Physics GCSE: Forces, Moments and Pressure

Moments

Why door handles are near the edge of doors.

Moments make things turn or rotate. They are caused by forces but are not forces themselves. Like forces, moments have a direction. We say they are either clockwise or anti-clockwise, to show which way they will make something turn.

The bigger the force causing the turning effect the bigger the moment will be.

The further the force is from the pivot the bigger the moment will be.

The size of a moment can be calculated using:

Moment = Force x Distance

• Force is measured in newtons, N.
• Distance is measured in either m or cm.
• If the distance is in m then the moment will be measured in Nm.
• If the distance is in cm then the moment will be measured in Ncm.

(Note: The force needs to be at right angles to the lever or rotating object.)

Here are some pictures involving moments.

For each picture:

Click on the moment and the direction that you think is correct then mark your answer.

In many situations there is more than one moment acting. To find the net or resultant moment the moments have to be added or subtracted, depending on their direction. Here is a worked example:

Moment due to A = 20 x 2 = 40 Nm anti-clockwise

Moment due to B = 10 x 1 = 10 Nm anti-clockwise

Moment due to C = 20 x 3 = 60 Nm clockwise

The total anticlockwise moment is 40 +10 = 50 Nm.

The total clockwise moment is 60 Nm.

So the resultant moment is 60 Nm - 50 Nm = 10 Nm clockwise.

Now here is one for you to try. Fill the correct numbers into the boxes: