Forces and motion, Year 7: 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 car travels at a steady 72 km/h. What is its speed in m/s? (number only)
20
提示Change kilometres to metres and hours to seconds.
为什么72 km/h = 72 000 m per 3600 s = 20 m/s. A shortcut is to divide by 3.6.
- What can an unbalanced force do to a moving object?
Change its speed or direction
提示Think of what a resultant force does to motion.
为什么An unbalanced force changes the motion of the object: it speeds it up, slows it down or turns it.
- Which non-contact force acts between two magnets?
Magnetic force
提示It can push or pull across a gap, like gravity does.
为什么Magnets push or pull each other across a gap without touching, just as gravity does.
- Is air resistance a contact force or a non-contact force?
A contact force
提示Think about what is touching the object.
为什么Air resistance is caused by the air particles hitting the surface of the moving object.
- A newtonmeter reads 0.2 N with nothing hanging on it. What is the name of this fault?
A zero error
提示The scale does not start where it should.
为什么Before using the instrument, adjust the reading to 0 or subtract the error from every result.
- Roughly what is the weight of a 1 kg bag of sugar on Earth, in newtons? (number only)
10
提示On Earth, 1 kg is pulled by about 10 N.
为什么Gravity pulls each kilogram with a force of about 10 N, so 1 kg weighs about 10 N (more exactly 9.8 N).
- On a force diagram 1 cm stands for 2 N. How long, in cm, is the arrow for a 10 N force? (number only)
5
提示Divide the force by the size of one centimetre.
为什么10 ÷ 2 = 5 cm.
- Why is oil put on a bicycle chain?
To reduce friction
提示Think of the surfaces rubbing together.
为什么A lubricant forms a thin layer between the parts so they slide past each other more easily and wear less.
- A spring has a natural length of 12 cm. Under a load it is 19 cm long. What is its extension in cm? (number only)
7
提示Extension is the increase in length.
为什么Extension = stretched length − natural length = 19 − 12 = 7 cm.
- What happens to a spring that is stretched beyond its elastic limit?
It does not go back to its original length
提示Think about the meaning of 'elastic'.
为什么Past the limit the spring is permanently stretched. Below it, the spring returns to its original length when the load is removed.
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