ASVAB Mechanical Comprehension, how machines behave

Two sheets of paper hang a few centimeters apart. When a person blows a stream of air between them, the sheets

1. move apart from each other
   This is what most people expect, but the moving air lowers the pressure between the sheets.
2. move toward each other
   Fast air between the sheets has lower pressure than the still air outside, which pushes them together.
3. stay exactly where they are
   The pressure difference does move them.
4. both swing in the direction of the air
   The airflow runs along the sheets, between them, not against their faces.

ID 716125 · Report a problem

A solid metal block hangs from a string and is lowered into a deep tank of water. After it is completely submerged, it is lowered farther. As it goes deeper, the buoyant force on it ____.

1. increases until it equals the block's weight
   A sinking metal block always displaces less weight of water than it weighs; that is why it sinks.
2. increases
   Pressure grows with depth on all sides, but the difference between top and bottom stays the same, so the buoyant force does not increase.
3. stays the same
   Buoyant force equals the weight of water displaced, and the block's volume does not change with depth, so it stays the same.
4. decreases
   Nothing about a solid block's displaced volume shrinks as it goes deeper, so the buoyant force does not decrease.

ID 813030 · Report a problem

A bicycle wheel is spun rapidly and held by the ends of its axle. Compared with the same wheel when it is not spinning, the spinning wheel is ____ to tilt to a new angle.

1. harder
   A spinning wheel has angular momentum, which resists any change in the direction of its axle.
2. impossible
   It can still be tilted with enough torque; it is harder, not impossible.
3. easier
   Spin adds resistance to tilting; it does not remove it.
4. just as easy
   A wheel at rest tilts easily; spinning changes that.

ID 813263 · Report a problem

A 150-lb person stands flat-footed on a floor and then balances on the toes of one foot. Compared with standing flat-footed, the person now exerts on the floor:

1. more force and more pressure
   The force is still the person's 150-lb weight; standing on toes does not add weight.
2. the same pressure but less force
   Pressure changes when the area changes, and the weight stays 150 lb.
3. less force and less pressure
   Both are backward: weight is unchanged and the area is smaller.
4. the same force but more pressure
   Body weight (the force) is unchanged, but it now acts on a much smaller area, so the pressure rises.

ID 813439 · Report a problem

A wooden crate is slid across a floor on its large flat side and then on its narrow end, which has one-third the contact area. Compared with the flat side, the sliding friction on the narrow end is:

1. about one-third as great
   The smaller area carries the same weight at higher pressure, so friction does not shrink.
2. about the same
   Friction depends on the normal force (the crate's weight) and the surfaces, not on the contact area.
3. reduced to nearly zero
   Changing the side does not remove friction.
4. about three times as great
   Higher pressure on a smaller area evens out; friction does not triple.

ID 813446 · Report a problem

A sailor raises a load 3 ft using a block and tackle with 4 rope segments supporting the load. How much rope must the sailor pull?

1. 0.75 ft
   0.75 ft divides by 4, the right number for the force, not the distance.
2. 3 ft
   3 ft would mean no mechanical advantage, a single fixed pulley.
3. 7 ft
   7 ft adds the segments to the height (4 + 3) instead of multiplying.
4. 12 ft
   With 4 supporting segments the MA is 4, so the rope moves 4 × 3 ft = 12 ft.

ID 813581 · Report a problem

Four splitting wedges are driven into the same log. Ignoring friction, which one needs the least force to drive in?

1. 6 in long, 2 in thick
   6 ÷ 2 = 3, the smallest advantage of the four.
2. 8 in long, 1 in thick
   A wedge's mechanical advantage is length ÷ thickness: 8 ÷ 1 = 8, the largest here.
3. 8 in long, 2 in thick
   8 ÷ 2 = 4; being long is not enough if the wedge is also thick.
4. 4 in long, 1 in thick
   4 ÷ 1 = 4; thin, but too short.

ID 813583 · Report a problem

An aircraft on its takeoff roll accelerates uniformly from rest and reaches 120 knots in 30 seconds. What is its speed 10 seconds after brake release?

1. 4 knots
   4 is the gain per second, not the speed after 10 seconds.
2. 12 knots
   12 knots divides 120 by 10 instead of using the 30-second rate.
3. 40 knots
   Speed grows evenly: 120 knots ÷ 30 s = 4 knots per second, so 10 s gives 40 knots.
4. 60 knots
   60 knots is the speed at the halfway time, 15 s, not at 10 s.

ID 813586 · Report a problem

A net force gives a cart an acceleration of 6 ft/s². If the net force is doubled and the cart's mass is tripled, what is the new acceleration?

1. 4 ft/s²
   a = F/m: doubling F and tripling m gives 6 × 2 ÷ 3 = 4 ft/s².
2. 6 ft/s²
   6 ft/s² assumes the two changes cancel; ×2 and ÷3 do not cancel.
3. 9 ft/s²
   9 ft/s² multiplies by 3 and divides by 2, swapping the roles of force and mass.
4. 36 ft/s²
   36 ft/s² multiplies by both 2 and 3.

ID 813609 · Report a problem

A 10-lb weight hangs from a spring scale inside an elevator. While the elevator is speeding up on its way upward, the scale reads:

1. more than 10 lb
   Accelerating upward, the scale must supply the weight plus the extra force for the acceleration.
2. less than 10 lb
   A lower reading occurs when the elevator accelerates downward.
3. zero
   Zero is free fall, when the cable is cut.
4. exactly 10 lb
   The scale reads exactly 10 lb only at constant speed or at rest.

ID 813816 · Report a problem

A machine lifts a 400-lb load 2 ft while the operator applies 50 lb over a distance of 20 ft. What is the machine's efficiency?

1. 10%
   10% divides the distances (2 ÷ 20) and ignores the forces.
2. 12.5%
   12.5% divides the forces (50 ÷ 400) and ignores the distances.
3. 80%
   Work out 400 × 2 = 800 ft-lb, work in 50 × 20 = 1,000 ft-lb, and 800 ÷ 1,000 = 80%.
4. 125%
   125% divides work in by work out; no machine gives more than 100%.

ID 813820 · Report a problem

A uniform 12-ft plank weighing 60 lb lies on a flat roof with 4 ft sticking out past the edge. How far past the edge can a 40-lb child walk before the plank starts to tip?

1. 1⅓ ft
   1⅓ ft swaps the weights: 40 × 2 ÷ 60.
2. 2 ft
   2 ft is where the plank's own weight acts, measured inside the edge.
3. 3 ft
   The plank's weight acts at its middle, 2 ft inside the edge: 60 × 2 = 40 × x gives x = 3 ft.
4. 4 ft
   4 ft is the full overhang; the child reaches the tipping point before the end.

ID 813837 · Report a problem

A claw hammer pulls a nail by pivoting on its head. The hand pulls on the handle 12 inches from the pivot, and the nail sits in the claw 2 inches from the pivot. If the hand pulls with 30 lb, how much force acts on the nail?

1. 5 lb
   5 lb divides by the lever ratio instead of multiplying.
2. 30 lb
   30 lb ignores the lever's advantage.
3. 60 lb
   60 lb multiplies the pull by the 2-inch claw distance instead of by the arm ratio 12 ÷ 2.
4. 180 lb
   MA = 12 ÷ 2 = 6, so the nail gets 30 × 6 = 180 lb.

ID 857801 · Report a problem

In a sealed hydraulic system, pushing on the small piston raises the fluid pressure

1. equally at every point inside
   A confined fluid passes the added pressure undiminished, equally to every point inside the system.
2. only near the small piston
   The fluid passes pressure throughout; it does not stay at the source.
3. only at the bottom of the system
   Depth adds a little pressure, but the applied pressure reaches every point.
4. most at the large piston
   The large piston gets a larger force, not a larger pressure.

ID 716133 · Report a problem

A 10-lb weight is dropped from a shelf 8 feet above the floor. Ignoring air resistance, how much kinetic energy does it have just before it hits the floor?

1. 40 ft-lb
   40 ft-lb takes half of weight × height, borrowing the ½ from ½mv² where it does not belong.
2. 80 ft-lb
   All the potential energy, 10 lb × 8 ft = 80 ft-lb, has become kinetic energy just before impact.
3. 160 ft-lb
   160 ft-lb doubles weight × height, turning the ½ of ½mv² into a 2.
4. 640 ft-lb
   640 ft-lb multiplies by the height twice (10 × 8 × 8), as if squaring the height like a speed.

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