Forces and motion, Year 7: more practice

20

HintChange kilometres to metres and hours to seconds.

The why72 km/h = 72 000 m per 3600 s = 20 m/s. A shortcut is to divide by 3.6.

Change its speed or direction

HintThink of what a resultant force does to motion.

The whyAn unbalanced force changes the motion of the object: it speeds it up, slows it down or turns it.

Magnetic force

HintIt can push or pull across a gap, like gravity does.

The whyMagnets push or pull each other across a gap without touching, just as gravity does.

A contact force

HintThink about what is touching the object.

The whyAir resistance is caused by the air particles hitting the surface of the moving object.

A zero error

HintThe scale does not start where it should.

The whyBefore using the instrument, adjust the reading to 0 or subtract the error from every result.

10

HintOn Earth, 1 kg is pulled by about 10 N.

The whyGravity pulls each kilogram with a force of about 10 N, so 1 kg weighs about 10 N (more exactly 9.8 N).

5

HintDivide the force by the size of one centimetre.

The why10 ÷ 2 = 5 cm.

To reduce friction

HintThink of the surfaces rubbing together.

The whyA lubricant forms a thin layer between the parts so they slide past each other more easily and wear less.

7

HintExtension is the increase in length.

The whyExtension = stretched length − natural length = 19 − 12 = 7 cm.

It does not go back to its original length

HintThink about the meaning of 'elastic'.

The whyPast the limit the spring is permanently stretched. Below it, the spring returns to its original length when the load is removed.