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

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sics 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

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sics 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 onlyDefine and apply the torque of a coupleDefine and apply the moment of force

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sics 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

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sics Couple - definition

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

x

x

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sics 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

6 N

2 m torque = force x perp. distance= 6 N x 2 m= 12 Nm

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sics 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

F

x

p

F

p x

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sics Moment of a Force - definition

The moment of a force is the force multiplied by the perpendicular distance from a stated point

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sics Example

A wheelbarrow is loaded as shown

a) Calculate the force that the gardener needs to exert to hold the legs off the floor

b) 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

1.20m

0.50m

0.20m

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sics Questions

Physics 1 SAQ 5.5 & 5.6 Page 56 questions 1 to 4

OCR AS Physics Page 41 questions 4 - 7

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sics Assessment Question

OCR AS Physics Page 53 question 5

Density

Select and use the equation for density

ρ = m/V

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sics Density

Density = mass

volume

where density (ρ) is in kg m-3

mass (m) kg

volume (V) m3

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sics Density

Density of solids and liquids varies little with temperature

Density of gases varies considerably

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sics 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

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sics Pressure

Pressure = force

area normal to force

where pressure (p) is in Pa or N m-2

force (F) N

area (A) m2

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sics 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

Liquid of density ρ

h

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sics Floating

For equilibrium, force upward = weight downward

So, calculate force upward from weight downward

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sics Pressure questions

OCR AS Physics Page 45

questions 1 - 2