External force and momentum change for an object (7-23)

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Question

Duration of a time interval over which the external forces act
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Review

Since the speed of light in a vacuum \(c\) is the maximum speed an object can acquire,this expression says that the maximum kinetic energy that an object can acquire is \((1/2)mc^2\). This contradicts the idea that there should be no limit on an object’s kinetic energy. Clearly, we need an improved equation for kinetic energy.

There is a similar problem with the Newtonian expression for momentum. We saw in Section 7-6 that the change in momentum of an object is determined by the external forces on the object and the duration of the time interval over which the forces act: