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Science 8

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Mechanical systems

Ten questions of mixed difficulty, covering Mechanical systems. Print it, or work through it on screen — the answer key starts on its own page.

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Mechanical systems

Science 8 · maddyhelps.com

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Circle the best answer for each question. Show your work in the space provided.

  1. Heading up a steep hill, a cyclist shifts so the chain runs on a smaller gear at the pedals and a larger gear at the back wheel. What does this change?

    1. a) Nothing, since the same chain still connects the two gears
    2. b) Pedalling is easier, but the wheel turns fewer times per pedal turn
    3. c) Pedalling is harder, but the wheel turns more times per pedal turn
    4. d) Pedalling is easier, and the wheel turns more times per pedal turn

  2. In a hydraulic jack, the small piston has an area of 2 cm² and the large piston an area of 50 cm². The small piston is pushed down 10 cm. How far does the large piston rise?

    1. a) 4 cm
    2. b) 0.4 cm
    3. c) 10 cm
    4. d) 250 cm

  3. An excavator's arm must hold a heavy bucket of rock perfectly still in mid-air. Why do its cylinders use oil (hydraulics) rather than air (pneumatics)?

    1. a) Air cannot carry pressure along a hose from place to place
    2. b) Oil squeezes into less space, so it stores up more force
    3. c) Oil barely compresses, so the arm does not sink or bounce
    4. d) Oil flows through narrow hoses more easily than air does

  4. A worker pushes down on a crowbar with a force of 150 N, and the other end lifts a crate with a force of 600 N. What is the crowbar's mechanical advantage?

    1. a) 90 000
    2. b) 450
    3. c) 0.25
    4. d) 4

  5. Three gears sit in a row. Gear P turns clockwise and meshes with gear Q, and gear Q meshes with gear R. Which way does gear R turn?

    1. a) Clockwise, the same way as gear P
    2. b) Either way, depending on the size of Q
    3. c) Counterclockwise, opposite to gear P
    4. d) Clockwise only if R is larger than P

  6. A driving gear with 12 teeth turns a driven gear with 36 teeth. The driving gear makes 6 full turns. How many turns does the driven gear make?

    1. a) 2 turns
    2. b) 3 turns
    3. c) 18 turns
    4. d) 6 turns

  7. You squeeze a water balloon at one spot, and it bulges out everywhere else. What does this show about a liquid in a closed container?

    1. a) The liquid becomes denser where it is squeezed
    2. b) The liquid is squeezed into a much smaller space
    3. c) Pressure on it travels only straight ahead from your hand
    4. d) Pressure on it is passed on equally in all directions

  8. A bicycle is a system made of subsystems. Which subsystem carries the force of the rider's legs to the back wheel?

    1. a) The brake levers, brake cables and brake pads
    2. b) The handlebars, stem and front fork
    3. c) The pedals, front gear, chain and rear gear
    4. d) The frame, seat and seat post

  9. Two frictionless ramps lead up to the same platform, 1 m high. One is 2 m long, the other 4 m long. Compared with pushing a box up the short ramp, the long ramp takes

    1. a) half the force over twice the distance, and the same work
    2. b) half the force over twice the distance, and twice the work
    3. c) the same force over twice the distance, and twice the work
    4. d) half the force over twice the distance, and half the work

  10. A flag is raised with a rope that runs over a single fixed pulley at the top of the pole. What does the pulley do for the person raising the flag?

    1. a) It changes the pull's direction, not its size
    2. b) It doubles the distance the flag moves
    3. c) It halves the force needed to lift the flag up the pole
    4. d) It takes the flag's weight off the rope

Answer key · Mechanical systems

Science 8 · maddyhelps.com

  1. b) Pedalling is easier, but the wheel turns fewer times per pedal turn — With a small driving gear and a large driven gear, the back wheel turns fewer times for each turn of the pedals, so each pedal stroke covers less ground, and in return it takes less force. Force and speed trade off: no gear gives both easier pedalling and more distance per turn.
  2. b) 0.4 cm — The oil pushed out of the small cylinder, 2 cm² × 10 cm = 20 cm³, must fit into the large cylinder, where it spreads over 50 cm², so the large piston rises 20 ÷ 50 = 0.4 cm. The large piston pushes with 25 times the force but moves only a twenty-fifth as far; multiplying by 25 instead gives 250 cm.
  3. c) Oil barely compresses, so the arm does not sink or bounce — A liquid hardly compresses, so a push on the oil moves the piston at once and holds it firmly where it stops. Air does carry pressure along a hose, which is how air tools and truck air brakes work, but it squashes like a spring, so a heavy load would sink and bounce.
  4. d) 4 — Mechanical advantage is output force ÷ input force = 600 N ÷ 150 N = 4, so the crowbar multiplies the push four times. Dividing the other way gives 0.25, which would mean the crowbar gives out less force than it takes in; and because newtons divide by newtons, the answer has no units.
  5. a) Clockwise, the same way as gear P — Two meshing gears always turn in opposite directions, so Q turns counterclockwise, and R, meshing with Q, turns clockwise again. Size changes how fast a gear turns, never which way; a middle gear used like Q is called an idler gear.
  6. a) 2 turns — Each tooth on the small gear pushes one tooth on the large gear, so 6 turns × 12 teeth = 72 teeth, which turns the 36-tooth gear 72 ÷ 36 = 2 times. The driving gear turns three times for each turn of the driven gear; 18 comes from multiplying by 3 instead of dividing.
  7. d) Pressure on it is passed on equally in all directions — Pressure applied to a trapped fluid spreads through all of it, pushing equally on every part of the container, so the balloon bulges on every side, not just opposite your hand. The water is not squeezed smaller: liquids barely compress, which is exactly what lets a hydraulic system pass a push along a pipe.
  8. c) The pedals, front gear, chain and rear gear — The drive subsystem transmits force and motion: the pedals turn the front gear, the chain links it to the rear gear, and the rear gear turns the wheel. The brakes slow the bike and the steering points it, while the frame supports the rider and holds the other subsystems together.
  9. a) half the force over twice the distance, and the same work — Without friction, the work to push the box up either ramp equals the work to lift it straight up, its weight × 1 m. The long ramp spreads that same work over twice the distance, so it needs half the force: a machine can trade force for distance, but it cannot reduce the work.
  10. a) It changes the pull's direction, not its size — With one fixed pulley you pull down to lift the flag up, which is far easier than climbing the pole, but the force needed is the flag's weight plus a little to overcome friction. Halving the force takes a movable pulley, one that rises with the load.