Question

Consider a particle moving along the world line \boldsymbol{x}=L \tanh (t / L) (a) Sketch this world line and describe the particle's motion in words. (b) Calculate the components of

the particle's 4-velocity and check that u - u = -1. (Itis probably easiest to find u' first.) (c) Calculate the components of the 4-force on the particle in terms of its mass m. Why is this crazy at t = 0?

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