AQA GCSE Gears and moments(Physics)

Gears

A gear is used to change the moment of a turning effect. 

cog and moment
Formula for calculating moments using force x distance. All 3 rearrangments are shown along with formulae triangle

Different sized gear cogs. 

The larger the diameter of the gear cog, the greater the distance between the pivot and where the line of action of the force will be.

Small and large cog, with different radius,

As the distance increases, then the size of the moment will increase. 

So, a larger cog, will have a bigger moment. 

Calculating Moments using gear cogs.

If we look at an example of a small cog turning a large cog:

Large and small cogs rotating

A force of 40N is applied to the small cog. Calculate the moment that the small cog produces. 

5cm = 0.05m

Clockwise moment of small cog = 0.05m x 40N = 2Nm

Remember on the previous page that the forces on each gear cog are equal, but opposite in direction. This means that the large cog also experiences a 40N force. 

10cm = 0.1m

Anticlockwise moment of large cog = 40N x 0.1m = 4Nm

If we look at our original defintion of a gear from the top of this page “A gear is used to change the moment of a turning effect. “

The gear cog has increased the moment in this case. So, the turning effect has increased. 

If you started with a large cog, which turns a small cog, then the output moment will be smaller than the input moment.

Practice questions

1.State the definition of a gear

2.Describe the relationship between size of gear cog and the magnitude of the moment produced

3. A small gear cog is 5cm in diameter and is turned with a force of 40N. This small cog turns a larger cog which has a diamter of 20cm. Calculate the size of the output moment from the larger gear cog.