1. A LEVEL PHYSICS Year 1 Introducing Moments A* A B C
I can define a ‘moment’ (Grade D)
I can use the principle of moments to calculate balanced forces (Grade B)
I can describe why the centre of mass of an object is important (Grade A)

2. To start
A picture is hung from a hook using a single piece of string. The string makes an angle of θ to the vertical on each side of the hook. If the tension in each half of the string is T write an expression for the weight of the picture, W, in terms of T.

3. Moments
The moment of a force about any point is defined as the force x the perpendicular distance from the line of action of a force to the point.
In which direction do the turning forces act? How can we increase the magnitude of the force?

4. Moment = Force x Perpendicular Distance
Determine the units for momentum

7. Why are most door knobs on one side of a door?

9. For an object at equilibrium the sum of the clockwise moments must equal the sum of the anticlockwise moments (principle of moments).
Write an expression to describe the system at equilibrium below:

10. Are the following balanced or unbalanced?
John is 1m from the pivot, Peter is 1.4m from the pivot?
John is 0.4m from the pivot, Peter is 0.56m from the pivot?
John is 1.2m from the pivot, Peter is 1m from the pivot?
John is 0.5m from the pivot, Peter is 1m from the pivot?
2. A counterweight is used to make a crane balance.
a) How far away must the 5,000N load be in order for the crane to be balanced?
b) If the load is increased to 10,000N how far away should it be?

11. Solutions to Calculating Moments Exercise A*/A
1. (a) 24 Nm ACW (b) 300 Nm CW (c) 0.3 Nm CW
2. (a) Nm ACW (b) 1.5 Nm ACW
Nm CW
Solutions to Calculating Moments Exercise B/C
1. (a) 28 Nm ACW (b) 0.4 Nm CW
2. (a) 20 Nm CW (b) 36 Nm CW
Nm CW

12. Next Lesson: Advanced Moments
Lesson after that: Motion along a straight line