Physics- P3 (chapters 1 and 2)AQA
The turning effect of a force is called a moment:
Moment of a force (Nm)=force (N) * Perpendicular distance from the pivot to the line of action of the force(m)
To increase a moment:
· Either the force must increase
· Or the distance to the pivot must increase
For example it is easier to undo a wheel nut by pushing a long spanner than a short one. That’s because the long spanner increases the distance between the line of action of the force and the pivot.
Note ‘perpendicular distance’ means the shortest distance from the line that the force acts along.
Centre of Mass
Although an object is made up of many particles, its mass can be thought to be concentrated at one single point. This point is called the centre of mass. Any body that is freely suspended will come to rest with its centre of mass directly below the point of suspension.
You can find the centre of mass of a thin sheet of material as follows:
· Suspend the thin sheet from a pin held in its clamp stand. Because it is freely suspended, it is able to turn
· When it comes to rest hang a plumbline from the same pin
· Mark the position of the plumb line against the sheet.
· Hang the sheet with the pin at another point and repeat
· The centre of mass is where the lines that marked the postion of the plumbline cross.
The position of the centre of mass depends on the shape of the objects, and sometimes lies outside the object. For a symmetrical object, its centre of mass lies on the axis of symmetry. If the object has more than one axis of symmetry, the centre of mass is where the lines meet.
Moments in balance
For an object in equilibrium: the sum of the anticlockwise moments about any point is equal to the sum of the clockwise moments of that point. If an object is in equilibrium it is balanced, not turning. We can take the moments about any point and will find total clockwise moment = total anticlockwise moment. There are lots of everyday examples of the principles of moments, such as seesaws and balance scales.
The stability of an object is increased by making its base wider and its centre of mass lower.
An object will tend to topple over if the line of action of its weight is outside its base.
The line of action of the weight of an object acts through it’s centre of mass. If the line of action of the weight lies outside the base of an object, there will be a moment and the object will tend to topple over. The wider the base of an object and the lower its centre of mass, the further it has to tilt before the line of action of its weight moves…