434 RelativityQ39.9By a curved line. This can be seen in the middle of Speedo’s world-line in Figure 39.12, where heturns around and begins his trip home.Q39.10According to pu=γm, doubling the speed uwill make the momentum of an object increase by thefactor 2422221 2cucu−−LNMOQP.Q39.11As the object approaches the speed of light, its kinetic energy grows without limit. It would take aninfinite investment of work to accelerate the object to the speed of light.Q39.12There is no upper limit on the momentum of an electron. As more energy Eis fed into the objectwithout limit, its speed approaches the speed of light and its momentum approaches Ec.Q39.13Recall that when a spring of force constant kis compressed or stretched from its relaxed position adistance x, it stores elastic potential energy Ukx=122. According to the special theory of relativity,any change in the total energy of the system is equivalent to a change in the mass of the system.
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