Given,
Initial linear velocity =u=7 m/s
Initial angular velocity ω=0
Initially due to slipping, friction will act opposite to the direction of motion, due to which torque will act. This will produce angular acceleration (in clockwise dirction). The angular speed starts increasing and linear speed starts decreasing.
After some time sphere starts pure rolling. Let its linear velocity be v and angular velocity ω′.
Then, v=ω′R ...(1)
In that situation, there will be no sliding between sphere and ground.
Hence, fk=0,α=0
ω′=constant, v=constant
By conservation of angular momentum about point P (τnet=0)
Li=Lf
⇒−muR=−mvR−Iω′
⇒muR=mvR+(25mR2)(vR)
⇒u=75v
⇒v=5u7=5 m/s