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

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.