ASVAB Mechanical Comprehension Practice Test 987758 Results

Your Results Global Average
Questions 5 5
Correct 0 3.27
Score 0% 65%

Review

1 If the green box weighs 20 lbs. and is 7 ft. from the fulcrum, how far from the fulcrum would a 75 lbs. force need to be applied to balance the lever?
58% Answer Correctly
1.87 ft.
3.73 ft.
2 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 = \( \frac{R_ad_a}{R_b} \) = \( \frac{20 lbs. \times 7 ft.}{75 lbs.} \) = \( \frac{140 ft⋅lb}{75 lbs.} \) = 1.87 ft.

2 If you lift a 19 lbs. rock 31 ft. from the ground, how much work have you done?
71% Answer Correctly
-12 ft⋅lb
1 ft⋅lb
589 ft⋅lb
279 ft⋅lb
Solution
Work is force times distance. In this case, the force is the weight of the rock so:
\( W = F \times d \)
\( W = 19 \times 31 \)
\( W = 589 \)
3

Which class of lever is used to increase force on an object in the same direction as the force is applied?

52% Answer Correctly

all of these

third

first

second
Solution

A second-class lever is used to increase force on an object in the same direction as the force is applied. This lever requires a smaller force to lift a larger load but the force must be applied over a greater distance. The fulcrum is placed at one end of the lever and mechanical advantage increases as the object being lifted is moved closer to the fulcrum or the length of the lever is increased. An example of a second-class lever is a wheelbarrow.

4 What is the efficiency of a machine has work input of 60 ft⋅lb and work output of 24 ft⋅lb?
67% Answer Correctly
60%
0%
40%
20%
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:
\( Efficiency = \frac{Work_{out}}{Work_{in}} \times 100 \) \( = \frac{24 ft⋅lb}{60 ft⋅lb} \times 100 \) \( = 40% \) %
5

A block and tackle with four pulleys would have a mechanical advantage of:

79% Answer Correctly

4

1

2

0
Solution

Two or more pulleys used together constitute a block and tackle which, unlike a fixed pulley, does impart mechanical advantage as a function of the number of pulleys that make up the arrangement.  So, for example, a block and tackle with three pulleys would have a mechanical advantage of three.