Moments
In a nutshell
A force applied to an object can cause a turning effect, this turning effect is called a moment. The point around which the rotation happens is called the pivot.
Equations
WORD EQUATION
| SYMBOL EQUATION
|
moment=force×distance | M=F×d |
Variable definitions
QUANTITY NAME
| SYMBOL
| UNIT NAME
| UNIT
|
| | | newton−metre
| |
| | | | |
distance
| | | |
Moment calculations
A moment is the turning effect of a force, it depends on both the size of the force being applied and the distance which is normal to the direction of force. In other words, this is the perpendicular distance between the line of action of the force and the pivot. The line of action is the line along which the force acts, so the perpendicular distance is at right angles to the line of action of the force.
The size of a moment can be calculated using the following formula.
moment=force×distance
M=F×d
As force is measured in newtons and distance in metres; moments are measured in newton-metres, Nm.
Example
If a child is sat at one end of a seesaw, what is the moment generated by their weight if they are sat 2 m away from the pivot and they weigh 300 N?
First write out the quantities needed and make sure they are in the correct for,:
f=300Nd=2m
Next, write down the equation you need to use:
moment=force×distance
Then, substitute the values into the equation:
Moment=300×2=600
Don't forget to include your units:
600Nm
A 300N child sat on a see-saw 2m away from the pivot has a moment of 600Nm.
Principle of moments
Sometimes objects can become balanced about their pivot, this happens when the clockwise moments equal the anti-clockwise moments. The clockwise moments will cause a clockwise turning effect, and the anti-clockwise moments will cause a turning effect in the opposite direction. This means that the object balances out in both directions and experiences no overall turning effect.
For an object in balance, the principle of moments holds:
total anti−clockwise moments=total clockwise moments
