I dont see many astrophysics problems on here so I thought I would ask one.

Consider a spacecraft of mass m whose engine is a perfectly absorbing laser sail that is initially at rest in space. No gravity is being exerted on it. We aim a laser at the sail and cause it to accelerate into deep space.

Derive a formula for its kinetic energy, as a function of its mass and the total amount of energy Ei that it absorbs from the laser beam during some time interval t. Assume that the spacecrafts velocity remains NON-relativistic.

I.E. You're firing radiation and using the pressure of light to accelerate it.

HINT: Radiation pressure=$\displaystyle F_(rad)= ((<S>A)/c)cos(theta)$

I've been stuck on this for a while. Obviously once you have the force then you can divide by mass to get the acceleration, but I dont see how Kinetic energy can be found.