Explain why were you asked to use the angular range 10°-20° and exclude 0-5 in Rutherford

The change is the same for each ball.

Explanation:

As we know that rate of the change in momentum is known as force on an object

So here when striped ball strikes with solid ball then during the collision the force exerted on each other must be equal and opposite

So here we can say

since time of collision is same for both so we can say

so momentum change for both balls will be same in magnitude