TURNING EFFECT OF FORCE by Sukesh Kamath

Moment of force

sukeshkamath — 2016-02-01 02:48:05 UTC

The moment of a force or torque, r is defined as the turning effect of the force about a pivot and is the product of the force (F) and the perpendicular distance (d) from the line of action of the force to the pivot.

SI unit of moment of a force is Newton-metre (Nm). It is a vector quantity.

Principle of moments

sukeshkamath — 2016-02-01 03:11:55 UTC

For any object that is in equilibrium, the sum of the clockwise moment about any point provided by the forces acting on the object equals the sum of the anticlockwise moment about the same point.

How to tackle questions:

Draw a diagram and mark on all of the information you have been given and allocate the unknown a symbol (usually F or d) - in an examination annotate the one on the paper.
Identify the 'pivot point', 'turning point' or fulcrum - the point around which the whole system turns.
Identify all of the forces acting.
- If they act through the support you can ignore them (as the support will produce a reaction force that will cancel them out!)
- You may have to calculate some of the forces as they may not be given to you - you may be given 'mass' instead of weight, for example.
Calculate the perpendicular distance from each of the forces to the turning point - in advanced level questions you are rarely given the correct distance!
Work out all of the clockwise moments and add them together
Work out all of the anticlockwise moments and add them together
State the Principle of Moments
Equate the clockwise and anticlockwise moments
Find the unknown
Check that this unknown is actually the value you are asked for in the question - sometimes they ask you for a distance that requires this information before you can do the last step!
Check that you have included the correct unit in your answer and that it is to the correct number of significant figures.

Torque of Couple

sukeshkamath — 2016-02-01 03:43:06 UTC

Couples

A couple is two equal forces which act in opposite directs on an object but not through the same point so they produce a turning effect.

torque of coupe = one of the forces X perpendicular distance between the forces.

Equilibrium

sukeshkamath — 2016-02-01 03:52:36 UTC

Conditions for the Equilibrium of Three Non-Parallel Forces

If we say that an object is under the influence of forces which are in equilibrium, we mean that the object is not accelerating - there is no net force acting.The object may still be travelling - but at a constant velocity - but in most questions the object will be stationary.

The following pointers will help you to solve problems that involve a body acted on by three co-planar forces.

The lines of action of the three forces must all pass through the same point.

ii)

The principle of moments: the sum of all the clock-wise moments about any point must have the same magnitude as the sum of all the anti-clockwise moments about the same point.

iii)

a) The sum of all the forces acting vertically upwards must have the same magnitude as the sum of all the forces acting vertically downwards
b) The sum of all the forces acting horizontally to the right must have the same magnitude as the sum of all the forces acting horizontally to the left.

So - if you resolve all of the forces the 'ups' will equal the 'downs' and the 'rights' will equal the 'lefts'.

Example problem with three forces

sukeshkamath — 2016-02-01 04:12:06 UTC

sukeshkamath — 2016-02-01 05:48:41 UTC