ASVAB Mechanical Comprehension Simple Machines Practice Test 693927 Results

Your Results Global Average
Questions 5 5
Correct 0 2.98
Score 0% 60%

Review

1 How much resistance could a 45 lb. effort force lift using a block and tackle pulley that has 4 ropes supporting the resistance?
82% Answer Correctly
90 lbs.
540 lbs.
180 lbs.
183 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 4. With a MA of 4, a 45 lbs. effort force could lift 45 lbs. x 4 = 180 lbs. resistance.

2 If A = 12 ft., B = 2 ft., C = 9 ft., the green box weighs 25 lbs. and the blue box weighs 30 lbs., what does the orange box have to weigh for this lever to balance?
44% Answer Correctly
50 lbs.
26.67 lbs.
2 lbs.
8.89 lbs.
Solution
In order for this lever to balance, the torque acting on each side of the fulrum must be equal. So, the torque produced by A must equal the torque produced by B and C. Torque is weight x distance from the fulcrum which means that the following must be true for the lever to balance:

fAdA = fBdB + fCdC

For this problem, this equation becomes:

25 lbs. x 12 ft. = 30 lbs. x 2 ft. + fC x 9 ft.

300 ft. lbs. = 60 ft. lbs. + fC x 9 ft.

fC = \( \frac{300 ft. lbs. - 60 ft. lbs.}{9 ft.} \) = \( \frac{240 ft. lbs.}{9 ft.} \) = 26.67 lbs.

3 What is the efficiency of a machine has work input of 155 ft⋅lb and work output of 46 ft⋅lb?
68% Answer Correctly
7%
120%
15%
30%
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:
\( Efficiency = \frac{Work_{out}}{Work_{in}} \times 100 \) \( = \frac{46 ft⋅lb}{155 ft⋅lb} \times 100 \) \( = 30% \) %
4 If the radius of the axle is 5 and the radius of the wheel is 6, what is the mechanical advantage of this wheel and axle configuration?
52% Answer Correctly
0.83
6
1.2
-1
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

5 If the handles of a wheelbarrow are 1.0 ft. from the wheel axle, how many pounds of force must you exert to lift the handles if it's carrying a 130 lbs. load concentrated at a point 0.5 ft. from the axle?
52% Answer Correctly
0
65
14
87
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 effort force:
\( F_e = \frac{F_r d_r}{d_e} \)
\( F_e = \frac{130 \times 0.5}{1.0} \)
\( F_e = \frac{65.0}{1.0} \)
\( F_e = 65 \)