Answer:
The potential for r > rb is equal to zero.
Explanation:
For r > rb, the potential is:

Then, the net potential is:



The momentum of the rolling ball will have less momentum than before the collision and the stationary ball will have more momentum after the collusion.
Answer:
Total time to complete the race = 45 sec
Explanation:
We have given that each lap is 60 m long
Average speed for the first lap = 2 m/sec
So time taken to complete the first lap 
Average speed for second lap = 2 m/sec
So time taken to cover the second lap 
So total time to complete the race = 30 +15 =45 sec