Question

a 10.0 kg ball falling at 10.0 m/s hits a mattress and comes to a complete stop in 1 s. what force does the mattress exert to stop the ball

Answer 1

Using Newton's second law;

F = ma, where m = mass, a = acceleration or deceleration

a = Δv/t = (v-u)/t, but v= 0, u = 10 m/s, t = 1.
Then,
a = (0-10)1 = -10 m/s^2

Substituting;
F = ma = 10*-10 = -100 N

The  mattress exerts 100 N to stop the ball.