Sunday, April 22, 2012

Everyday physics questions

Some of these are better than others. Feel free to discuss in the comments. I doubt I'll post my answers unless some sort of argument breaks out. At best, these might be interesting, and at worst, they should give non-physicists some idea of what's going through the physicists head when the conversation turns boring:




  • when one goes to return the shampoo bottle to its place, its moment of inertia is generally much larger than when you picked it up. The moment of inertia is essentially a measure of a rigid body's resistance to rotation, and therefore one might naively think it would be more stable after use. However, that's clearly not the case. So the first question is why? The second is: is there some limit in which the increase to the moment of inertia would be the dominant effect so that one could construct a shampoo bottle that is unconditionally stable about its bottom?
  • Walking down the street, one observes someone who is otherwise well covered and obscured by clothing, but who has some cleavage apparent. As this person walks towards you, you observe the limited cleavage heaving up and down a bit. The question is: can one legitimately estimate the endowment by the time-scale of the heaving or instead does one just instinctively increase ones estimate (given there's so little else to go on) simply because there is motion at all? In other words, are the variables too many...speed, type of walking motion, type of support clothing etc...to provide any real estimate? [Sorry for the crassness...]
  • Seeing a Smart car, I am struck by the fact that it is so small but yet the fuel economy isn't particular good, compared to other micro-cars. If one considers good economy cars and then considers mopeds, the fuel economy roughly increases by a factor of two. What is the ultimate (ie physical) limit of fuel economy? Clearly certain assumptions have to go into such a limit, but how many?

3 comments:

Anonymous said...

Like saying mass measures resistance to acceleration, so that big boulder on top of the hill should be really stable.

Davidmac said...

The efficiency question is obviously just an F=ma limit that must include a drag coefficient. It comes down to how light of a vehicle with low drag can you design to carry a specific mass of humanity. I have been considering two designs, one that uses an ICE to run at single speed to produce electricity and with a small battery or capacitor stack, and one that is totally solar. The first is much more easily scaled than the second. Not to mention more reliable in its output.

rparthasarathy said...

For question #3 (cars), you might find this piece I wrote relevant / entertaining: tpt.aapt.org/resource/1/phteah/v50/i7/p395_s1 (or http://dx.doi.org/10.1119/1.4752039) . Drag + thermodynamics are crucial, and surprisingly easy to estimate.
I'm not touching question #2...