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posted Jun 23, 2016, 2:33 PM by Upali Salpadoru   [ updated Jun 26, 2016, 7:35 PM ]

Have you ever wondered why a barrel rolls when you push?

Why can’t it move forward just like a box?

It all depends on what we call TORQUE.   

What is torque?

T 1.jpg

If you have used a spanner you would have noticed that the turning effect will change according to the position of the handle.  In the illustration, the spanner arm is about 45 below the horizontal.

What do you think is the turning effect here?

Is it  T= 50 x L      or     50 x d  ?

As the force is acting down, displacement is the perpendicular distance. (Not the length of the spanner)

As the spanner turns, when the force comes in line with Fulcrum or C the centre of the circle , what will be the turning effect?  It will be zer as there is no displacement between the force and fulcrum?

Torque gives the turning ability due to a force.
It is measured by the product of force and displacement.

So  Torque =  Force x perpendicular distance to the Fulcrum.

Example 1.


Fig. Balancing coins.

Some coins are placed on a uniform ruler as shown. Weigh of a coin is 1 N. Find the following:-

  1. Clockwise torque.

  2. B. Anti clockwise torque.

  3. Which way the ruler will turn.


  1. C.W Torque  =   2x 0.5   =  1 Ncm.

  2. ACW Torque = 3 x 0.25   = 0.75 Nm.

  3. It will turn Clockwise.

When the fulcrum is not given?

Example 2Acrobats.jpg

Liz, who weighs 320 N is sitting on a uniform plank 1m. from B.  weighing 450 N. .Length of plank is 4m.

Find the weight on the heads of  two people.

It is convenient to assume the Fulcrum to be either at A , or at B..

Let us assume the fulcrum to be at A.

Then Clockwise Torque would be = Support force at B x 4 Nm.

ACW torques  =  (320 X 3) + 450 X 2 Nm.

As the forces are balanced

B X4 =   960 + 900

    B   =       1860/4   ………. = 465 N,

Up forces are =   A + 465 N

Down Forces are =  320 + 450 N

As the [arallel forces are equal

   A + 465  =  320 + 450

       A      =  770 - 465………………………= 305 N

 What is a couple?



A couple is a pair of equal and opposite forces which are not in a straight line.

The arrows here show the forces that form a couple to turn the wheel.

What is the magnitude of the force?

It is equal to  10 x 0.3 =   3.0 Nm.

Q. 1.0

The seesaw has a weight of  300 N.  While Jenny balances him.



a. The weight of Jenny

b.. The support force at F.

Q. 2.02.0.jpg

A rod is hanging from two force measurers A and B as shown. The centre of mass is at G. The weight of rod is 25N.

Find the readings on A and B.

10 x2 = 20 marks.

Q, 4.0


A tyre weighing 40 N and a radius of 0,35 m. is on a ramp.

A.   Label the numbered forces.

4x5= 20 marks.

B- Find the following

    1.. Normal reaction due to the weight.

    2. The force that pulls the tyre along the slope.

    3.. The torque that turns the wheel

                                                  5x3= 15 marks.