An engine does 10 j of work and exhausts 25 j of waste heat during each cycle.
what is the thermal efficiency?
i did it like
eff = = 0.67 = 67%
but my homework said it was incorrect
so i tried it this way
eff = = 0.4 = 40%
but that is also incorrect, so what am i doing wrong?

The thermal efficiency is defined as follows

,

and the energy which is put into the system is

.

In your case 

So  which gives an efficiency of

.

Answertraveling around a circular trackexplanation"traveling around a circular track" is the best answer as you could be traveling around a circular track at a constant speed. your velocity would therefore have the same magnitude but will always be constantly direction. since acceleration is the rate of change of velocity, the rate of change will only involve the direction of velocity and not the magnitude.wrong choices: "an object at rest" does not accelerate at all"increasing speed while traveling around a curve" means that the acceleration will not be constant due to only change in direction; it will be constant due to both magnitude and direction"an object in free fall" does not necessarily change direction