36 Mechanical Comprehension (MC) Practice Questions & Answers

1. 1. A 12-ft board is balanced on a fulcrum at its center. An 80-lb child sits 3 ft from the fulcrum on one side. How far from the fulcrum must a 60-lb child sit on the other side to balance the board?

   • A.4.00 ft✓ Answer
   • B.3.00 ft
   • C.2.25 ft
   • D.2.00 ft

   Balance requires equal moments about the fulcrum: 80 lb x 3 ft = 240 ft-lb, so the 60-lb child needs 240 / 60 = 4 ft. Choosing 3 ft assumes equal distances balance unequal weights; 2.25 ft inverts the ratio (60 x 3 / 80); 2 ft comes from measuring the 4 ft from the end of the board (6 - 4) instead of from the fulcrum.

   Ref QB-NSJYBCSource: Official ASVAB Mechanical Comprehension subtest (officialasvab.com): law of the lever, moments about the fulcrum (force x distance) (clockwise moments = counterclockwise moments)Report a problem with this question

2. 2. In a wheelbarrow, the wheel's axle is the pivot, the load sits in the bed, and the user lifts at the ends of the handles. Which class of lever is a wheelbarrow?

   • A.First class, because the pivot sits between the load and the effort
   • B.First class, because the effort and the load move in opposite directions
   • C.Third class, because the effort sits between the pivot and the load
   • D.Second class, because the load sits between the pivot and the effort✓ Answer

   Lever class is set by what sits in the middle. In a wheelbarrow the axle (pivot) is at one end, the effort at the other, and the load between them, which makes it second class; the effort arm is longer than the load arm, so it multiplies force. First class needs the pivot in the middle, and in a wheelbarrow the effort and load both move upward, not in opposite directions.

   Ref QB-7A4G4ESource: Official ASVAB Mechanical Comprehension subtest (officialasvab.com): law of the lever, moments about the fulcrum (force x distance) (classes of levers: second class has the load between fulcrum and effort)Report a problem with this question

3. 3. A person sweeps with a broom, holding the top of the handle still with one hand as the pivot and pushing with the other hand partway down the handle; the bristles carry the load. What does this arrangement give the user?

   • A.More force at the bristles in exchange for a slower, shorter hand movement
   • B.No change in force or speed, because the pivot sits at the end of the handle
   • C.More force and more speed at the bristles, because two hands share the work
   • D.More speed and sweep at the bristles in exchange for a larger hand force✓ Answer

   With the effort between the pivot and the load, the broom is a third-class lever: the effort arm is shorter than the load arm, so its mechanical advantage is below 1. The hand must push harder than the force delivered at the bristles, but the bristles move farther and faster than the hand. No machine gives more force and more speed at the same time, because output work cannot exceed input work.

   Ref QB-PSWAHDSource: Official ASVAB Mechanical Comprehension subtest (officialasvab.com): law of the lever, moments about the fulcrum (force x distance) (third-class lever: effort between fulcrum and load, mechanical advantage less than 1)Report a problem with this question

4. 4. A 6-ft crowbar rests on a fulcrum 1 ft from the end that is under a 500-lb rock. A worker pushes down on the far end of the bar. Ignoring the bar's weight, how much force must the worker apply to lift the rock?

   • A.500 lb
   • B.2,500 lb
   • C.100 lb✓ Answer
   • D.83 lb

   Arms are measured from the fulcrum: the effort arm is 6 - 1 = 5 ft and the load arm is 1 ft, so the mechanical advantage is 5 and the force is 500 / 5 = 100 lb. 83 lb comes from using the whole 6-ft bar as the effort arm; 2,500 lb multiplies by the advantage instead of dividing; 500 lb ignores the lever's advantage altogether.

   Ref QB-8AFT9FSource: Official ASVAB Mechanical Comprehension subtest (officialasvab.com): law of the lever, moments about the fulcrum (force x distance) (ideal mechanical advantage = effort arm / resistance arm)Report a problem with this question

5. 5. A bolt must be tightened to 90 ft-lb of torque. The mechanic pushes on the wrench at right angles to the handle, at a point 18 inches from the center of the bolt. How much force is needed?

   • A.90 lb
   • B.135 lb
   • C.60 lb✓ Answer
   • D.5 lb

   Torque = force x distance from the pivot, with the distance in feet to match ft-lb: 18 in = 1.5 ft, so force = 90 / 1.5 = 60 lb. 5 lb divides by 18 without converting inches to feet; 135 lb multiplies torque by distance instead of dividing; 90 lb treats the torque number as if it were the force.

   Ref QB-QS9NPESource: Official ASVAB Mechanical Comprehension subtest (officialasvab.com): torque = force x perpendicular distance from the axis of rotationReport a problem with this question

6. 6. A rope runs over a single pulley fastened to a ceiling beam. One end is tied to a 100-lb crate and a worker pulls down on the other end. Ignoring friction, what does the pulley do for the worker?

   • A.It halves the rope travel, so the worker pulls 100 lb half as far
   • B.It halves the force, so the worker pulls 50 lb but twice as far
   • C.It doubles the force, because both rope ends pass over the wheel
   • D.It changes the direction of pull, but the worker still pulls 100 lb✓ Answer

   A single fixed pulley does not move with the load, so only one rope segment supports the crate and the mechanical advantage is 1. It lets the worker pull down instead of lifting up, but the force (100 lb) and the rope travel (equal to the lift) are unchanged. Halving the force needs a movable pulley, where two segments share the load.

   Ref QB-9AT5FGSource: Official ASVAB Mechanical Comprehension subtest (officialasvab.com): pulleys and block and tackle, mechanical advantage = number of rope segments supporting the movable block (a single fixed pulley has a mechanical advantage of 1)Report a problem with this question

7. 7. A block and tackle lifts a 300-lb engine. Three rope segments hold up the movable lower block, and the free end of the rope comes down from the fixed upper block to the worker's hands. Ignoring friction and the weight of the blocks, how hard must the worker pull?

   • A.150 lb
   • B.75 lb
   • C.100 lb✓ Answer
   • D.300 lb

   The mechanical advantage equals the number of rope segments supporting the movable block, here 3, so the pull is 300 / 3 = 100 lb. 75 lb counts the downward free end as a fourth supporting segment; 150 lb treats the rig like a single movable pulley with an advantage of 2; 300 lb assumes pulleys only change direction.

   Ref QB-RSMYUFSource: Official ASVAB Mechanical Comprehension subtest (officialasvab.com): pulleys and block and tackle, mechanical advantage = number of rope segments supporting the movable blockReport a problem with this question

8. 8. A worker uses a block and tackle with a mechanical advantage of 4 to raise a load 3 ft. Ignoring friction, how much rope must the worker pull through the blocks?

   • A.7.00 ft
   • B.3.00 ft
   • C.0.75 ft
   • D.12.00 ft✓ Answer

   A machine trades force for distance: with an advantage of 4, each foot of lift needs 4 ft of pull, so 3 x 4 = 12 ft. 3 ft forgets the trade-off and assumes the rope moves as far as the load; 7 ft adds the lift and the advantage; 0.75 ft divides by the advantage, which would mean getting more work out than was put in.

   Ref QB-AA6GMHSource: Official ASVAB Mechanical Comprehension subtest (officialasvab.com): pulleys and block and tackle, mechanical advantage = number of rope segments supporting the movable block (distance pulled = load rise x mechanical advantage)Report a problem with this question

9. 9. A windlass has a crank handle that turns in a circle of radius 15 in. The rope winds onto a drum that is 6 in in diameter. Ignoring friction, what force on the handle will lift a 400-lb load?

   • A.2,000 lb
   • B.400 lb
   • C.160 lb
   • D.80 lb✓ Answer

   Wheel-and-axle advantage is handle radius / drum radius. The drum's radius is half its 6-in diameter, 3 in, so the advantage is 15 / 3 = 5 and the force is 400 / 5 = 80 lb. 160 lb uses the drum's diameter as if it were its radius (advantage 2.5); 2,000 lb multiplies by the advantage; 400 lb ignores the advantage.

   Ref QB-SSYA2HSource: Official ASVAB Mechanical Comprehension subtest (officialasvab.com): wheel and axle, mechanical advantage = wheel (crank) radius / axle (drum) radiusReport a problem with this question

10. 10. Three gears sit in a row: gear A meshes with gear B, and gear B meshes with gear C. No two gears share a shaft. If gear A turns clockwise, which way does gear C turn?

    • A.Clockwise, because each mesh reverses direction and there are two meshes✓ Answer
    • B.Clockwise if gear B is the same size as gear A, counterclockwise otherwise
    • C.Counterclockwise, because gear B reverses A and passes that reversal to C
    • D.Counterclockwise, because the last gear always turns opposite to the first

    Every mesh between two external gears reverses the direction: A clockwise turns B counterclockwise, and B counterclockwise turns C clockwise. Gear B acts as an idler, so A and C turn the same way. Gear size changes speed, not direction, and the 'last gear is always opposite' rule fails whenever the number of meshes is even.

    Ref QB-YNEFV5Source: Official ASVAB Mechanical Comprehension subtest (officialasvab.com): gears, meshing gears turn in opposite directions; speed ratio rpm1 x teeth1 = rpm2 x teeth2 (an idler gear makes the driver and the output turn the same way)Report a problem with this question

11. 11. A 20-tooth gear turning at 300 rpm meshes with and drives a 60-tooth gear. How fast does the 60-tooth gear turn?

    • A.100 rpm✓ Answer
    • B.150 rpm
    • C.300 rpm
    • D.900 rpm

    Teeth pass the mesh point at the same rate on both gears, so rpm x teeth is the same: 300 x 20 = 6,000 = rpm x 60, giving 100 rpm. The larger gear turns slower (and with more torque). 900 rpm inverts the ratio; 300 rpm assumes meshing gears share a speed; 150 rpm takes the 40-tooth difference over 20 as the ratio.

    Ref QB-G6MMF6Source: Official ASVAB Mechanical Comprehension subtest (officialasvab.com): gears, meshing gears turn in opposite directions; speed ratio rpm1 x teeth1 = rpm2 x teeth2Report a problem with this question

12. 12. Gear A (12 teeth) turns at 600 rpm and meshes with gear B (36 teeth). Gear C (10 teeth) is fixed to the same shaft as gear B, and gear C meshes with gear D (40 teeth). How fast does gear D turn?

    • A.800 rpm
    • B.50 rpm✓ Answer
    • C.200 rpm
    • D.180 rpm

    Work one stage at a time. A to B: 600 x 12 / 36 = 200 rpm. C shares B's shaft, so it also turns at 200 rpm. C to D: 200 x 10 / 40 = 50 rpm. 180 rpm skips the shared shaft and pairs A directly with D; 200 rpm stops at gear C; 800 rpm inverts the second stage.

    Ref QB-ZNSS27Source: Official ASVAB Mechanical Comprehension subtest (officialasvab.com): gears, meshing gears turn in opposite directions; speed ratio rpm1 x teeth1 = rpm2 x teeth2 (gears on the same shaft turn at the same rpm; multiply stage ratios)Report a problem with this question

13. 13. A hoist uses a single-thread worm that drives a 50-tooth worm wheel. Which statement about this drive is correct?

    • A.The wheel advances one tooth per worm turn, so it turns 50 times slower✓ Answer
    • B.The wheel and the worm turn at one rate; only the axis is changed
    • C.The wheel advances 50 teeth per worm turn, so it turns 50 times faster
    • D.The wheel advances two teeth per worm turn, so it turns 25 times slower

    A single-thread worm pushes the wheel forward by one tooth for each full turn, so a 50-tooth wheel needs 50 worm turns per revolution: a 50:1 reduction with a large gain in torque, which is why worm drives suit hoists. The worm also turns the drive through a right angle, but that does not keep the speeds equal, and only a two-thread worm would move two teeth per turn.

    Ref QB-H6YXM7Source: Official ASVAB Mechanical Comprehension subtest (officialasvab.com): gears, meshing gears turn in opposite directions; speed ratio rpm1 x teeth1 = rpm2 x teeth2 (worm gear: one tooth per worm revolution for a single-thread worm)Report a problem with this question

14. 14. Pulley A turns clockwise and drives pulley B with a crossed belt. Pulley B is fixed to the same shaft as pulley C, and pulley C drives pulley D with an open (uncrossed) belt. Which way does pulley D turn?

    • A.It turns clockwise; neither kind of belt changes the direction of rotation
    • B.It turns clockwise; the shared shaft reverses the rotation a second time
    • C.It turns clockwise; the open belt reverses the rotation a second time
    • D.It turns counterclockwise; only the crossed belt reverses the rotation✓ Answer

    A crossed belt reverses rotation, so B turns counterclockwise. Pulleys on one shaft turn together, so C is also counterclockwise. An open belt keeps the same direction, so D turns counterclockwise. Only the crossed belt produced a reversal; the shared shaft and the open belt pass the direction along unchanged.

    Ref QB-2P5588Source: Official ASVAB Mechanical Comprehension subtest (officialasvab.com): belt drives, an open belt keeps the direction of rotation and a crossed belt reverses itReport a problem with this question

15. 15. A motor pulley 4 in in diameter turns at 1,800 rpm and drives a pulley 12 in in diameter through an open belt. Ignoring slip, how fast does the larger pulley turn?

    • A.1,800 rpm
    • B.600 rpm✓ Answer
    • C.5,400 rpm
    • D.200 rpm

    The belt moves at one speed, so rpm x diameter is constant: 1,800 x 4 = rpm x 12, giving 600 rpm; the larger pulley turns slower. 5,400 rpm inverts the ratio; 1,800 rpm assumes a belt keeps both pulleys at the same rpm; 200 rpm uses the square of the diameter ratio (9), as if pulley speed depended on area.

    Ref QB-J6BAT8Source: Official ASVAB Mechanical Comprehension subtest (officialasvab.com): belt and pulley drives, rpm x diameter is the same for both pulleysReport a problem with this question

16. 16. A grinding wheel turns at a steady speed. Point P is on the rim, and point Q is halfway between the center and the rim. Which statement is correct?

    • A.Q makes more turns per minute, because its circular path is shorter
    • B.P and Q make the same turns per minute, but P travels faster✓ Answer
    • C.P and Q make the same turns per minute, but Q travels faster
    • D.P and Q travel at the same speed, because the wheel is one solid piece

    Every point on a rigid wheel completes a turn in the same time, so the rpm is shared. In that time P travels a circle twice the radius of Q's, so P covers twice the distance and has twice the linear speed. Linear speed grows with distance from the center.

    Ref QB-3PGFD9Source: Official ASVAB Mechanical Comprehension subtest (officialasvab.com): rotating parts, linear speed = rotational speed x radiusReport a problem with this question

17. 17. A 300-lb crate is pushed up a smooth ramp that is 10 ft long and rises 6 ft; the ramp covers 8 ft of floor. Ignoring friction, what force parallel to the ramp is needed?

    • A.300 lb
    • B.180 lb✓ Answer
    • C.225 lb
    • D.500 lb

    Work along the ramp equals work lifting straight up: F x 10 ft = 300 lb x 6 ft, so F = 180 lb. The advantage is slope length / height, not floor length / height. 225 lb uses the 8-ft floor distance instead of the 10-ft slope; 500 lb inverts the ratio; 300 lb assumes a ramp gives no advantage.

    Ref QB-K6PMY9Source: Official ASVAB Mechanical Comprehension subtest (officialasvab.com): inclined plane, ideal mechanical advantage = length of slope / heightReport a problem with this question

18. 18. Two frictionless ramps lead up to the same loading dock, 4 ft high. Ramp X is 8 ft long and ramp Y is 16 ft long. A worker pushes the same cart up each ramp. How does the work compare?

    • A.Same force on both, because the cart ends up at the same height
    • B.Same work on both; Y needs half the force but over twice the distance✓ Answer
    • C.Less work on Y, because Y needs only half as much pushing force
    • D.More work on Y, because the cart is pushed twice as far along it

    Without friction, the work on a ramp equals the weight times the height gained, which is the same for both ramps. The longer ramp Y halves the force (advantage 16/4 = 4 versus 8/4 = 2) but doubles the distance, so force x distance is unchanged. A machine can reduce force but never the total work.

    Ref QB-4PUSJASource: Official ASVAB Mechanical Comprehension subtest (officialasvab.com): inclined plane, ideal mechanical advantage = length of slope / height (work input equals work output in an ideal machine)Report a problem with this question

19. 19. A screw has 4 threads per inch, so its pitch is 0.25 in. How far does the screw advance into the wood in 16 full turns?

    • A.16.00 in
    • B.64.00 in
    • C.0.25 in
    • D.4.00 in✓ Answer

    Each full turn advances a screw by one pitch, so 16 x 0.25 in = 4 in. 64 in multiplies the turns by the threads per inch instead of by the pitch; 16 in assumes one inch per turn; 0.25 in takes a single pitch as the total travel.

    Ref QB-M62Y5BSource: Official ASVAB Mechanical Comprehension subtest (officialasvab.com): screw as an inclined plane, one turn advances the screw one pitch (pitch = 1 / threads per inch)Report a problem with this question

20. 20. A worker pushes a 150-lb crate 20 ft across a level floor with a steady horizontal force of 50 lb. How much work does the worker do on the crate?

    • A.1,000 ft-lb✓ Answer
    • B.70 ft-lb
    • C.4,000 ft-lb
    • D.3,000 ft-lb

    Work is the force applied times the distance moved in the direction of that force: 50 lb x 20 ft = 1,000 ft-lb. The crate's weight acts straight down and is not the pushing force, so 3,000 ft-lb (weight x distance) and 4,000 ft-lb (weight plus push, times distance) are wrong; 70 ft-lb adds force and distance instead of multiplying.

    Ref QB-BX5BFWSource: Official ASVAB Mechanical Comprehension subtest (officialasvab.com): work, energy and power, work = force x distance in the direction of the forceReport a problem with this question

21. 21. A hoist lifts a 200-lb load 15 ft straight up in 6 seconds. What power does the hoist deliver?

    • A.18,000 ft-lb/s
    • B.1,200 ft-lb/s
    • C.500 ft-lb/s✓ Answer
    • D.3,000 ft-lb/s

    Power is work divided by time. The work is 200 lb x 15 ft = 3,000 ft-lb, and 3,000 / 6 s = 500 ft-lb/s. 3,000 ft-lb/s forgets to divide by time; 18,000 ft-lb/s multiplies by time; 1,200 ft-lb/s multiplies the weight by the time instead of by the distance.

    Ref QB-TEY5VVSource: Official ASVAB Mechanical Comprehension subtest (officialasvab.com): power = work / timeReport a problem with this question

22. 22. A jack takes in 800 ft-lb of work and does 600 ft-lb of useful work raising a car. What is the jack's efficiency?

    • A.25%
    • B.100%
    • C.133%
    • D.75%✓ Answer

    Efficiency is useful output work divided by input work: 600 / 800 = 0.75, or 75%. The missing 200 ft-lb is lost mostly to friction as heat. 133% divides input by output, which would mean the machine creates energy; 25% is the fraction lost, not the efficiency; 100% describes only an ideal, frictionless machine.

    Ref QB-CXGNMXSource: Official ASVAB Mechanical Comprehension subtest (officialasvab.com): efficiency = output work / input workReport a problem with this question

23. 23. A cart's speed doubles from 10 ft/s to 20 ft/s while its mass stays the same. How does its kinetic energy change?

    • A.It stays exactly the same
    • B.It becomes 8 times as large
    • C.It becomes 4 times as large✓ Answer
    • D.It becomes 2 times as large

    Kinetic energy is 1/2 x mass x speed squared, so it grows with the square of speed: doubling speed multiplies it by 2 x 2 = 4. Choosing 2 times treats KE as proportional to speed; 8 times cubes the speed; staying the same confuses kinetic energy with mass. This is why stopping distance rises so sharply with speed.

    Ref QB-UEBG2XSource: Official ASVAB Mechanical Comprehension subtest (officialasvab.com): kinetic energy = 1/2 m v^2Report a problem with this question

24. 24. A pendulum is pulled to one side and released from rest. Ignoring air resistance, at what point in its swing is the bob moving fastest?

    • A.Halfway down, where the falling and swinging motions are balanced
    • B.At the far end, where the bob has gathered speed over the whole swing
    • C.At the lowest point, where the most potential energy has become kinetic✓ Answer
    • D.At the release point, where the bob has the most stored energy to use

    The bob's total energy stays constant. At the release point and at the far end it is momentarily at rest with maximum potential energy; as it drops, potential energy turns into kinetic energy, which peaks where the bob is lowest. Past the bottom it climbs and slows again, so the far end is a stopping point, not the fastest point.

    Ref QB-DXUYSYSource: Official ASVAB Mechanical Comprehension subtest (officialasvab.com): conservation of mechanical energy, potential energy (mgh) converts to kinetic energyReport a problem with this question

25. 25. A ball on a string is whirled in a flat, horizontal circle. Seen from above, which way does the ball move at the instant the string breaks?

    • A.Straight outward, along the line from the center through the ball
    • B.Straight inward toward the center, where the string was pulling
    • C.Along a straight line tangent to the circle at the point of release✓ Answer
    • D.Along a curved path that keeps following the circle for a short time

    The string supplies the centripetal force that pulls the ball toward the center and keeps bending its path. At every instant the ball's velocity points along the tangent, so when that force disappears the ball continues in a straight line along the tangent. Nothing pushes it outward, and without the string nothing keeps it curving.

    Ref QB-VEPT7YSource: Official ASVAB Mechanical Comprehension subtest (officialasvab.com): circular motion, centripetal force acts toward the center; a released object moves along the tangentReport a problem with this question

26. 26. A worker slides a crate up a ramp by pushing on it parallel to the ramp's surface. In which direction does the friction force on the crate act?

    • A.Up the slope, parallel to the ramp, in the direction of the push
    • B.Down the slope, parallel to the ramp, opposite the crate's motion✓ Answer
    • C.Straight out of the ramp surface, at right angles to the slope
    • D.Straight down toward the ground, in the same direction as gravity

    Sliding friction acts along the contact surface, opposite the direction the object slides. The crate moves up the slope, so friction points down the slope, parallel to it. Gravity pulls straight down and the ramp's normal force points out of the surface, but neither of those is friction.

    Ref QB-EX7BYZSource: Official ASVAB Mechanical Comprehension subtest (officialasvab.com): friction acts parallel to the contact surface, opposite the direction of motionReport a problem with this question

27. 27. Al and Ben carry a 240-lb load hung from a 12-ft pole, Al at one end and Ben at the other. The load hangs 3 ft from Al's end. Ignoring the pole's weight, how much of the load does Al support?

    • A.80 lb
    • B.180 lb✓ Answer
    • C.60 lb
    • D.120 lb

    Take moments about Ben's end: the load is 12 - 3 = 9 ft from Ben, so Al's force x 12 ft = 240 lb x 9 ft, giving 180 lb; Ben carries the other 60 lb. The person closer to the load carries more. 60 lb swaps the two shares; 120 lb splits the load evenly as if it were centered; 80 lb divides the load by the 3-ft distance.

    Ref QB-WE26DZSource: Official ASVAB Mechanical Comprehension subtest (officialasvab.com): structural support and load distribution, taking moments about a supportReport a problem with this question

28. 28. A tall storage cabinet is to be filled with heavy tools. Which way of loading it makes the cabinet hardest to tip over?

    • A.Heaviest tools on the middle shelves, to balance the top and the bottom
    • B.Heaviest tools on the bottom shelves, to lower the center of gravity✓ Answer
    • C.Heaviest tools on the top shelves, to press the cabinet onto the floor
    • D.Weight spread evenly on every shelf, to keep the center of gravity high

    An object tips when its center of gravity moves past the edge of its base. A lower center of gravity must be raised and shifted farther before it passes the edge, so loading the heavy items low makes the cabinet more stable. Heavy items up high raise the center of gravity and make tipping easier, no matter how hard they press down.

    Ref QB-7XQYRRSource: Official ASVAB Mechanical Comprehension subtest (officialasvab.com): center of gravity and stability, a lower center of gravity and wider base resist tippingReport a problem with this question

29. 29. A spring stretches 2 in when a 10-lb weight hangs from it. Two of these springs are hooked end to end (in series), and the same 10-lb weight hangs from the bottom one. How far does the pair stretch in total?

    • A.4 in✓ Answer
    • B.5 in
    • C.2 in
    • D.1 in

    In series, each spring carries the full 10 lb, so each stretches 2 in and the total is 4 in; springs in series are softer. 1 in is the answer for two springs side by side (in parallel), where each carries only 5 lb; 2 in assumes the pair stretches like one spring; 5 in reports the spring constant (10 lb / 2 in = 5 lb per inch) as a distance.

    Ref QB-PEJT6RSource: Official ASVAB Mechanical Comprehension subtest (officialasvab.com): springs, Hooke's law F = kx; springs in series share the full load and their stretches addReport a problem with this question

30. 30. A bimetal strip is made of brass bonded to steel. Brass expands more than steel when heated. When the strip is heated, how does it bend?

    • A.It curves toward the steel side, with the brass on the outside of the curve✓ Answer
    • B.It stays straight but grows longer, because both metals expand together
    • C.It curves toward the brass side, with the steel on the outside of the curve
    • D.It curves toward the brass side, because brass gets heavier as it warms

    Bonded together, the brass grows longer than the steel, and the only way a longer layer can stay attached to a shorter one is on the outside of a curve. So the strip bends toward the metal that expands less, the steel. Heating does not change a metal's weight; this bending is what operates many thermostats.

    Ref QB-8V56XSSource: Official ASVAB Mechanical Comprehension subtest (officialasvab.com): properties of materials, thermal expansion; a bimetal strip bends toward the metal with lower expansionReport a problem with this question

31. 31. Why do bridge trusses and roof frames use triangles rather than rectangles?

    • A.A triangle uses less material because it has one fewer side than a rectangle
    • B.A triangle lets its corners pivot freely, so it bends instead of breaking
    • C.A triangle puts every member in tension, so no part is ever in compression
    • D.A triangle keeps its shape as long as none of its members change length✓ Answer

    Three sides of fixed length allow only one shape, so a triangle cannot rack or fold even if its joints are pinned. A rectangle with pinned corners can lean over into a parallelogram without any member changing length. In a real truss some members are in tension and others in compression, and saving material is not why the shape is chosen.

    Ref QB-RCBBHTSource: Official ASVAB Mechanical Comprehension subtest (officialasvab.com): structures, a triangle is the rigid basic unit of a trussReport a problem with this question

32. 32. In a hydraulic jack, the small piston is 2 in in diameter and the large piston is 8 in in diameter. A force of 32 lb is applied to the small piston. Ignoring friction, what force does the large piston exert?

    • A.128 lb
    • B.512 lb✓ Answer
    • C.2 lb
    • D.32 lb

    By Pascal's principle the pressure is the same on both pistons, so force scales with piston area. Area grows with the square of the diameter: (8 / 2)^2 = 16, and 32 x 16 = 512 lb. 128 lb uses the diameter ratio (4) instead of the area ratio; 32 lb assumes equal pressure means equal force; 2 lb divides by the area ratio instead of multiplying.

    Ref QB-7VSTRRSource: Official ASVAB Mechanical Comprehension subtest (officialasvab.com): fluids and hydraulics, Pascal's principle: F2 = F1 x (A2 / A1)Report a problem with this question

33. 33. Three open tanks hold water to the same depth of 10 ft. Tank X is a narrow pipe, tank Y is a wide box, and tank Z flares outward toward the top. How does the water pressure at the bottom of the tanks compare?

    • A.It is equal in all three, because pressure depends on the depth✓ Answer
    • B.It is greatest in Y, because the wide box holds the most water
    • C.It is greatest in Z, because the flared sides press down on the bottom
    • D.It is greatest in X, because the narrow pipe concentrates the weight

    Liquid pressure at a point depends on the liquid's density and the depth below the surface, not on the shape of the container or the total amount of liquid. All three tanks hold water 10 ft deep, so the bottom pressure is the same. A larger tank has more total force on a larger bottom area, but the pressure (force per area) is unchanged.

    Ref QB-QCYYBSSource: Official ASVAB Mechanical Comprehension subtest (officialasvab.com): fluids and hydraulics, hydrostatic pressure depends on depth and density, not container shapeReport a problem with this question

34. 34. A 10-lb metal block hangs from a spring scale. When the block is lowered until it is fully under water, it displaces 3 lb of water. What does the scale read while the block is submerged?

    • A.3 lb
    • B.13 lb
    • C.7 lb✓ Answer
    • D.10 lb

    By Archimedes' principle the buoyant force equals the weight of the water displaced, 3 lb, and it pushes up, so the scale reads 10 - 3 = 7 lb. 10 lb ignores buoyancy; 3 lb mistakes the buoyant force for the scale reading; 13 lb adds the buoyant force as if it pushed down.

    Ref QB-6VEGKQSource: Official ASVAB Mechanical Comprehension subtest (officialasvab.com): fluids and hydraulics, Archimedes' principle: buoyant force = weight of fluid displacedReport a problem with this question

35. 35. Water flows steadily through a pipe that narrows from 4 in in diameter to 2 in in diameter. Compared with its speed in the wide section, how fast does the water move in the narrow section?

    • A.Two times slower
    • B.Two times faster
    • C.Four times faster✓ Answer
    • D.The same speed

    The same volume of water must pass every section each second, so speed x cross-sectional area is constant. Halving the diameter cuts the area to (1/2)^2 = 1/4, so the water must move 4 times as fast. Two times faster uses the diameter ratio instead of the area ratio; slower flow or equal speed ignores the continuity of flow.

    Ref QB-PCMN6RSource: Official ASVAB Mechanical Comprehension subtest (officialasvab.com): fluids and hydraulics, continuity of flow: velocity x cross-sectional area is constantReport a problem with this question

36. 36. Air flows through a horizontal tube that has a narrow section in the middle. Compared with the air in the wide parts of the tube, what is true of the air in the narrow section?

    • A.It moves faster and its pressure is lower✓ Answer
    • B.It moves slower and its pressure is higher
    • C.It moves slower and its pressure is lower
    • D.It moves faster and its pressure is higher

    Continuity forces the air to speed up where the tube narrows. By Bernoulli's principle, where a fluid flows faster its pressure is lower, because the higher pressure in the wide section is what accelerates the air into the narrow one. Many people expect squeezing to raise the pressure, but in steady flow the narrow section has the lowest pressure.

    Ref QB-5V36EPSource: Official ASVAB Mechanical Comprehension subtest (officialasvab.com): fluids and hydraulics, Bernoulli's principle: faster flow has lower pressureReport a problem with this question

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