thebrain13 The information you have been given is complete and accurate you just need to rearrange the equation to extract the velocity if you are happy to work in energy terms if you don't know how to do this you really need to learn a bit more algebra if you want to understand the question you have asked properly.

Going back to your original question.

You have to remember that the velocity of an object does not depend on strictly on the force that is acting on it it depends on the force AND the length of time that it is acting. If I give you a brief push with a force you wont move very far or fast but if I push you steadily for a long time you will go fast and a long way.

So to make your original question accurate you need to specify for example that the force is constant and always in the same direction and that the body starts from rest relative to some point.

Then you need to rearrange the equation to get what you want

The fundamental newton equation is, force equals mass times acceleration, so the acceleration equals the force divided by the mass.

If then you assume that you start with zero velocity the velocity you get to is the acceleration times the time that it has been operating.

so the velocity is the time, multiplied by the force and divided by the mass.

To convert this equation to a relatavistic form is not all that easy because as the velocity gets significantly close to the velocity of light the mass is increasing continuously by the lorenz factor (stated above) and a fixed force (like that produced by a rocket motor) gradually produces less acceleration and you need to do some integrating to get to the final velocity.

However if you are thinking about a rocket accelerating there is an added complication. The rocket is using up fuel by chucking mass out the back, so the mass of the main object is decreasing for all the time that the force is applied and this means that the relatavistic decrease in final velocity is not quite as much as you might expect.

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