Re: How can energy be lost during energy conversions?

Thank you Emily. An excellant question. Conservation of Energy is one of the key underpinnings of physics. While its roots go back to ancient Greece to philosophers like Thales of Miletus, Karl Mohr (1837) gave this principle one of its earliest general statements. But conservation of energy holds in a closed system. In your question, you list the primary sources of "loss". Additional experiments would now be required to characterize those mechanisms and identify how much energy is converted in the form by friction. Whether that friction is "rolling friction" due to the wheels or axels or aerodynamic friction (drag) caused by motion of one body through air is difficult to determine without more data about your experiment. My suspicion is that it is rolling friction due to the wheel/axel system.

If the loss mechanism is aerodynamic drag, energy is converted into motion of the air molecules and surface molecules of the trolley - in other words, heat.

But as the good experimentalist you are, there is one other factor which must be calculated as part of your experiment: the confidence limit or estimated error. No measurement is perfect. Timing, whether limited by eye- hand coordination, or electronically with a photocell system, has a finite limit of precision. Measurements of the trolly mass and the distance the trolley moved also have some inherent error. That will roll up to an estimated uncertainty in your measured kinetic energy. Understanding the uncertainties in each measurement help the experimenter place a limit on how good one can test a hypothesis - and in this case, to tell you whether the "lost energy" exceeds your estimated energy and is therefore truely a demostration that energy converted form (thermal energy for friction).

I've attached some links below which may help. And keep asking questions. That's the heart of science.

http://en.wikipedia.org/wiki/Conservation_of_energy

http://www.grc.nasa.gov/WWW/K-12/airplane/falling.html

http://instruct1.cit.cornell.edu/courses/virtual_lab/LabZero/Experimental_Error.shtml

http://www.lhup.edu/~dsimanek/errors.htm