A knight on horseback holds a 10-m lance.The horse can run at 0.78c.(It wins most of its races!)How long will the lance appear to a person that is standing still on the ground as the horse runs past?
A) 7.1 m
B) 15 m
C) 10 m
D) 6.3 m

Question

Quizplus · Accepted Answer

We can use the length contraction formula:
L' = L / sqrt(1 - v^2/c^2)
Where L = 10 m (length of the lance)
v = 0.78c (velocity of the horse)
c = speed of light
Plugging in the values, we get:
L' = 10 / sqrt(1 - 0.78^2) = 6.3 m
Therefore, the lance will appear to be 6.3 m long to a person standing still on the ground as the horse runs past.

A Knight on Horseback Holds a 10-M Lance