1. 1.2.3 Equilibrium Objectives Draw and use a triangle of forces.
Week 6
1.2.3 Equilibrium
Objectives
Draw and use a triangle of forces.
Complete Practical 6 to prove the equilibrium of 3 forces satisfies the triangle of forces idea
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2. Dynamics is involved with looking at the forces on objects that are in motion whereas ……
Statics looks at the forces acting on objects that are stationary
If we are considering forces acting on an object they will always be acting in the same plane but not necessarily the same direction.
These are called coplanar forces.
An example would be the screen all the arrows are in the same plane as the screen NOT coming out of it or going into it!

3. YES NO
YES NO
YES NO NO
So equal forces in opposite directions doesn’t always mean equilibrium. There can be a net turning moment.

4. There is NO NET FORCE in any direction
In order to solve problems concerning objects that are static or in EQUILIBRIUM you need to remember the following points.
There is NO NET FORCE in any direction
Resultant Vertical force is Zero
Resultant Horizontal force is Zero
2. There is NO NET MOMENT about any point ie.
clockwise moment = anticlockwise moment
To make life easier we use two standard practices:
a) So that we can think about the forces easily we turn them into vertical and horizontal components only
b) Consider the moments about one point through which 2 forces are acting

5. If there are more than two forces in several directions we need to remember that for an object to truly be in equilibrium, two conditions must be satisfied.
If you draw the forces nose to tail they form a closed loop
2) All the forces must have lines of action that go through the same point, if just one force doesn’t go through that point it will not be in equilibrium as it would spin.

6. How scale drawings are used to add forces together
Week 6
How scale drawings are used to add forces together
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7. Triangle of forces Week 6
This diagram shows the equilibrium of 3 forces which complete the triangle. D
In the case of finding the resultant above A+ B = D
Where as in this case A + B + C = 0 therefore A + B = -C
Which means that C is the exact opposite of the resultant of
the two forces A and B when they combine!
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8. Climber stationary on a rock face
Week 6
Climber stationary on a rock face
Forces acting on climber
If X is 400N calculate Y.
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9. 400 Week 6 Use the “Triangle of forces” © Pearson Education Ltd 2008
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10. Week 6 Question 2 page 35 - Kite © Pearson Education Ltd 2008
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11. 1.2.4 Centre of Gravity Objectives
Week 6
1.2.4 Centre of Gravity
Objectives
• Define the centre of gravity of an object.
• Describe a simple experiment to determine the centre of gravity of an object.
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12. Finding theCentre of gravity of a jigsaw piece
Week 6
Finding theCentre of gravity of a jigsaw piece
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13. Week 6
Centre of gravity of a cup – not all shapes are nice and regular and the Cof G may not actually be in the material of the object itself!
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14. 1.2.5 Turning Forces Objectives • Explain what a couple is.
Week 6
1.2.5 Turning Forces
Objectives
• Explain what a couple is.
• Define and apply the torque of a couple and the moment of force.
• Apply the principle of moments to solve problems.
• Explain that the net force and net moment on an extended object in equilibrium are both zero.
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15. Week 6
Typical loading diagram – it may be assumed that the building is safe but it is not just about the walls being strong enough – the wrong design could cause it to topple.
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16. Moment of force = Fx units are Nm
Week 6
Moment of force = Fx units are Nm
A moment is the turning effect of a single force on an object about a pivot point.
The distance is the perpendicular distance between the line of action of the force and the pivot point
If an object is in equilibrium
the clockwise moment and
anticlockwise moment must
be equal.
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17. The moment of a force is the turning effect of the force
The moment of a force is defined as the product of the force and it’s perpendicular distance from the point of rotation
Moment = force * perpendicular distance
Nm N m
If a rigid body is in equilibrium, the sum of the clockwise moments will ALWAYS be equal to the sum of the anticlockwise moments.

18. Week 6
A Couple – Two forces not in line that can only cause rotation because they are equal and opposite in magnitude but act through different points of an object. Therefore producing no linear acceleration
The torque is the turning effect of the couple and is calculated as the product of the magnitude of one forces and the distance between them.
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19. Week 6 Carrying a load Idealised sketch
Can you explain why the forearm is a poor solution to the problem of carrying loads?
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