution

A concentrated load acts on a relatively small area of a structure, a static uniformly distributed load doesn't create specific stress points or vary with time, a dynamic load varies with time or affects a structure that experiences a high degree of movement, an impact load is sudden and for a relatively short duration and a non-uniformly distributed load creates different stresses at different locations on a structure.

2 What is the efficiency of a machine has work input of 210 ft⋅lb and work output of 168 ft⋅lb?
67% Answer Correctly
80%
20%
160%
0%
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:
\( Efficiency = \frac{Work_{out}}{Work_{in}} \times 100 \) \( = \frac{168 ft⋅lb}{210 ft⋅lb} \times 100 \) \( = 80% \) %
3 How much resistance could a 40 lb. effort force lift using a block and tackle pulley that has 8 ropes supporting the resistance?
81% Answer Correctly
320 lbs.
323 lbs.
106 lbs.
352 lbs.
Solution

The mechanical advantage (MA) of a block and tackle pulley is equal to the number of times the effort force changes direction. An easy way to count how many times the effort force changes direction is to count the number of ropes that support the resistance which, in this problem, is 8. With a MA of 8, a 40 lbs. effort force could lift 40 lbs. x 8 = 320 lbs. resistance.

4 The radius of the axle is 5, the radius of the wheel is 6, and the blue box weighs 95 lbs. What is the effort force necessary to balance the load?
53% Answer Correctly
6.2 lbs.
7.2 lbs.
5 lbs.
79.17 lbs.
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 6 and the output radius (where the resistance is being applied) is 5 for a mechanical advantage of \( \frac{6}{5} \) = 1.2

MA = \( \frac{load}{effort} \) so effort = \( \frac{load}{MA} \) = \( \frac{95 lbs.}{1.2} \) = 79.17 lbs.

5 If the green box weighs 35 lbs. and is 5 ft. from the fulcrum, how far from the fulcrum would a 45 lbs. force need to be applied to balance the lever?
58% Answer Correctly
0.97 ft.
175 ft.
3.89 ft.
0 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 db, our missing value, and plugging in our variables yields:

db = \( \frac{R_ad_a}{R_b} \) = \( \frac{35 lbs. \times 5 ft.}{45 lbs.} \) = \( \frac{175 ft⋅lb}{45 lbs.} \) = 3.89 ft.