Answer:
 1470kgm/s
Step-by-step explanation:
Given parameters: 
Mass of the rock = 50kg 
Time taken for the free fall = 3s 
Unknown: 
Change in momentum = ? 
Solution: 
The change in momentum will be difference between the ending momentum and finishing momentum. 
 Momentum is the product of mass and velocity
 Momentum = mass x velocity 
Initial momentum = 0, the velocity is 0 
Final momentum = mass x final velocity 
 let us find the final velocity;
 V = U + gt 
V is the final velocity 
U is the initial velocity
 g is the acceleration due to gravity = 9.8m/s² 
t is the time 
 V = 0 + 9.8x3 = 29.4m/s 
So;
 Change in momentum = 50 x 29,4 = 1470kgm/s