Turning Forces and Centres of Gravity Turning Forces

Turning Forces and Centres of Gravity

Turning Forces • Have you ever tried to open a metal cap bottle by pushing or pulling it? ?

• You may have noticed that this is very difficult to do, so how would you make it easier? • We normally use a bottle opener to take the metal cap off a bottle

• The bottle opener makes your force bigger because it is a lever and it has a turning effect • Turning forces allow us to apply much greater forces than if we were pushing or pulling look at some of the following examples

• It is difficult to move the large rock by pushing or pulling it • However it is much easier to use a turning force • Do this for yourself in class!

• If you were trying to loosen a bolt would you succeed in pulling it or pushing it? • If you’re digging hard ground what would you need?

• All of the example we have just seen are Levers, A Lever is a rigid body which is free to turn about a fixed point called the FULCRUM

Look at the fulcrum in the following examples The fulcrum is where the lever turns about The door turns about the hinges so this is the fulcrum

Can you think what the fulcrum is in these examples

• The turning effect of levers makes our lives a lot easier as they turn our small forces into much bigger forces

• The Turning effect of a force is called the moment of the force it depends on two things 1. The size of the force (in Newtons) 2. The distance (in meters) from the fulcrum to where the force is applied

Moment of a force = (Force x Perpendicular distance from the force to the fulcrum) • Don’t worry if you’re confused look at the following examples to try to understand

What is the moment of the force? • Moment = Force x Distance • The force is 40 N the distance is 0. 2 m • Moment = 40 X 0. 2 = 8 N/m

Try the following examples

How can you increase the moment of a force? ? • Two volunteers needed to close the door!! • Why can one person close it easily while another has difficulty? ? • Two volunteers needed to cut some celery!! • Why can one person cut it easily while another has difficulty? ?

From what you have just seen can you suggest a solution to the following problem? ? • In this diagram the bolt cannot be loosened

3 Forces act on a lever, the effort, the load and the fulcrum • The load is the thing the lever is trying to move, the effort is the force put on the lever we have already looked at what the fulcrum is (p 288)

• Look at the diagrams and see if you can tell which arrows show the load, which show the effort and which show the fulcrum

The Law of the Lever • When a lever is balanced the clockwise and anticlockwise moments are equal Think of the two people balancing on the see -saw in this picture is the anticlockwise moment (on the left) equal to the clockwise moment (on the right)?

Example

Try this with meter sticks in class use different masses and a meter stick to test the law of the lever Open page 291

The following examples show you can work out weight and distance using the law of the lever

The centre of gravity • The point at which all the weight of a body appears to act is called its centre of gravity • We can find the centre of gravity with a simple experiment (p 293)

Equilibrium

An object will fall over if its centre of gravity moves outside its base • Look at the bus in the following example do you think it is a good idea to have the centre of gravity so high? ? • How would you make the bus more stable? ?

How to be a good gymnast!

Stable design • The most stable objects have a wide base and a low centre of gravity

Why is this cone a good stable design?