Two boxers are fighting. Boxer 1 throws his 5 kg fist at boxer 2 with a speed of 9 m/s.
Boxer 2 stops the punch with a force of 45,000 N.
How much time did it take for boxer 1's hand to stop moving?

0.001 s

Explanation:

The force applied on an object is equal to the rate of change of momentum of the object:

where

F is the force applied

is the change in momentum

is the time interval

The change in momentum can be written as

where

m is the mass

v is the final velocity

u is the initial velocity

So the original equation can be written as

In this problem:

m = 5 kg is the mass of the fist

u = 9 m/s is the initial velocity

v = 0 is the final velocity

F = -45,000 N is the force applied (negative because its direction is opposite to the motion)

Therefore, we can re-arrange the equation to solve for the time:

+15-35=-20

going in opposite directions