Forces and motion, Year 8: more practice

20

提示Moment = force × distance from the pivot.

为什么50 × 0.4 = 20 N m.

8

提示Forces in opposite directions are subtracted.

为什么20 − 12 = 8 N, in the forward direction.

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.

20000

提示Pressure = force ÷ area.

为什么600 ÷ 0.03 = 20 000 Pa (20 000 N/m²).

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.

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.

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.

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.

5

提示The forces must be balanced.

为什么A floating object is in equilibrium, so upthrust equals weight: 5 N.

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.