1.2.3 Equilibrium

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

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

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

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

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

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

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

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

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

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

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

Assessment Question OCR AS Physics Page 53 question 5

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

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

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

Density questions OCR AS Physics Page 43 questions 1 - 4

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

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

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

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

Pressure questions OCR AS Physics Page 45 questions 1 - 2