Posts Tagged ‘Conservation of Energy’
A couple of weeks ago we did a lab straight from the textbook. (Here is the Lab Instructions, typed up and put it into my words.) I’d been looking for a good Conservation of Energy Lab. I wanted to use the Vernier devices, but there wasn’t anything in their book that I liked. Rather than make something up from scratch, I decided to work directly from the Holt Physics textbook.
The lab had two parts; the first was to calculate the spring force constant using a Hooke’s Law device. I didn’t have that device, so I created my own by first designing a simple indicator on Solidworks (3D CAD Software) and then printing eight of them in the 3D printer. That worked out great. (I will try to remember to post my Hooke’s device design, you can build them for about $0.25 each.) Unfortunately, it was the only part of the lab that worked out at all.
We were able to use a ring stand, ruler, and indicator to successfully calculate the spring force constant. The second part of the lab was supposed to demonstrate conservation of mechanical energy by bouncing the weight and measuring the high and low point. Quite honestly, it just didn’t make sense. At first it did, but the more I thought about it, the less sense it made.
To begin with, it was nearly impossible to measure the bottom and top of the bounce with any accuracy. It was pure guesswork and the kids were really struggling.
This is a great bunch of kids, I warned them ahead of time that this was the first time using this lab, there might be some hiccups. They were understanding and really tried to make this work, but they were totally frustrated. I told them I would grade the lab on their effort, spring constant results, qualitative analysis, and attempt at explaining the results.
Clearly, I need a much better lab for next year. I was originally thinking of calculating the spring force constant, then determining the weight of an unknown object based on distance the spring stretches. That makes sense for the chapter on oscillations, but not for Conservation of Mechanical Energy. I don’t have an air track, but I’m really good at McGuyvering solutions, as you all well know.
Please throw your awesome labs my way, I need help.
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.
This is an experiment I did in my Conceptual Physics class to demonstrate Conservation of Energy. I take a clean plastic peanut butter jar and put in about an inch of copper BB’s. We then wrap the jar in a towel and have a student start shaking the jar as hard as they can. The towel serves two purposes; it stops the transfer of heat from the students’ hands to the jar, and it helps deaden the deafening sound this little activity generates. Use the non-contact thermometer to take a reading of the BB’s before beginning and then after every two minutes of shaking. I usually set up two or three jars and have the students pass the jar to someone else after the temperature is taken. Chart the temperature on the board. Don’t expect to get anything else done while this is happening. There is too much noise and everybody wants to see how vigorously they can shake the jars.
The students are putting energy into the system because they are shaking the jar. They are actually taking the chemical energy from the food they ate and turning that into kinetic energy. The kinetic energy is transfered into themal energy (heat) because of the friction of the BB’s banging into each other. I just thought of this, but it would be a good idea to have another jar set up as a control.