ASVAB Mechanical Comprehension Practice Test 8

Question 1 easy

To lift a 300-pound crate 5 feet high into a storage rack, how long must an inclined plane be to reduce the required lifting effort to one-third of the weight (100 pounds)?

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Correct answer: C — 15 feet long

Mechanical Advantage (MA) = Resistance Force / Effort Force = 300 lbs / 100 lbs = 3 (or ratio of $$\frac{1}{3}$$ effort). For an inclined plane, MA = Length / Height. Therefore, Length = Height × MA $$= 5\text{ feet} \times 3 = 15$$ feet.

Question 2 easy

A heavy steel beam weighing 8,500 newtons is raised using a block and tackle system. If an input force of only 425 newtons is required to pull the rope, what is the mechanical advantage of the system?

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Correct answer: A — 20

Mechanical Advantage (MA) = Output Force (Weight) / Input Force (Effort) = $$\frac{8{,}500\text{ N}}{425\text{ N}} = 20$$

Question 3 easy

Three interconnected water tanks (Tank A, Tank B, and Tank C) receive water from left to right. Water enters Tank A at 10 gallons/minute, flows into Tank B at 12 gallons/minute, flows into Tank C at 15 gallons/minute, and exits Tank C at 8 gallons/minute. Which tank is at risk of overflowing?

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Correct answer: C — Tank C

Compare the inflow and outflow rates for each tank:

Tank A: Inflow = 10 gal/min, Outflow = 12 gal/min (Draining faster than filling)
Tank B: Inflow = 12 gal/min, Outflow = 15 gal/min (Draining faster than filling)
Tank C: Inflow = 15 gal/min, Outflow = 8 gal/min (Inflow exceeds outflow by 7 gal/min)

Therefore, Tank C will accumulate water and is at risk of overflowing.

Question 4 easy

Wheel X has a diameter of 6 inches and Wheel Y has a diameter of 18 inches. If both wheels are attached to the same rotating axle and turn at the exact same rotational speed (RPM), how does the linear outer edge speed of Wheel Y compare to Wheel X?

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Correct answer: C — Three times faster than Wheel X

When wheels share the same axle, they complete the same number of revolutions per minute (RPM). Since linear speed $$v = C \times \text{RPM} = \pi \times d \times \text{RPM}$$, linear speed is directly proportional to diameter. Wheel Y's diameter (18 inches) is 3 times larger than Wheel X's diameter (6 inches), so its outer edge travels 3 times faster.

Question 5 medium

An industrial machine's heavy duty suspension spring compresses 4 inches when subjected to a load of 1,200 pounds. What is the spring constant (k) in pounds per inch?

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Correct answer: A — 300 lbs/in

According to Hooke's Law: $$F = k \times x$$, where F is force (1,200 lbs) and x is displacement/compression (4 inches). Solving for the spring constant k:

$$k = \frac{F}{x} = \frac{1{,}200\text{ lbs}}{4\text{ inches}} = 300\text{ lbs/in}$$.

Question 6 medium

Refer to the setup: A 900-pound heavy box rests on a uniform 20-pound wooden plank balanced between two scales (Scale Left and Scale Right). If the box is placed three-quarters $$(\frac{3}{4})$$ closer to the left scale than the right scale (meaning it is $$\frac{1}{4}$$ of the plank length from the left end), what will the left scale register?

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Correct answer: B — 685 lbs

1. Calculate the weight carried by the left scale from the 900 lb box:

Since the load is at $$\frac{1}{4}$$ distance from the left support, it exerts $$\frac{3}{4} (75\%)$$ of its weight on the left scale: $$900\text{ lbs} \times 0.75 = 675$$ lbs.

2. Calculate the weight carried by the left scale from the uniform plank:

Since the 20 lb plank is uniform, its weight is split equally between the two scales: $$20\text{ lbs} / 2 = 10$$ lbs.

3. Total weight on Left Scale = 675 lbs + 10 lbs = 685 lbs.

Question 7 medium

A bicycle features a 52-tooth front chainring gear connected to a 13-tooth rear sprocket gear. If the cyclist pedals at a steady cadence of 90 revolutions per minute (rpm), what is the rotational speed of the rear wheel?

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Correct answer: A — 360 rpm

Gear Ratio = Front Gear Teeth / Rear Gear Teeth = $$\frac{52}{13} = 4$$. The rear sprocket turns 4 times faster than the front pedals. Rear Wheel Speed = Pedal Cadence × Gear Ratio $$= 90\text{ rpm} \times 4 = 360$$ rpm.

Question 8 medium

Ignoring friction and rope mass, what mechanical advantage is provided by a single fixed pulley attached to an overhead beam?

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Correct answer: C — 1

A single fixed pulley changes only the direction of the applied pulling force, not its magnitude. Therefore, the effort required equals the weight lifted, yielding a mechanical advantage (MA) of exactly 1.

Question 9 medium

If five spur gears are linked in a straight sequence and Gear 1 rotates counterclockwise, in which direction will Gear 5 rotate?

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Correct answer: B — Counterclockwise

Adjacent meshed gears rotate in opposite directions. For an odd number of meshed gears in a linear sequence (G1, G2, G3, G4, G5), the odd-numbered gears (G1, G3, G5) all turn in the same direction. Since Gear 1 turns counterclockwise, Gear 5 also turns counterclockwise.

Question 10 hard

Three objects weighing 40 pounds, 50 pounds, and 70 pounds are placed on the left side of a balanced seesaw at distances of 2 feet, 4 feet, and 5 feet from the central fulcrum, respectively. An adult weighing 180 pounds sits on the right side. How far from the fulcrum must the adult sit to keep the seesaw horizontally balanced?

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Correct answer: A — 3.5 feet

For rotational balance, Total Left Torque = Total Right Torque.

Left Torque = (40 lbs × 2 ft) + (50 lbs × 4 ft) + (70 lbs × 5 ft) = $$80 + 200 + 350 = 630$$ lb·ft.

Right Torque = 180 lbs × Distance (d).

$$180 \times d = 630 \Rightarrow d = \frac{630}{180} = 3.5$$ feet.

Question 11 hard

Which primary mechanical function does a fluid pump serve in hydraulic and mechanical piping systems?

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Correct answer: C — To transfer liquids against gravity or pressure differentials by adding mechanical energy

A mechanical pump converts mechanical energy into hydraulic energy, imparting force to liquids to transport them upward (uphill), across distance, or against higher system resistance/pressure.

Question 12 hard

A single fixed pulley is mounted to a ceiling joist. Assuming friction in the axle is negligible, how much downward force must a worker exert on the rope to lift a 75-pound cargo box vertically off the floor at a constant speed?

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Correct answer: B — 75 pounds

A single fixed pulley has a mechanical advantage of 1 (MA = 1). It does not magnify input force; it only changes the direction of force. Thus, lifting a 75-pound object requires an input pulling force equal to its weight: 75 pounds.

Question 13 hard

In an ideal mechanical system without friction, how does the total work output performed using a simple machine compare to the work done directly without a machine?

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Correct answer: C — The work required remains exactly the same

According to the Law of Conservation of Energy, simple machines cannot reduce the total work required to accomplish a task (Work = Force × Distance). While a machine can reduce the force needed, it proportionally increases the distance over which that force must be applied, keeping total work constant ($$\text{Work}_{\text{in}} = \text{Work}_{\text{out}}$$).