Subject: How can energy be lost during energy conversions?

At school I did a simple classroom experiment to demonstrate how gravitational 
potential energy is converted into kinetic energy when a dynamics trolley of a 
certain mass is released from the top of a ramp. The speed of the dynamics 
trolley as it moved down the ramp was detected using speed sensors. After 
completing the experiment I did some calculations to determine the 
gravitational potential energy of the trolley at the start of the experiment 
(at the top of the ramp before being released) and the kinetic energy it had 
as it passed through the speed sensors. I then used these 2 figures to work 
out the efficiency of the transfer of gravitational potential energy into 
kinetic energy. I found from my calculations that the conversion was not 100% 
efficient. My question is, how was energy lost during this conversion? Was 
this energy lost due to friction between the wheels of the dynamics trolley 
and surface of the ramp? If this is the case, was this energy lost in the form 
of heat energy? Did this friction cause both the wheels of the trolley and the 
surface of the ramp to gain heat (or just one of the aforementioned)? Where 
does this heat energy then go - is it simply lost to the surroundings or can 
it somehow be usefully reused/recycled? I also wonder whether energy could 
have been lost during the conversion of gravitational potential energy into 
kinetic energy due to air resistance? I believe that air resistance can reduce 
the speed of a moving object (thus it is useful to streamline the shape of a 
racing car) but am unsure whether this effectively reduces its amount of 
kinetic energy. Would one or both of these factors (+ any others) be 
responsible for the inefficient conversion of gravitational potential energy 
into kinetic energy? If, in fact, air resistance is also a reason for energy 
loss, what happens to this lost energy? Is it perhaps lost as heat energy and 
warms the surroundings? Thank you     

Re: How can energy be lost during energy conversions?