## August 26, 2012

### Why a cricketer lowers his hands while catching a ball?

• We know how to calculate the kinetic energy of moving objects -- isn't that enough? No. It turns out that many situations involving collisions do not obey the simple conservation of Mechanical Energy. Why not? Because it takes energy to bend, break, mutilate and deform objects, energy which disappears from the kinetic and gravitational potential energy.

• But a different quantity is conserved, even during collisions. The linear momentum of an object is defined as
```                   p  =  (mass) * (velocity)
```
It is a vector quantity, and the total linear momentum of a bunch of objects will remain the same, before and after a collision.

• Momentum is connected to force by impulse, which is simply
```            impulse   =  (force) * (time)
```
if the force has a constant magnitude during its action. If the force changes with time, then one must integrate to find the impulse:
```                         /
impulse   =  |  (force) dt```
```                         /
```

• The Momentum-Impulse Theorem states that the change in momentum of an object is equal to the impulse exerted on it:
```           (change in momentum)    =  (impulse)

p      -   p          =  (force) * (time)
final      initial

m*v      -  m*v         =  (force) * (time)
final       initial```

Impulse  It is effect of force acting for a short interval of time.
According to Newton’s 2nd law
Rate of change of momentum = Force applied.
If time period dt is  increased than Impact force decreases.

Why a cricketer lowers his hands while catching a ball

If suppose a 0.16 kg cricket ball hits a bat at 100 km/hr and then comes off the bat at 100 km/hr in 0.001 sec. The average force on ball is 8800 N which is enough to lift 880 kg of mass off the ground.  That is why it hurts to get struck by ball. Therefore while catching cricketer lowers his hands to increase time interval of change in momentum thereby impact force reduces and the chances of any injury and avoiding reaction of hand that can drop the catch .