Moments

Question 1

What is a moment ?

Answer 1

Force applied multiplied by the perpendicular distance of a force from a point
They can either be clockwise or anticlockwise

Question 2

What are the rules for equilibrium ?

Answer 2

Sum of forces up = the sum of forces down
Sum of forces left = sum of forces right
Sum of moments clockwise = sum of moments anticlockwise

Question 3

What if the object is in equilibrium and the resultant force is 0

Answer 3

The resultant moment about that point must also be 0

Question 4

What is the principle of moments ?

Answer 4

Sum of moments clockwise is the sum of moments anticlockwise

Question 5

Steps to tackling a moments question

Answer 5

Sketch a simplified diagram
Choose a point to take moments about (where one of the unknown forces act )

Question 6

What is a couple ?

Answer 6

A pair of equal and opposite forces acting on a body but not acting on the same line
When a single force acts on an object through a point other than its centre of mass the force has two effects:
The object accelerates in the direction of the force (2nd law) and it rotates about its centre of mass
If we want the object to rotate but don’t want its centre of mass to accelerate we need another force, parallel to the first but in the opposite direction

Question 7

What does it mean when an object is stable ?

Answer 7

It returns to its original position when you tilt it on a surface
If you tilt it further and the line of action of the weight of the body coming down from its centre of mass is outside the point at where it tilts it will topple