Question

A spaceship at rest in a certain reference frame S is given a speed increment of 0.50c. Relative to its new rest frame, it is then given a further 0.50c increment. This process is continued until its speed with respect to its original frame S exceeds 0.999c. How many increments does this process require?

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When a particle is moving with speed uu^\prime in the positive xx^\prime direction in an inertial reference frame SS^\prime that itself is moving with speed vv parallel to the xx direction of a second inertial frame SS, the speed uu of the particle as measured in SS is:

u=u+v1+uv/c2u=\dfrac{u^\prime +v}{1+u^\prime v/c^2}

at each increment the speed of frame of the spaceship is 0.50c0.50c , relative to the new frame. The spaceship starts from rest, so at the first increment u=0u^\prime=0, and the speed of the frame is v=0.50cv=0.50c, so:

u1=0+v1+0=0.50cu_1=\dfrac{0 +v}{1+0}=0.50c

in the second increment u=u1=0.50cu^\prime=u_1=0.50c and v=0.50cv=0.50c, so:

u2=0.50c+0.50c1+(0.50c)(0.50c)/c2=0.80cu_2=\dfrac{0.50c +0.50c}{1+(0.50c)(0.50c)/c^2}=0.80c

in the third increment u=u2=0.80cu^\prime=u_2=0.80c and v=0.50cv=0.50c, so:

u3=0.80c+0.50c1+(0.80c)(0.50c)/c2=0.9286cu_3=\dfrac{0.80c +0.50c}{1+(0.80c)(0.50c)/c^2}=0.9286c

in the fourth increment u=u3=0.9286cu^\prime=u_3=0.9286c and v=0.50cv=0.50c, so:

u4=0.9286c+0.50c1+(0.9286c)(0.50c)/c2=0.9756cu_4=\dfrac{0.9286c+0.50c}{1+(0.9286c)(0.50c)/c^2}=0.9756c

in the fifth increment u=u4=0.9756cu^\prime=u_4=0.9756c and v=0.50cv=0.50c, so:

u5=0.9756c+0.50c1+(0.9756c)(0.50c)/c2=0.9918cu_5=\dfrac{0.9756c+0.50c}{1+(0.9756c)(0.50c)/c^2}=0.9918c

in the sixth increment u=u5=0.9918cu^\prime=u_5=0.9918c and v=0.50cv=0.50c, so:

u6=0.9918c+0.50c1+(0.9918c)(0.50c)/c2=0.9973cu_6=\dfrac{0.9918c+0.50c}{1+(0.9918c)(0.50c)/c^2}=0.9973c

in the seventh increment u=u6=0.9973cu^\prime=u_6=0.9973c and v=0.50cv=0.50c, so:

u7=0.9973c+0.50c1+(0.9973c)(0.50c)/c2=0.9990cu_7=\dfrac{0.9973c+0.50c}{1+(0.9973c)(0.50c)/c^2}=0.9990c

Thus, seven increments are needed.

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