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Science 8

Science 8 · Worksheets

Mixed review · Fluids, machines and light

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

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Mixed review · Fluids, machines and light

Science 8 · maddyhelps.com

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

  1. You step out of a dark movie theatre into bright sunlight, and your pupils shrink. Which part of the eye makes them smaller?

    1. a) The retina
    2. b) The iris
    3. c) The cornea
    4. d) The lens

  2. Muddy pond water looks cloudy when it is stirred. Left in a jar overnight, it clears as a layer of silt settles to the bottom. The muddy water was

    1. a) a solution
    2. b) a saturated solution
    3. c) a pure substance
    4. d) a suspension

  3. A bathroom window of frosted glass lets daylight into the room, but nobody can see clearly through it. The frosted glass is

    1. a) reflective
    2. b) transparent
    3. c) opaque
    4. d) translucent

  4. A student pushes a box 4.0 m up a ramp with a force of 250 N. The box, which weighs 600 N, rises 1.5 m. What is the ramp's efficiency?

    1. a) 111%
    2. b) 240%
    3. c) 42%
    4. d) 90%

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

  6. In a hydraulic lift, the small piston has an area of 2 cm² and the large piston an area of 50 cm². A 30 N push on the small piston makes what force on the large piston?

    1. a) 82 N
    2. b) 1.2 N
    3. c) 750 N
    4. d) 30 N

  7. A student dissolves 12 g of salt in water to make 200 mL of solution. What is the concentration in grams per 100 mL?

    1. a) 12 g per 100 mL
    2. b) 0.06 g per 100 mL
    3. c) 24 g per 100 mL
    4. d) 6 g per 100 mL

  8. A ray of light hits a flat mirror, making an angle of 25° with the mirror's surface. What is the angle of reflection?

    1. a) 50°
    2. b) 65°
    3. c) 130°
    4. d) 25°

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

  10. A hiker weighs 720 N. In boots, her weight rests on 0.06 m² of snow; in snowshoes, it rests on 0.36 m². What pressure does she put on the snow in snowshoes?

    1. a) 0.0005 Pa
    2. b) 2000 Pa
    3. c) 12 000 Pa
    4. d) 259 Pa

Answer key · Mixed review · Fluids, machines and light

Science 8 · maddyhelps.com

  1. b) The iris — The pupil is just an opening; the iris is the coloured ring of muscle around it, which narrows the pupil in bright light and widens it in dim light, as a camera's aperture does. The lens focuses the light, and the retina detects it.
  2. d) a suspension — In a suspension, solid particles are spread through a liquid but are big enough to settle out when it stands, and a filter can catch them. The dissolved particles in a solution never settle, however long it stands, so water that clears overnight was never a solution.
  3. d) translucent — Translucent materials let light through but scatter it, so shapes behind them look blurred. Clear window glass is transparent, letting light through without scattering it, and an opaque material, such as a wooden door, lets no light through at all.
  4. d) 90% — Input work = 250 N × 4.0 m = 1000 J and output work = 600 N × 1.5 m = 900 J, so efficiency = 900 ÷ 1000 × 100% = 90%. An answer over 100% means something is upside down: friction always turns some input work into heat, so no real machine gets out more work than goes in.
  5. a) 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.
  6. c) 750 N — The pressure is the same everywhere in the liquid, and the large piston has 50 ÷ 2 = 25 times the area, so it pushes with 25 × 30 N = 750 N. It is the pressure that stays the same, not the force, and mixing the two up gives 30 N.
  7. d) 6 g per 100 mL — 200 mL is two lots of 100 mL, so each 100 mL holds 12 ÷ 2 = 6 g of the salt, which is the solute. Answering 12 g forgets that the salt is spread through 200 mL, not 100 mL, and 24 g doubles where it should halve.
  8. b) 65° — The law of reflection measures angles from the normal, not from the mirror, so the angle of incidence is 90° − 25° = 65°, and the angle of reflection equals it. 25° is the angle to the surface, the usual trap, and 130° is the whole angle between the incoming and reflected rays.
  9. a) 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.
  10. b) 2000 Pa — Pressure is force per area: p = F/A = 720 N ÷ 0.36 m² = 2000 Pa, a sixth of the 12 000 Pa her boots press with. 12 000 Pa comes from using the boots' area by mistake; the same weight on six times the area gives a sixth of the pressure, which is why snowshoes keep her from sinking.