A bicyclist starts a timed race at $6.0 \mathrm { mi } / \mathrm { h } .$ In order to win,he must average $21 \mathrm { mi } / \mathrm { h } .$ Assuming constant acceleration from the start,how fast must he be traveling at the end of the race?
A)36 mi/h
B)30 mi/h
C)24 mi/h
D)42 mi/h

Question

Quizplus · Accepted Answer

To average 21 mi/h starting from 6 mi/h and ending at an unknown speed, the final speed must be twice the desired average minus the starting speed due to the symmetry of acceleration. Thus, $2 \times 21 \mathrm{mi/h} - 6 \mathrm{mi/h} = 42 \mathrm{mi/h}$.

A Bicyclist Starts a Timed Race At 6.0mi/h.6.0 \mathrm { mi } / \mathrm { h } .6.0mi/h.

A Bicyclist Starts a Timed Race At $6.0 \mathrm { mi } / \mathrm { h } .$