Dot Physics

Note to self: don’t do the mechanical equivalent of heat lab again. It doesn’t really work that well and there are better labs to do.

So, what is the mechanical equivalent of heat lab? It is actually a pretty cool idea. Take and object and drop it. What happens to the kinetic energy the object had right before it hit the ground? Most of it goes into thermal energy of the object and surroundings. In this lab, the students measure the change in gravitational potential energy for a falling object (where object is really lead shot or something) and then measure the change in temperature in that object. This is why it is an interesting lab. It connects two things the students don’t usually think of as connect, thermal energy and kinematics.

Let me just say that I first started doing this lab when I was cleaning out the physics store room. I found these cardboard tubes and I was like “huh?” After some poking around there was one with a little sheet of paper that said what they were used for. Here is how the lab works.

Measure the Specific Heat of the material

First, I hate the term ‘specific heat’. I don’t know why, but I tend to confuse it with specific heat capacity or heat capacity. Anyway, this is part is really just to make the lab a little more involved so that it can take up more of the allotted lab time. Basically, students take some metal (same as in the tube) and heat it up. They then put it in a known amount and temperature of water and measure the final temperature. Normal thermodynamics type stuff that students seem to think of as chemistry for some reason.

The only good thing about this part of the lab is that they can easily check their value with google (which they do without even mentioning it). Also, this is a great case for propagation of error practice.

Comparing Change in Gravitational Potential Energy and Change in Thermal Energy

Here the students take a tube (that I prepare beforehand) and turn it over like 75 times. Since the tube is cardboard with a cork end cap, it doesn’t take much of the thermal energy in the collisions. Students can assume most of the energy goes into increasing the thermal energy of the shot.

Why I don’t like the lab

First, the students see this tube as very “black boxy”. Or in this case, black tube. In order to prevent massive spillage of shot, I prepare and close the tubes ahead of time. They don’t measure the mass of the shot. They don’t even actually know what metal is in there except for what is written on the side of the tube.

Even the height the shot falls is indicated by two large X’s on the side of the tube. They just need to turn the thing over and count.

In the end the numbers turn out well enough for this kind of lab, but the students don’t seem convinced.

Finally, this lab has much more pre-lab prep than normal. I really like to let students set up their own stuff and clean up afterwards. That gives them more control over their experiment. However, for this lab, I need to get out stuff to heat water and other stuff that is not normally in the room.

It’s a cool experiment, but I just don’t think I will do it again (or at least not next semester). Really, it is a good opportunity for student to measure can calculate the uncertainty for both the change in thermal energy and the change in gravitational potential energy, but that relies on both some guesses about uncertainties and info from me.

Comments

  1. #1 Alex
    July 18, 2009

    The physical science course I teach contains a lab very similar to this except that the led shot is held in a styrofoam cup which is then repeatedly dropped from desk height to the ground. Students then calculate specific heat assuming that all PE goes into thermal energy.

    Like you I really like the idea of this lab however I tend to run out of time at the end of the semester and end up skipping this lab. I’d be interested to hear if you have any comparable lab that you like better.

  2. #2 Rhett
    July 20, 2009

    @Alex,

    I don’t really have a comparable lab for this. I guess you could do something like the joule experiment where a falling mass is tied to a string that stirs some water. You could then measure the change in temp of the water, but this is really the same idea.

    If I think of another work-energy lab or come across one, I will let you know.