A spear is thrown at you at a very high speed. As it passes, you measure its length as one-half its normal length. From this measurement, you conclude that the moving spear's mass must be
A) twice its rest mass.
B) four times its rest mass.
C) zero.
D) one-half its rest mass.

Question

Quizplus · Accepted Answer

The spear's length contraction indicates it is moving at a relativistic speed. According to the theory of relativity, as an object's speed approaches the speed of light, its relativistic mass increases. If the length is measured as half, the Lorentz factor is 2, meaning the relativistic mass is twice the rest mass.

A Spear Is Thrown at You at a Very High