1. Learning Objective Describe and calculate moments To be able to:
Key Words:
Turning force, distance, lever

2. Exam question Wednesday, 22 May 2019 Moments
Date and title in books. You need a pen, pencil & ruler.
Starter Activity – 5 minutes
Exam question

3. Learning Outcomes Describe the principal of moments
Target 1-9
State what is meant by a moment
(F/H)
Target 4-9
Calculate a moment
(H)
Target 5-9
Describe the principal of moments
Extension: Application of Knowledge
Target 8/9
Explain how gears and levers work
Learning Outcomes

4. Balanced and unbalanced forces
Boardworks GCSE Separate Sciences: Physics
Mechanics
When forces in opposite directions have the same value, they cancel each other out. The forces are balanced.
The object is either stationary or moves at a constant speed.
If the forces on an object are not balanced, there is a resultant force, and the object accelerates in the direction of this force. 4

5. Turning effects from balanced forces
Boardworks GCSE Separate Sciences: Physics
Mechanics 5

6. Boardworks GCSE Separate Sciences: Physics
What is a moment?
Boardworks GCSE Separate Sciences: Physics
Mechanics
When a force applied to an object makes it turn about a pivot, this turning effect is called a moment.
The size of the moment depends on two factors. What are they?
The bigger the force, the larger the turning effect.
The larger the perpendicular distance from the pivot to the point where the force is applied, the larger the turning effect. 6

7. Perpendicular distances
Boardworks GCSE Separate Sciences: Physics
Mechanics
Teacher notes
This multiple-choice quiz could be used as a plenary activity to assess students’ understanding of moments. 7

8. The moment equation
Boardworks GCSE Separate Sciences: Physics
Mechanics
The size of a moment is found using the moment equation:
perpendicular distance from pivot (m)
moment (Nm) = force (N) ×
When children push a roundabout to make it accelerate,
the acceleration can be increased without changing the force if the children increase their perpendicular distance from
the pivot.
force
perpendicular distance 8

9. Clockwise and anticlockwise moments
Boardworks GCSE Separate Sciences: Physics
Mechanics
When Mark and Tim lift their feet off the ground, what will happen to the see-saw?
Tim
Mark
The force of gravity acting on each person creates a turning effect on the see-saw. On Tim, it creates an anticlockwise moment. On Mark, it creates a clockwise moment.
The clockwise moment is larger than the anticlockwise moment, so the see-saw will turn in a clockwise direction. 9

10. The principle of moments
Boardworks GCSE Separate Sciences: Physics
Mechanics
If an object is balanced about a pivot, then we can say that:
sum of anticlockwise moments
sum of clockwise moments =
This is the principle of moments.
When this principle is fulfilled, the object does not turn or rotate, but it may still be moving. The forces on the object may not be balanced, or the object may be moving at a constant speed.
Teacher notes
Student attention could be drawn to the fact that in this image, the see-saw is perfectly horizontal even though neither child has their feet on the ground. The moments are balanced, showing that the two children are the same weight. 10

11. Using the principle of moments
Boardworks GCSE Separate Sciences: Physics
Mechanics
Teacher notes
This activity tests pupils’ understanding of both the moment equation and the principle of moments. Set up a combination of weights on the left-hand side of the scales and ask pupils to work out what to add to the right-hand side to balance the scales. Pupils could also be asked to think of as many different solutions as possible to each problem.
Arrangements of weights on the left (and some possible answers) might include:
400g at 10cm (400g at 10cm; 200g at 20cm; 800g at 5cm)
100g at 17cm (100g at 17cm is the only solution)
700g at 20cm (using all four hooks: 600g at 10cm with 400g at 20cm; 1000g at 12cm with 100g at 20)
100g at 20cm, 12cm and 6cm (200g at 19cm is the only solution) 11

12. Boardworks GCSE Separate Sciences: Physics
Levers
Boardworks GCSE Separate Sciences: Physics
Mechanics
The explorer Minnesota Adams needs to move a 1000 N boulder that is blocking a pyramid. She can only apply a 20 N force, but she also has a 6 m metal pole. What does she do?
Minnesota puts one end of the pole under the boulder.
She props the pole on a stone 0.1 m from the boulder.
She pushes down on the other end of the pole and the boulder rolls away.
Teacher notes
The boulder weighs 1000 N but is only 0.1 m from the stone that is acting as a fulcrum or pivot. This means the anticlockwise moment is 1000 × 0.1 = 100 Nm. When Minnesota pushes down on the other end of the pole, she applies a maximum force of 20 N at 5.9 m from the pivot. The clockwise moment is 20 × 5.9 = 118 Nm. There is a resultant moment and the boulder is lifted up. This knocks the boulder out of place and makes it roll away from the tomb entrance.
Why did this work? 12

13. Calculations with moments
Boardworks GCSE Separate Sciences: Physics
Mechanics 13