ASVAB Mechanical Comprehension Practice Test 219687 Results

Your Results Global Average
Questions 5 5
Correct 0 3.22
Score 0% 64%

Review

1 A = 6 ft., the green box weighs 30 lbs., and the blue box weighs 60 lbs. What does distance B need to be for this lever to balance?
65% Answer Correctly
360 ft.
1 ft.
6 ft.
3 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 6 ft. = 60 lbs. x dB

dB = \( \frac{30 \times 6 ft⋅lb}{60 lbs.} \) = \( \frac{180 ft⋅lb}{60 lbs.} \) = 3 ft.

2

When all forces acting on a system cancel each other out, this is called:

80% Answer Correctly

equilibrium

stasis

rest

potential energy
Solution

When a system is stable or balanced (equilibrium) all forces acting on the system cancel each other out. In the case of torque, equilibrium means that the sum of the anticlockwise moments about a center of rotation equal the sum of the clockwise moments.

3 If this lever is in equilibrium with an effort force of 8.89 ft. lb. at the blue arrow and a resistance force of 8 ft. lb. at the green box, what is its mechanical advantage?
48% Answer Correctly
0.81
0.3
0.9
1.8
Solution

Mechanical advantage (MA) is the ratio by which effort force relates to resistance force. If both forces are known, calculating MA is simply a matter of dividing resistance force by effort force:

MA = \( \frac{F_r}{F_e} \) = \( \frac{8 ft.}{8.89 ft.} \) = 0.9

In this case, the mechanical advantage is less than one meaning that each unit of effort force results in just 0.9 units of resistance force. However, a third class lever like this isn't designed to multiply force like a first class lever. A third class lever is designed to multiply distance and speed at the resistance by sacrificing force at the resistance. Different lever styles have different purposes and multiply forces in different ways.

4 What is the efficiency of a machine has work input of 225 ft⋅lb and work output of 213 ft⋅lb?
67% Answer Correctly
190%
95%
1%
23%
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:
\( Efficiency = \frac{Work_{out}}{Work_{in}} \times 100 \) \( = \frac{213 ft⋅lb}{225 ft⋅lb} \times 100 \) \( = 95% \) %
5

Which of the following is the formula for torque?

61% Answer Correctly

τ = F/r

τ = rF

τ = r/F

τ = F/r2
Solution

Torque measures force applied during rotation: τ = rF.  Torque (τ, the Greek letter tau) = the radius of the lever arm (r) multiplied by the force (F) applied. Radius is measured from the center of rotation or fulcrum to the point at which the perpendicular force is being applied. The resulting unit for torque is newton-meter (N-m) or foot-pound (ft-lb).