If you rub a balloon on a piece of fabric or carpeting and then hold it against your head, your hair can stand on end. use what you have learned in this lesson to explain why this happens.

I believe this happens because when you rub the balloon on the carpet or fabric, the connection or contact of that jumbles up the atoms and they receive the energy which makes your hair as the question said 'stand on end' 

So when you rub a balloon on a piece of fabric or carpeting and then hold it against your head it has static electrocute that causes your hair to stand on end

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 

If the heat engine is a perfect carnot cycle engine, thenefficiency = (t2-t1)/t2   where t1=temperature of cold reservoir ( ° k) = 135            t2=temperature of hot reservoir ( ° k)   = 325thereforeefficiency = (t2-t1)/t2 = (325-135)/325 = 0.5846 = 58%  note: this is the theoretical maximum efficiency of a perfect heat engine.   real-life engines do not achieve this value.