Answer: 800 N
Step-by-step explanation:
Force is change of momentum (p) over change in time: F = ma = dp/dt
change in time dt= 0.10 s
mass = 2 kg
initial velocity v0=20 m/s
final velocity vf= -20 m/s
change in momentum dp= m(vf-v0)
dp=2(-20-20)= -80 N/s
F=dp/dt=-80/.10=-800 N
Since they only want the magnitude, you can drop the negative sign so the answer is just 800 N.