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