Abicycle rider increases his speed from 5 m/s to 15 m/s in while accelerating at 2.5 m/s2. how long does this take ?

4 seconds

Explanation:

Average acceleration is change in velocity over change in time:

a = Δv / Δt

Δt = Δv / a

Δt = (15 m/s - 5 m/s) / 2.5 m/s²

Δt = 4 s

Yes, it is possible to change the transitional kinetic energy of an object without changing its rotational energy. For example dropping a spinning ball into the side of the table, the ball will keep its rotational motion while its transitional kinetic energy is changing.