Forces and motion, Year 8: turning forces and pressure

A moment

HintIn everyday speech the same word means a short time.

The whyIts 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

HintTry pushing a door open right next to the hinge.

The whyThe 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

HintMultiply each force by its distance from the pivot.

The why300 × 2 = 600 N m on one side and 400 × 1 = 400 N m on the other. The larger moment wins.

5 N m

HintMultiply the force by the distance.

The why10 × 0.5 = 5. The unit is the newton metre, because newtons have been multiplied by metres.

It increases

HintThere is more liquid above, pressing down.

The whyThe 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

HintAsk where the water is being squeezed hardest.

The whyThe 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

HintThink about where a diver would feel the biggest squeeze.

The whyPressure increases with depth, so the lowest part of the wall is pushed hardest and needs the most strength.

In all directions

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

The whyThe 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

HintWe live at the bottom of a very deep layer of gas.

The whyThe 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

HintThere is less air above you the higher you go.

The whyWith 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

HintInside, there is no longer anything pushing back.

The whyNormally 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

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

The whyAs 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.