1. 1.2.3 Equilibrium

2. Objective
Draw and use a triangle of forces to represent the equilibrium of three forces acting at a point on an object

3. Forces
Make a list of all the different types of forces you can think of:
Pushes and pulls
Weight
Friction
Drag
Upthrust
Contact/Reaction Force
Tension
Write a sentence or two about each force, with examples

4. Triangle of Forces
Consider the Worked Example on page 61 of “Physics 1”

5. Turning Forces
Explain that a couple is a pair of forces that tends to produce rotation only
Define and apply the torque of a couple
Define and apply the moment of force

6. Couple
A couple occurs when two forces are equal and opposite to each other, but are not in the same straight line
Upward force balances downward force
Resultant equals zero
Tend to produce rotation

7. Couple - definition
A couple is a pair of equal and parallel but opposite forces, which tend to produce rotation only x x

8. Torque Describes the turning effect of a couple
Measured in Newton metres
Used to describe turning forces e.g. car engines
torque = one of the forces x perpendicular distance between the forces
6 N
2 m
torque = force x perp. distance
= 6 N x 2 m
= 12 Nm
6 N

9. Moment of a Force
The moment of a force is the turning effect of a single force
Measured in Newton metres
Principle of Moments states that:
For a body in rotational equilibrium, the sum of the clockwise moments equals the sum of the anticlockwise moments x F p p x F

10. Moment of a Force - definition
The moment of a force is the force multiplied by the perpendicular distance from a stated point

11. Example A wheelbarrow is loaded as shown
Calculate the force that the gardener needs to exert to hold the legs off the floor
What force is exerted by the ground on the legs of the barrow (taken both together) when the gardener is not holding the handles?
400N
0.50m
1.20m

12. Questions Physics 1 OCR AS Physics SAQ 5.5 & 5.6
Page 56 questions 1 to 4
OCR AS Physics
Page 41 questions 4 - 7

13. Assessment Question
OCR AS Physics
Page 53 question 5

14. Select and use the equation for density ρ = m/V

15. Density Density = mass volume where density (ρ) is in kg m-3
mass (m) kg
volume (V) m3

16. Density Density of solids and liquids varies little with temperature
Density of gases varies considerably

17. Density questions
OCR AS Physics
Page 43
questions 1 - 4

18. Pressure Select and use the equation for pressure – p = F/A
where F is the force normal to the area A

19. Pressure Pressure = force area normal to force
where pressure (p) is in Pa or N m-2
force (F) N
area (A) m2

20. Pressure at a depth in a fluid
volume = Ah
mass m = Ahρ
weight = mg = Ahρg = force F
pressure on base of column
= Ahρg / A
= hρg A h
Liquid of density ρ

21. Floating For equilibrium, force upward = weight downward So,
calculate force upward from weight downward

22. Pressure questions
OCR AS Physics
Page 45
questions 1 - 2