PDA

View Full Version : Fun with Newton's 1st Law and discrepencies within it.

Elgin Clock
09-14-2007, 01:56 AM
I. Every object in a state of uniform motion tends to remain in that state of motion unless an external force is applied to it.

Now, this makes sense to me, but I used to think the opposite was also true.
An object at rest, will stay at rest unless an external force is applied to it as well, but now I'm not so sure.

The other day in my Chemistry class, we were discussing atoms and how they are seen, and looked at through a microscope. The only way to see them is to slow them down enough to be able to see with the naked eye.
This is done by taking them to a temperature which is approximately Zero Degrees C.

However, it was stated in the class Zero Degrees C can not ever be reached. So instead of stopping an atom, they slow it down enough to be able to capture it in the field of view on the microscope.

When we apply Newton's first law to an object in motion (say a robot at the end of a match hitting a wall) we say it has "stopped" and therefore Newton's Law was proven (It was acted upon by hitting a wall, and a multitude of other forces acting on it as well (gravity/friction/air resistance).

How do you prove Newton's first Law with an atom which can't be "stopped", but only slowed down as of right now with current technology?

Just something I pondered during a long class on Monday night.

Also, in the whole conservation of energy theory, IF one atom ever was able to stop completely, would we would need something to move to take it's place to make perfect balance to the system??

Maybe I'm confusing two different theory's here, but something just doesn't seem to add up at all.

Also, do atoms move in uniform motion? (I only ask because I see that uniform motion of an object is a requirement of Newton's First law?)

Does anything ever move uniformly in the world for that matter?

Any input would be appreciated so I don't go crazy during another class, or everytime we start talking about atom motion or Newton's laws. :eek:

weinbergmath
09-14-2007, 11:15 AM
Hi Elgin,

A couple of things that might help clear things up:

Observing individual atoms is made possible by scanning electron microscopes which utilize principles of quantum mechanics to make the image. The principles of QM are important when looking at things that are so small, and Newtonian mechanics doesn't describe appropriately how things move at that scale. In fact, at this small scale, particles can tunnel through walls, and can have their properties changed simply by changing the container holding them. Take a look at this link to see an image developed at IBM showing what happens when a circle of atoms is small enough to contain an electron.

We can very easily appreciate Newton’s laws when looking at a robot stopping because it is a large object, and Newton’s laws are good enough to describe the objects we encounter in everyday life. When discussing atoms, however, we won’t necessarily observe a direct confirmation that the 1st law applies because quantum mechanics describes interactions on that scale, not Newton’s laws.

The other thing to remember is that in a gas, atoms/molecules are constantly bouncing off of each other, and so uniform motion does not generally occur except in the very brief periods between collisions.

Hope this helps at least a little bit!

Evan

Tristan Lall
09-14-2007, 12:10 PM
The other day in my Chemistry class, we were discussing atoms and how they are seen, and looked at through a microscope. The only way to see them is to slow them down enough to be able to see with the naked eye.
This is done by taking them to a temperature which is approximately Zero Degrees C.

However, it was stated in the class Zero Degrees C can not ever be reached. So instead of stopping an atom, they slow it down enough to be able to capture it in the field of view on the microscope.I think you mean absolute zero, or 0 K = -273.15°C. (Thermodynamically, it's possible to define a negative temperature (http://en.wikipedia.org/wiki/Negative_temperature), but it's not colder than absolute zero—it's a different mess entirely.)

Also, Evan's comments are on track: Newtonian mechanics is intended to work at the macroscopic level, at speeds well under the speed of light. Under those conditions, Newton's laws yield so good an approximation that it's essentially indistinguishable from the results predicted by quantum mechanics or special relativity.

Also, in the whole conservation of energy theory, IF one atom ever was able to stop completely, would we would need something to move to take it's place to make perfect balance to the system??The act of stopping the atom (because it can't spontaneously stop) transfers its energy to something else, ensuring that energy is conserved. That something else might be its surroundings, if it's being cooled, or might be another body, if it is colliding. In either case, what was formerly the kinetic energy of the atom is being converted to some other form (which isn't necessarily kinetic).

lukevanoort
09-14-2007, 12:13 PM
Newton's Laws and all other classical physics type stuff (si=v*t, si+v*t+a*t^2=sf, etc) are all approximations that work pretty well in everyday life, at everyday speeds. Classical physics doesn't really work well at very small scales, or at very high speeds. This is where quantum physics comes in, and, in addition to explaining some odd things, it makes really REALLY strange predictions and is just generally weird. If you ever feel like not only confusing yourself for one class, but instead for several weeks, look in to quantum physics, in particular topics like vacuum fluctuations and quantum coupling.