Forces and motion, Year 8: turning forces and pressure

A moment

提示In everyday speech the same word means a short time.

为什么Its size depends on the force and on how far from the pivot the force acts: moment = force × distance.

A bigger distance gives a bigger turning effect

提示Try pushing a door open right next to the hinge.

为什么The moment is the force multiplied by its distance from the pivot, so a small push far from the hinge turns the door as well as a large push close to it.

Towards the 300 N child

提示Multiply each force by its distance from the pivot.

为什么300 × 2 = 600 N m on one side and 400 × 1 = 400 N m on the other. The larger moment wins.

5 N m

提示Multiply the force by the distance.

为什么10 × 0.5 = 5. The unit is the newton metre, because newtons have been multiplied by metres.

It increases

提示There is more liquid above, pressing down.

为什么The deeper you go, the greater the weight of liquid overhead, so the squeeze is greater. Divers feel it in their ears within a few metres.

The bottom hole

提示Ask where the water is being squeezed hardest.

为什么The pressure is greatest at the greatest depth, so the jet from the lowest hole comes out fastest and travels furthest.

Water pressure is greatest at the bottom

提示Think about where a diver would feel the biggest squeeze.

为什么Pressure increases with depth, so the lowest part of the wall is pushed hardest and needs the most strength.

In all directions

提示A submarine is squeezed on its sides and underneath as well as on top.

为什么The particles of a liquid move and push every way, so the pressure acts sideways and upwards as well as downwards.

The weight of the air above us

提示We live at the bottom of a very deep layer of gas.

为什么The atmosphere is many kilometres deep, and all of it is pulled down by gravity. That weight squeezes the air at the bottom, so its particles press on every surface.

It decreases

提示There is less air above you the higher you go.

为什么With less air overhead, there is less weight pressing down. The air is also thinner, which is why climbers on very high peaks carry oxygen.

The pressure of the air outside

提示Inside, there is no longer anything pushing back.

为什么Normally the air inside pushes out as hard as the atmosphere pushes in. Remove the air inside and the outside push is unopposed.

The air pressure outside the eardrum drops

提示Some air is sealed inside your ear, still as it was on the ground.

为什么As the pressure in the cabin falls, the air behind the eardrum is at a higher pressure and pushes it outwards. The 'pop' is that air escaping to even things up.