Forces and motion, Year 8: more practice：全部卡片
整套按顺序列出——孩子看到之前，您可以先通读一遍。
- A 50 N force acts 0.4 m from a pivot. What is its moment in newton metres? (number only)
20
提示Moment = force × distance from the pivot.
为什么50 × 0.4 = 20 N m.
- A boat is pushed forwards by a 20 N force while water pushes back with 12 N. What is the resultant force in newtons? (number only)
8
提示Forces in opposite directions are subtracted.
为什么20 − 12 = 8 N, in the forward direction.
- A sledge is pulled forward by 40 N and friction is 40 N backwards. It is moving. What happens to its speed?
It stays the same
提示What is the resultant force?
为什么The resultant force is 0 N, so there is no change in motion: the sledge keeps moving at a steady speed in a straight line.
- A force of 600 N acts on an area of 0.03 m². What is the pressure in pascals? (number only)
20000
提示Pressure = force ÷ area.
为什么600 ÷ 0.03 = 20 000 Pa (20 000 N/m²).
- Is the pressure greater at the same depth in sea water or in fresh water?
Sea water
提示The two liquids differ in density.
为什么Pressure depends on depth and on the density of the liquid. Sea water is denser, so the pressure is greater at the same depth.
- Two points are at the same depth in a lake. How do the pressures there compare?
They are equal
提示What does the pressure in a liquid depend on?
为什么Pressure in a liquid depends on depth, not on the direction or the width of the container above the point.
- What instrument measures atmospheric pressure?
A barometer
提示Its name includes the Greek word for weight.
为什么Barometers measure the pressure of the air, which changes with the weather and altitude.
- When you drink through a straw, what pushes the liquid up it?
Atmospheric pressure on the liquid in the glass
提示Sucking removes air from the straw, so what pushes from outside?
为什么Sucking reduces the pressure in the straw, and the greater air pressure on the surface of the drink pushes the liquid up.
- A toy boat floats at rest and weighs 5 N. What is the upthrust on it in newtons? (number only)
5
提示The forces must be balanced.
为什么A floating object is in equilibrium, so upthrust equals weight: 5 N.
- A steel bolt sinks but a steel ship floats. What is the key difference?
The ship's shape means its average density is less than water's
提示Think about all the space inside the hull.
为什么A ship is hollow and encloses a large volume of air, so its overall density is less than that of water.
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