Forces and motion, Year 7: more practice

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.

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.

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.

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 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.

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).

5

提示Divide the force by the size of one centimetre.

为什么10 ÷ 2 = 5 cm.

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.

7

提示Extension is the increase in length.

为什么Extension = stretched length − natural length = 19 − 12 = 7 cm.

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.