Use Newton's second law, F=dp/dt, and the formula for relativistic momentum, Eq. (4.44), to show that the acceleration vector a=dv/dt produced by a force F acting on a particle of mass m is
a=γmF−γmc2v(F⋅v),
where F⋅v is the vector dot product between the force F and the particle velocity v. Thus the acceleration depends on the particle's velocity and is not in general in the same direction as the force.