# Unbanked Curve increasing speed

1. Mar 17, 2015

### Jake Wright

1. The problem statement, all variables and given/known data

A race car going around a flat, unbanked circular track gradually increases speed as it
completes one full trip around the track. Which of the following can explain why the car gains
speed?
(A)
Energy stored in the fuel is converted to mechanical energy.
(B)
A component of the frictional force exerted by the ground on the tires is directed toward
the center of the circle.
(C)
A component of the frictional force exerted by the ground on the tires is in the direction of
motion.
(D)
The car’s velocity and acceleration are perpendicular.

2. Relevant equations

(This is multiple correct so multiple answers may be correct)
Just common sense

3. The attempt at a solution

So A is very obvious, but I'm not sure what else is correct because A is not the only one that's correct.

2. Mar 17, 2015

### andrewkirk

Note that they use the word speed rather than velocity, and velocity is speed with a direction. Some of B,C,D are about changes in direction and hence affect velocity but not speed. To increase the speed of an object it needs to be pushed in the same direction as it is already moving. Think about Newton's third law of motion and what a car pushes on (that's not part of the car) to make itself go faster.