Science Buddies: "Ask an Expert"

Hi, I'm helping my daughter with her science project on marble roller coasters. We have calculated potential energy of a marble with the initial lift hill of the track at a height of .5842m (23") to be .025J. We then calculated a velocity of the 5 gram marble at the bottom of the lift hill to be 1.42 m/s. From the velocity we calculated a kinetic energy of .005 J. I have never studied Physics, but from the simulators I viewed online of the kinetic and potential energy balancing out (I know these assume no loss of energy) I was expecting the Kinetic energy to be greater. I was just wondering if that was a reasonable result or if I made an error somewhere along the way. Many thanks!

Hi sdbrow,

Using the data you posted I calculated 0.029 J and 0.005 J for the potential and kinetic energy values. So I don't think you made a math error. If you assume that the kinetic energy is the same as the potential energy (0.029 J) the velocity of the marble would have to be 3.4 m/s or about 2.4 times the velocity that you measured. This kind of loss might be reasonable if the track was rough or the marble bounced around (side to side or up and down). On the other hand, you may have an experimental error of some kind with the marble velocity measurement. You might want to try measuring the marble velocity a few more times to confirm the value.

I hope this helps. Please ask again if you have more questions.

A. Norman

Thank you so much, Mr. Norman. I got a little mixed up on the numbers that I sent yesterday. It was long day.
I mixed up some values from a test at a different height. I also got .029J for potential energy. My velocity on that particular one was actually 1.58 m/s, still low, I agree. I will try to remeasure the speed. I had used a stopwatch earlier taking 10 speeds and averaging them, but it was not very scientific. I will try to test with a video this time. I really appreciate your time and input.