# Gas station without pumps

## 2012 January 27

### Another plea for physics lab ideas

Filed under: home school — gasstationwithoutpumps @ 16:40
Tags: , , ,

I’m having trouble coming up with any sort of meaningful lab for Chapter 6 of Matter and Interactions.  This is the first chapter on energy, introducing it as $E= \gamma m c^2$ and discussing potential and kinetic energy.  There is a fair amount of math we can do (like deriving the formula for energy from  $dE/dx = F= dp_x/dt$), and plenty of calculations, but I don’t see any easy labs.  Labs become fairly easy again in Chapter 7, but I’d welcome any suggestions for things we could do with equipment I could build in the next few days.  (I have to keep the pace at a chapter every two weeks if we’re going to finish the mechanics part of the book before the AP exams.)

1. Do you have a roller skate or some sort of wheeled cart? You can investigate elastic potential energy by stretching some sort of rubber band across a table, and using it as a slingshot to launch the mass. You could investigate the relationship between k and v, m and v, delta x (stretch of slingshot) and v, and lots more. If you don’t have the motion sensor working, you could always just film it and use tracker to find velocity.

The other thing that might be interesting is exploring the physics in this video by the slingshot guy. Stretch a rubber band and measure how the force changes with time, then see how adding heat changes the spring constant and the energy stored in the rubber band. For fun, you could also correct some of the bad terminology/explanations in the video.

Comment by John Burk — 2012 January 27 @ 16:53

• We could rig up some sort of cart, though perhaps not a very low friction one. I do have ultrasonic rangefinders that worked earlier in the year, though not with small targets (https://gasstationwithoutpumps.wordpress.com/tag/ultrasonic-sensor/).

We could also use rubber bands or springs to launch small balls and estimate the launch velocity from the energy in the spring. They’d be too small and fast for the ultrasonic rangefinder, but I could try Tracker or we could try to estimate speed from how far they travel before hitting the ground (hmm, maybe not—too hard to launch at a known angle, and not enough flat ground in the neighborhood).

I think you’re right, though, that I should be able to come up with some sort of speed from kinetic energy from potential energy lab.

Comment by gasstationwithoutpumps — 2012 January 27 @ 18:02

• A skateboard with exercise bands would also work.

Comment by John Burk — 2012 January 27 @ 18:08

• Neither skateboards nor exercise bands exist in our household. Other than Lego, about the only thing with 4 working wheels is the lawnmower, and the wheels have pretty high friction.

Wait, we have a couple of mousetrap cars also, one with very low friction magnetic bearings. They’re probably too fragile to launch from a slingshot, and much of their mass is in the wheels, so we probably want to wait until we’ve covered angular momentum for them—I think that energy calculations that neglect the energy of the wheels spinning are likely to be too far off to be a good lab. Once we have covered that, computing the speed of the mousetrap car from the energy in the spring would be a good lab, I think.

I’ll keep looking around for something we can use as a good cart. Maybe I’ll go down to the thrift store tomorrow and pick up something we can use.

Comment by gasstationwithoutpumps — 2012 January 27 @ 18:29

• Another thought: all the “potential energy of a spring” stuff is at the beginning of Chapter 7, so these labs seem more appropriate for Chapter 7.

Chapter 6 has the definition of energy, relationships between energy and momentum, lots of atomic reactions (which we can’t do at home), and gravitational and electrostatic potential energy. I suppose we could slide a skate down ramps at various slopes and for various heights and look at kinetic energy for predicting speeds.

Comment by gasstationwithoutpumps — 2012 January 27 @ 22:05

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