Answer:
Step-by-step explanation:
Impulse = change in momentum 
Initial momentum = mass x initial velocity = 100 x 5 = 500 kg m/s 
final momentum = mass x final velocity = 100 x - 4 = -400 ( - ve sign due to reversal of direction ) 
change in momentum = final momentum - initial momentum 
= - 400 - 500 = - 900 kg m/s .
As it is - ve , it acts upwards . 
So magnitude of impulse on Perter = 900 kg m/s