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Why is it necessary to bend knees while jumping from a height?

Why is it necessary to bend knees while jumping from a height?

Asked Rahul. (via SMS from our mobile version http://m.askphysics.com )

Answer:

This is a consequence of the impulse momentum principle.

Jump from Atacama Dune

When we bent our knees on landing, the time of impact increases and reduces the average force exerted during the entire process. This prevents the chance of damage to our body.

Impulse momentum principle states that the impulse of a force is equal to the change in momentum produced.

Impulse is measured as the product of force and time interval. So, when we stop       ourselves, there is a fixed change in momentum. If we stop ourselves very fast, the time interval is less and the average force will be very large.

But, if we bent our knees during landing, the time involved in the proces of landing will be more and thus the average force will be less and hence the damage will also be less.

 

Displacement Problem

A particle has a displacement of 12 m towards east and 5 m towards north and then 6 m vertically up. Find the magnitude of the resultant of these displacements.

(Received via SMS from http://m.askphysics.com – the mobile version of AskPhysics)

Answer:

The resultant of the first two displacements – 12 m and 5m in mutually perpendicular directions is

\sqrt{\left ( 12^{2}+5^{2} \right )}=13 m

The resultant of 13m and 6m in mutually perpendicular directions is

\sqrt{\left ( 13^{2}+6^{2} \right )}=14.32 m

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