Why a fielder does takes his hand backward while catching?

Dhruvin Kaneria | Laws of physics are already inbuilt in us. We apply Newtonian mechanics unknowingly in almost all of our actions. What we need is curious eye and imagination in order to understand this phenomenon with the help of physics. When I was in 9th grade, I wondered
why a fielder withdraws his hands backward just before catching.So, I googled the thing, asked my teacher only to find 1 answer ‘the increase in time for change in momentum reduces the force of impact’. No satisfactory explanation was provided even on the internet.


The next thing I did was brainstorming. I asked myself questions like: which is the time that’s “increasing”? The overall time or the time of collision? How does it affect the force experienced by our hands? After an hour, I came up with my own reason, that at least satisfied me. So, here it is.


Let the initial velocity of the ball be u. So initial momentum of the ball is mu. After collision, the final momentum of the ball is o. So, the change in momentum is |mu|. Now, when the fielder moves his hands backward, he is actually moving his hands in the direction of ball so as to create a relative velocity. The relative velocity being less than u [as both bodies are moving in same direction restricts the change in momentum to something less than |mu|. So, the impulse of force experienced by the fielder is less.

Impulse 1 (without hands going backward) = mu

Impulse 2 (with hands going backwards) = m(u-h); where h is the velocity with which fielder moves backwards.

Certainly, u is greater than (u-h) and therefore impulse 1 is greater than impulse 2.

So when we take our hands backward, the velocity with which the ball strikes is not actually u but relative velocity u-h. Hence, change in momentum is less and so is impulse of force.
(Here I have considered impulse because time period is very less. The same thing can also be explained based on comparison of forces.)

So the explanation of RELATIVE VELOCITY is what I find more convincing. All other sources consider the “increase in time” as the reason. (Though mathematically, both reasons are similar; at the end of the day the fielder is finding it easy to catch the ball by withdrawing hands!!!)

Article submitted by Dhruvin Kaneria.