Class 8 Science — Pressure and how it is shared
Work out pressure from a force and an area, and explain why the same force presses harder when it is spread over less of a surface.
Name: ________________________
1.Boat B has a base area of 3.5 m² and carries 3 people. Each person weighs 700 N. What is the pressure on Boat B's base, in N/m²? Answer with a number.
2.Each of these is a way of choosing how much surface a force lands on. Sort them by the pressure that follows.
Groups: High pressure · Low pressure
- A broad shoulder strap
- The sharp edge of a knife
- The pointed end of a nail
- The broad base of a dam
3.Why is a household water tank put up on the roof rather than at ground level?
- a) Water becomes heavier when it is higher up
- b) Water flows faster in a warm tank than a cool one
- c) A taller column of water raises the pressure at the taps, giving a better stream
- d) A tank on the roof holds more water than the same tank on the ground
4.Two pipes are open at the top, with a balloon tied over the bottom of each. Both balloons are the same to start with. Tap the balloon that will bulge out more.
Write the number of the part.
5.The air above a patch of skin 15 cm by 15 cm presses down with about the same force as gravity on a 225 kg object. Why does that not crush us?
- a) Air is so light that it cannot really press on anything
- b) The pressure inside our bodies is equal to it and balances it
- c) The air pushes upwards as much as it pushes downwards, so it cancels out
- d) Our skin is too tough for air to press through
6.The pressure exerted by the air around us has a two-word name in the chapter. What is it?
7.Two bags weigh exactly the same, but one has narrow straps and one has broad straps. Which is more comfortable, and why?
- a) Neither, because the bags weigh the same
- b) The broad-strap bag, because the same weight is spread over a larger area of the shoulders
- c) The broad-strap bag, because broad straps make the bag lighter
- d) The narrow-strap bag, because narrow straps hold the weight closer to the body
8.Match each thing you can watch happen to what it tells you about pressure.
- Water spurts from holes made in the sides of a filled bottle
- Balloons on two pipes of different widths, filled to the same level, bulge the same amount
- A rubber sucker pressed onto smooth tiles will not pull off easily
- A paper plate is harder to lift under an unfolded sheet than a folded one
- a liquid presses on the sides of its container too, not just the bottom
- the air outside it presses harder than the thinned-out air trapped inside
- air pushes with a bigger force when it has a bigger area to push on
- it is the height of the water column that sets the pressure, not the width
Answer key — Class 8 Science — Pressure and how it is shared
- 1. 600
- 2. The pointed end of a nail → High pressure; The sharp edge of a knife → High pressure; A broad shoulder strap → Low pressure; The broad base of a dam → Low pressure
- 3. c) A taller column of water raises the pressure at the taps, giving a better stream
- 4. 1 — The balloon under the tall column of water
- 5. b) The pressure inside our bodies is equal to it and balances it
- 6. atmospheric pressure
- 7. b) The broad-strap bag, because the same weight is spread over a larger area of the shoulders
- 8. Water spurts from holes made in the sides of a filled bottle → a liquid presses on the sides of its container too, not just the bottom; Balloons on two pipes of different widths, filled to the same level, bulge the same amount → it is the height of the water column that sets the pressure, not the width; A rubber sucker pressed onto smooth tiles will not pull off easily → the air outside it presses harder than the thinned-out air trapped inside; A paper plate is harder to lift under an unfolded sheet than a folded one → air pushes with a bigger force when it has a bigger area to push on