Physics & Physical Science Demos, Labs, & Projects for High School Teachers

Conservation of Energy Lab

Posted on: March 11, 2010

I quite literally pulled this together one morning after realizing I didn’t have a lab in this chapter.  The kids have been struggling with the concept of Conservation of Energy.  Putting up the Physlet for the skate park really helped.  I like that you can display a bar graph of the energy in the system and watch it move back and forth between KE and PE.  It’s really cool, I put it on the smart board and kept modifying the track until the skater did a double loop.  It definitely helped the kids understand that the skater can’t go higher than he starts and the transfer between KE and PE.  There are lots of controls, you can add track, modify friction and gravity, change the viewing speed, etc.  We spent around 15 minutes in each class just playing and laughing at the simulation.

The lab I created used the electronic timers to see how fast a steel marble is going at the bottom of an aluminum track.  This is the same basic set-up as the landing zone lab, however a few problems lead to a few of improvements.

I’m not sure what’s wrong with the electronic timers, but at a certain speed, the balls won’t properly trigger the light sensor.  We had been using them on Timing II which returns the time elapsed between triggering two sensors.  When the ball was released from 25cm or higher, the timer kept failing.  We changed over to using a single sensor and Timing I, which returns the amount of time the sensor is dark.  This allowed us to get consistent data up to about 40cm high.  I have an old Pachinko machine, so I borrowed a few balls for my labs.  These are 11mm in diameter, which makes it easy to calculate the velocity of the ball at the bottom of the ramp.

I was trying to decide if I wanted them to graph energy or velocity.  In the end, I decided they would graph the theoretical KE and the observed KE.  The gap in the two is the energy lost to friction.  I thought that if they graphed velocity, they wouldn’t see the loss as an energy loss, only a speed loss.

The paper went through one major revision since I wrote it three days ago.  If you have comments or ideas for improving the lab, I want to hear from you.

Conservation of Energy

4 Responses to "Conservation of Energy Lab"

Didn’t get to try the applet (laptop’s broken) but I’m glad you found a way to help your kids visualize this because its so useful later. It’s a critical part of physical reasoning, it can help explain some things in calculus, and of course it’s such a useful and fundamental tool in solving practical physics problems.

By the way, I don’t know if I’ve ever mentioned it before but I’m interested in teaching math… not as a career I don’t think but maybe over the summer. I don’t think I’m good enough at physics to teach that, but if I ever do I’m gonna know where to look for all this lab info haha.


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When a solid ball rolls, 2/7 of its kinetic energy is rotational. I don’t know if you factored that in — if not, a lot of the “missing energy” is probably rotational k.e., not heat.

I’ve been thinking about your comment since you wrote it and I’m not sure I agree. First, 2/7 is a lot of energy. We would see that consistently as a pattern in the graph, but I don’t think we did. Unfortunately, that data is lost. Second, the ball is not rotating faster than it is moving, it is not wound up like a top or a gyroscope. Third, you can hear the ball rolling down the metal ramp, so the sound vibrations (loss) is also indicating frictional loss.

I’m not saying you are incorrect. I never thought about it before and I would love if you or someone else could either site some source so I can learn more or simply give me a deeper explanation.

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