# The Principle of Moments

**The Principle of
Moments: **for a body in equilibrium
(balanced!) the sum of the clockwise moments is equal to the sum of the
anticlockwise moments.

**clockwise moments = anticlockwise
moments**

Garfield has considerably more weight than Odie - so for them to 'teeter-totter' Odie has to sit a bigger distance away from the fulcrum. To play 'see-saw' their moments have to be equal (according to the Princlple of Moments):

F_{1}d_{1} = F_{2}d_{2}

As Odie has less weight (force) he needs more distance!

**How to tackle questions:**

Draw a **diagram** and mark on all of the information you have been given and allocate the unknown a symbol (usually F or d) - in an examination annotate the one on the paper.

Identify the 'pivot point', 'turning point' or **fulcrum** - the point around which the whole system turns.

Identify all of the** forces** acting.

- If they act through the support you can ignore them (as the support will produce a reaction force that will cancel them out!)

- You may have to calculate some of the forces as they may not be given to you - you may be given 'mass' instead of weight, for example.

Calculate the **perpendicular distance** from each of the forces to the turning point - in advanced level questions you are rarely given the correct distance!

Work out all of the **clockwise moments** and add them together

Work out all of the **anticlockwise moments** and add them together

State the **Principle of Moments**

**Equate** the clockwise and anticlockwise moments

**Find the unknown**

Check that this unknown is actually the value you are asked for in the question - sometimes they ask you for a distance that requires this information before you can do the last step!

Check that you have included the **correct unit **in your answer and that it is to the **correct number of significant figures.**

### Here is a link to a video to make you think about moments:

Click here to look at stability

Here is a link to a sample question on moments.

This is an interactive PowerPoint for you to work through

Try this interactive puzzle or this one!

### Questions