1. To start
A metre rule of mass 0.25 kg rests horizontally on two ping pong balls at the 5cm and 75 cm marks.
Sketch the arrangement
Calculate the support force on each ping pong ball
State any assumptions you have made
Challenge: Explain why different amounts of force can act on different supports.

3. A LEVEL PHYSICS Year 1 Centre of Mass A* A B C
I can explain the difference between a stable and unstable equilibrium (grade d)
I explain when a tilted object is about to topple (grade b)
I can explain why an object with a lower centre of mass is more stable (grade a)

4. Centre of mass
The centre of mass of a body is the point through which a single force on the body has no turning effect
The centre of mass of an object is the single point you can consider its whole weight to act through.

5. Tall/thin objects are easier to knock over than short/wide objects
Tall/thin objects are easier to knock over than short/wide objects. The centre of mass is higher on tall objects with a narrow base. An object is very stable if it has a low centre of mass and wide base area

7. In a stable equilibrium, when an object is displaced from its equilibrium position it will return to its equilibrium position.
This is because the COM is directly below the point of support when the object is at rest. The support force and weight are equal and opposite so when its displaced the weight returns the object back to equilibrium.
In unstable equilibrium, when the object is displaced it will not return to equilibrium.
This is because the COM is directly above the point of support when the object is at rest. If the object is displaced the COM is no longer above the point of support therefore the weight acts to move the obect further from equilibrium.
For each of the following scenarios would you describe as a stable or unstable equilibrium?

8. Tilting Fd < Wb/2 F d x x W Therefore for tilting to occur: b/2d x
b/2
Only work when just lifted from ‘support’ x
Therefore for tilting to occur:
Fd < Wb/2 W

9. Toppling
Sometimes called the line of action
A tilted object will topple if the action of the weight (centre of mass) passes beyond the pivot (outside the base).
Here the COM is directly over the pivot so there is no resultant moment. This is the furthest it can be tilted without toppling.

10. Quick check Standard Challenge
Ginny puts all her books on the top shelf of her bookcase. Harry puts all his books on the middle shelf. Which book case is more stable? Why?
An empty wardrobe of mass 50kg has a square base of 0.75m x 0.75m and a height of 2.0m. A horizontal force is applied to the top edge of the wardrobe to make it tilt. Calculate the force needed to lift the base off the floor along one side.
An empty wardrobe of mass 50 kg has a square base of area 0.81m2 and height of double the length of the base. A horizontal force is applied 20cm below the upper edge of the wardrobe. Calculate the force required to lift the base off the floor along one side.

11. Questions Standard Challenge Practice tilting and toppling questions:
Summary Questions
Q1+2 on page 109 of the AQA textbook.
Make sure you ask for help if you get stuck!
Mark your answers (in the back of the textbook)
Tilting and toppling on a slope.
Q 3 on page 109
Read the ‘On a slope’ section on page 108. We will also be going through this in more detail tomorrow.

12. To finish Exam question The task
A car of mass 10000N has a wheel base of 1.25m and a height of 1.70m. Calculate the force that must be that must be applied at 1.60m to tilt the car. (3 marks)
When placed on an adjustable slope, what angle would cause the car to topple. (3 marks)
In groups you are to discuss this question without writing anything for two minutes.
After two minutes you are to complete the exam question on your own
The lowest mark counts