Why do two isotopes of an element have the same atomic number but different mass numbers?

Because isotopes all contain the same number of protons.

here we know that ball will drop down by total distance 3.2 m

so here we know that

now using kinematics

now in the same time it will cover horizontal distance of 0.75 m

so speed of the ball will be

friction

explanation:

they us to move forward and backwards