Turning Forces and Centre of Gravity

Chapter 42

What is a turning force?

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Turning Forces

A turning force is something that would allow us to apply a much greater force than pulling or

pushing an object like moving a rock

It makes use of something called a lever

A lever is a rigid body which is free to turn about a fixed point called the fulcrum

Turning Forces

The Fulcrum is the part about which the lever turns

The effort is where the force is applied

The load is where the force takes effect

Everyday examples

Moments

The turning effect of the force is called the moment of the force and it depends on 2 things:

1. The size of the force (measured in Newton's)

2. The distance (in meters) from the fulcrum to where the force is applied

Some Math…

Moment = Force x Distance from the Fulcrum

Balancing Moments

If the moments are equal on both sides of the see saw it is balanced

Some Examples

A metre stick is suspended from its centre of gravity which is at its midpoint.A force of 3 N acts on the stick at the 90 cm mark and a force of F newtons acts on the stick at the 20 cm mark. Calculate force F.

The Law of The Lever

When a lever is balanced, the clockwise and anti-clockwise moments are equal

The Experiment

You will need :

1. A meter stick2. Retort stand3. Weights4. Table to record results

* 100g = 1N

Table

Method

1. Hang the meter stick from the retort stand at the 50cm mark or until it is balanced

2. Hang a 1N weight on the RHS and a 2N weight on the LHS so they balance eachother

3. Note the distances of each weight from the fulcrum and calculate the moment

4. Repeat placing the weights in different positions or adding more weight

Just for fun

Try and find the weight of an unknown object…

How do you think we can do this?

*Phet annimation

Homework

Q3, 4 Page 286

In your pairs bring in a piece of cardboard for next week!

Centre of GravityPage 281

Challenges!

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Centre of Gravity

The center of gravity of an object is the point through which all the weight of the

object appears to act

Investigation to find the centre of gravity of Irregularly Shaped Cardboard

Apparatus:

• Cardboard• Retort Stand• Cork• Pin• Weight• String• Pencil• Ruler

Equilibrium and The Stability of Objects

An object that is balanced and not moving is said to be in equilibrium. There are three states of equilibrium : stable, unstable and neutral

An object can be made more stable by:• giving it a low centre of gravity• giving it a wider base

An object will topple if a vertical line through its centre of gravity falls outside its base.

States of Equilibrium