## Rotational Equilibrium – A Numerical Problem

A father and his son are supposed to carry a load of 500 N. They decided to use a uniform pole that is 4.0 m long and weighs 60 N. The father and his son support the pole at the ends. Where should the load be placed so that the father supports three times as much as his son?

Posted by Daniel

Let’s represent the situation be the folowing diagram

Let the force exerted by son be f

Then the force exerted by father = 3f

Therefore the total force exerted =Total weight = 560N = 4f

Which gives f = 140 N, the force exerted by son

3f=420 N, the force exerted by father

Applying the principle of moments about the point where father holds the rod,

420 x 0 – 500 x x – 60 x 2 + 140 x 4 = 0

==> x = 440/500 = 0.88 m from the end where father holds the load

(If there is any problem you find in the solution given, please post as comment to this post.)

## Why are you hurt more when you punch a wall then when you punch a sponge?

Why are  you hurt more when you punch a wall then when you punch a sponge?

This is explained on the basis of impulse momentum principle.

Impulse of a force is the product of Force and time and is equal to the change in momentum produced.

The overall effect of a force is the change in momentum produced.

When we are punching, the effect on hitting is the stopping of the fist moving initially with a particular speed.

So, if we hit on a wall, the change in momentum takes place in a small interval of time and therefore the average force involved is very large and therefore it hurts.

When we hit on a sponge, the time taken will be more and the average force will be less and therefore it hurts less.

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