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Structures and forces

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

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Structures and forces

Science 7 · maddyhelps.com

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

  1. A shelf must hold 300 N and sag as little as possible. Tests on three materials: P breaks at 400 N and sags 2 mm under 300 N; Q breaks at 600 N and sags 15 mm under 300 N; R sags 1 mm under 200 N but breaks at 250 N. Which is best?

    1. a) Q: it bends the most, so it will not crack
    2. b) Q: it holds the heaviest load before breaking
    3. c) R: it sags the least of all three materials
    4. d) P: strong enough, and it sags less than Q

  2. A cupboard door must swing open, but the cupboard's frame must stay rigid. Which joints are best?

    1. a) Hinges on the door, and glue and screws on the frame
    2. b) Glue on the door, and hinges at the corners of the frame
    3. c) Hinges on both the door and the frame, so each can move
    4. d) Nails on both the door and the frame, so nothing wobbles

  3. Engineers say a crowd jumping in time at a concert is harder on a balcony than the same crowd standing still. Why?

    1. a) Jumping adds mass to each person, so the crowd weighs more
    2. b) A moving, dynamic load can push harder than the same weight at rest
    3. c) A crowd standing still puts no force on the balcony at all
    4. d) Jumping turns the compression in the balcony into torsion

  4. An astronaut has a mass of 70 kg on Earth. Gravity on the Moon is about one-sixth as strong as on Earth. On the Moon, her mass is

    1. a) 70 kg
    2. b) about 12 kg
    3. c) about 420 kg
    4. d) 0 kg

  5. An eggshell, a bike helmet and a pop can are all shell structures. What do shell structures have in common?

    1. a) Solid material all the way through, held in place by its weight
    2. b) Separate parts joined by hinges so that they can move freely
    3. c) A skeleton of beams and struts joined together into triangles
    4. d) A thin, hard outer layer that spreads a load over its whole surface

  6. A diving board must bend a long way under a diver and spring back to its shape without breaking. Which property matters most in its material?

    1. a) Heaviness
    2. b) Hardness
    3. c) Stiffness
    4. d) Flexibility

  7. A bag of flour has a mass of 5 kg. About how much force does it push down on a shelf with, here on Earth?

    1. a) About 5 N
    2. b) About 500 N
    3. c) About 50 N
    4. d) About 0.5 N

  8. Students test paper bridges. The specification: hold the most pennies for each sheet of paper used. Bridge A used 6 sheets and held 120 pennies; bridge B used 3 sheets and held 80. Which met the specification better?

    1. a) Bridge B, at about 27 pennies per sheet
    2. b) Bridge A, because it held 40 more pennies
    3. c) Bridge A, because more paper makes it sturdier
    4. d) Neither, since both bridges collapsed in the end

  9. A square frame of four sticks, pinned at the corners, folds sideways when pushed. Adding one stick from corner to corner stops this. Why?

    1. a) It splits the square into triangles, which cannot change shape
    2. b) It lets the frame bend a little without the pins pulling out
    3. c) It makes the frame heavier, so a push has less effect
    4. d) It adds another side, and more sides make a shape stronger

  10. A flat sheet of thin cardboard sags when you hold it by one edge, but corrugated cardboard, with a wavy layer glued between two flat sheets, stays stiff. Why?

    1. a) The folds act like many small arches and beams that resist bending
    2. b) The wavy layer is made of a much stronger kind of paper
    3. c) The glue between the layers is stiffer than the cardboard
    4. d) The wavy layer adds so much weight that the sheet cannot sag

Answer key · Structures and forces

Science 7 · maddyhelps.com

  1. d) P: strong enough, and it sags less than Q — First rule out any material that cannot do the job: R breaks at 250 N, below the 300 N the shelf must hold. Of the two that pass, P sags 2 mm and Q sags 15 mm, so P meets both parts of the specification; Q is the strongest, but extra strength was not what was asked for.
  2. a) Hinges on the door, and glue and screws on the frame — A hinge is a moving joint that lets the door turn, while glue, nails and screws make fixed joints that hold parts still. Choosing a joint starts with one question: does this part need to move?
  3. b) A moving, dynamic load can push harder than the same weight at rest — A load that moves or changes is dynamic: each landing pushes down with more force than the person's weight, and jumping in rhythm can make a structure sway more and more. The people's mass never changes; the forces they put on the balcony do.
  4. a) 70 kg — Mass is the amount of matter in her body, and it is the same everywhere. What drops to about one-sixth is her weight, the force of gravity on her, from about 700 N on Earth to about 120 N on the Moon; dividing the mass by 6 mixes up mass and weight.
  5. d) A thin, hard outer layer that spreads a load over its whole surface — A shell structure is a thin outer layer, often curved, that holds its shape and spreads any force across its whole surface. A skeleton of beams is a frame structure, like a transmission tower, and solid all the way through describes a solid structure, like a stone wall.
  6. d) Flexibility — Flexibility means bending under a load and returning to shape, which is exactly what a diving board does. A stiff board would give no spring, and a hard but brittle material such as glass would snap; strong is not the same as stiff.
  7. c) About 50 N — Earth's gravity pulls on each kilogram with about 10 N (9.8 N, more exactly), so 5 kg weighs about 5 × 10 = 50 N. Kilograms measure mass, the amount of matter, and newtons measure force; answering 5 N forgets to change one into the other.
  8. a) Bridge B, at about 27 pennies per sheet — The specification asks for pennies per sheet, so divide: A held 120 ÷ 6 = 20 and B held 80 ÷ 3 ≈ 27. A held more pennies only because it used twice the paper, and both bridges were always going to collapse, since loading until failure is how the maximum load is found.
  9. a) It splits the square into triangles, which cannot change shape — Once a triangle's three sides are fixed, its shape is fixed, so it cannot fold unless a side stretches or squashes. A square's corners can swing even when all four sides stay the same length, which is why towers, bridges and roof trusses are built of triangles.
  10. a) The folds act like many small arches and beams that resist bending — Corrugation turns a floppy sheet into a row of tiny arches and beams standing on edge, so bending it means squashing and stretching every fold at once. The paper is the same; only its shape changed, which is also why corrugated metal is used for roofs and culverts.