Inclined Tube Experiment(2)


Dear James D.Jones:
You were able to give me some wonderful and helpful information. Thank you so very much. However, I still have some questions. Yesterday, I went to go speak to a Physics professor at my school. He said to look up Newton's second Law. That's all he said. I looked it up, and it was difficult for me to see the significance it had with my experiment. Well, my question is, could you help me interpret that law and explain how it pertains to my experiment?

Could you please also tell me if there are any Laws or Physicists i can look up that may pertain to the type of experiment i am conducting. In other words, is there a certain Law(s) that sums up something that you explained to me in the last e-mail?

P.S.- One more thing, I still haven't decided what types of materials [of tubes] I should use. Could you suggest some materials that will give me varied results. (They have to be reasonable materials ie: no gold or something difficult to find!) Earlier i had also debated on whether to use a solid metal ball, or a liquid such as water. But i believe that the experiment will be interesting if I use both. What do you think?

Thank You so VERY MUCH,


Here is the deal with Newton's second law. He said that the acceleration an object has is proportional to the net force applied to it. In fact the acceleration is equal to the force divided by the object's mass.

Acceleration is the rate of change of velocity so the more net force applied to an object, the faster its velocity changes. With regard to your experiment, if you release an object without pushing it, so that it starts with zero speed and slides down a tube, assuming that air resistance is small enough to be neglected, It will go faster and faster until it shoots out the bottom at its final velocity. The rate at which it picks up speed, according the Newton's second law, is proportional to the net force on the object.

I keep saying NET force which means the total of all the forces on the object taking into account the different directions of the various forces. As we already know from this morning's email, the forces on the object are gravity, friction, air resistance and the force applied by the tube.

The physical laws governing friction and air resistance are way too complicated to help you very much. Stokes law deals with air resistance in an approximate way, and sliding or rolling friction depends on the laws governing the electrical interaction of atoms. I am afraid that what I gave you was the summary :).

If I had to do the project, I would stay away from liquids. They are too hard to measure and explain. Liquids wet the surfaces they touch an part of the material stays in the tube at zero velocity. Also if you want to demonstrate varied results for different materials, you should probably not use a ball since the only effect of the material will be to change the friction, and rolling objects have so little friction that changes will be hard to detect.

I would suggest that you use a piece of plastic pipe for one tube and perhaps a cardboard tube such as maps are mailed in, or wrapping paper is stored on, for another. The plastic pipe should have a quite smooth, hard surface and the cardboard tube should be rougher and softer. The plastic pipe is available at any hardware store and the cardboard tube may be around the house or available with some wrapping paper for Christmas on it at Wal-Mart or some such place.

For an object to send down the tubes, I would recommend a short length of heavy metal pipe which will fit somewhat loosely inside the tubes. Using a hollow pipe segment to slide down your tubes rather than a solid object will reduce the effect of air resistance so that it does not complicate your results.

How are you planning to set up the experiment so as to measure the exit velocity of the object? How will you test the effect of different elevation angles and tube lengths? How are you going to make sure that for each run of the experiment you do everything exactly the same so that the change in results is due only to the things you are controlling, like material, length and angle. These are some things you need to think about as you design your experiment.