Moments

Question 1

Centre of Gravity

Answer 1

Centre of mass: Of an object is the single point that you can consider its whole weight to act through
An object will always balance around this point
The centre of gravity tells us how stable an object will be

Question 2

Centre of Gravity for Regular Shaped Objects

Answer 2

Centre of gravity is found where lines of symmetry meet
If an object is describe as uniform (Look for in questions) -> It means it’s weight (vertically downwards) is acting at the centre of the object

Question 3

Experiment to find the centre of gravity of an irregular object

Answer 3

Hang the object freely from a point, usually one corner
Draw a vertical line downwards from the point of suspension, using a plumb bob (ball on a string) to get the line exactly vertical
Hang the object from a different point
Draw another vertical line
Repeat this for a third line
The point where the three line intersect is the centre of gravity

Question 4

Moments - Of a force

Answer 4

The turning effect of a force about some axis or a point
Definition: Moment = Force x perpendicular distance to the line of action of force from the axis or point of rotation (pivot
Moment = Fx
F: Force in Newtons
x: Perpendicular distance in metres
Units are Newton-metre (Nm)

Question 5

Perpendicular Distance

Answer 5

Perpendicular distance from the pivot
Either:
Use trigonometry to find the perpendicular distance
or
Resolve the force, F, into it’s perpendicular components

Question 6

The Principle of Moments

Answer 6

When a body is in equilibrium, the resultant force acting on it is zero, AND the resultant moment is zero

Question 7

What does the Principle of Moments State

Answer 7

For a body in equilibrium, the sum of the anticlockwise moments about any point is equal to the sum of clockwise moments about that same point
This can be used to solve problems for an object in rotational equilibrium
Σclockwise moments = Σanticlockwise moments
AND F(R) = 0

Question 8

Problems with two support points

Answer 8

If not told to, take pivots at a specific point -> Choose a sensible place to put the pivot
If there is a force acting through the pivot -> Perpendicular distance is zero so moment is zero
Do not use pivots where the force is acting
To find the force acting at the point F(R) = 0N -> Only if the object is in equilibrium Σforces = 0N

Question 9

Couples

Answer 9

A couple is a pair of forces equal in size, acting parallel to each other but in opposite directions
A couple does not produce a linear force but does produce a turning effect

Question 10

Conditions for a Couple

Answer 10

Equal in magnitude
Parallel but opposite in direction
Separated by a perpendicular distance, d

Question 11

Torque

Answer 11

Vector
The turning effect of a couple is called torque
Torque of a couple = size of one of the force x perpendicular distance between the forces
The pivot is the midpoint between the two forces
Torque = F x d - Units are Nm