When you jump from a height to the ground, you let your legs bend at the knees as your feet hit the floor. Why we do this in terms in physics in momentum?

Force:

Force is defined as the product of mass and acceleration. But it can also be defined as the rate of change of momentum with respect to time. That is: F =dp/dt

Explanation:

Now as we land on the floor, we bend legs at knees. This is to slow the change in the momentum of the body with time, so we apply less force on the ground and in return ground applies less force on the legs. This makes landing smooth. While we catch the ball we move hand backward even after catching it for small-time, this makes catching easier we experience less force.