Surprising Physics: Acceleration Faster Than Freefall

[toocool_sashi]

If you slide it more, at some point it rotates and falls down and that point comes BEFORE the rod is completely out of the iron plate. Now consider this position just before when it falls. The forces acting on it are :-
1. the magnetic force by the plate which gets canceled by the normal force.
2. the frictional force by the plate
3. gravity.

I did not quite understand ur expirement clearly but i think uve forgotten 1 force...the force which ur applying 2 slide it?? or is it in equilibrium after the CoM is out of the square plate and u have also removed any physical contact with it?

 

[vinter]

I did not quite understand ur expirement clearly but i think uve forgotten 1 force...the force which ur applying 2 slide it?? or is it in equilibrium after the CoM is out of the square plate and u have also removed any physical contact with it?

My mistake. i think I did not explain the question properly.
When I said that you are sliding the rod slowly, I meant that you are sliding it very little and then leavin it to see what happens to it. The forces that I have listed are the forces acting on it in this situation, i.e., when you have left it to see what happens. I hope this makes it clearer.

 

[GENIERE]

I think that's an old one that was originally done with the old type milk bottles (which had much larger necks than coke bottles). Anybody remember those things? ask grandpa about them.

Yep! Grandpa remembers them, delivered twice a week to the milkbox on the porch along with the bread.

I'm certain I saw the experiment before I reached 6th grade.

...

 

[StatusX]

The forces acting on it are :-
1. the magnetic force by the plate which gets canceled by the normal force.
2. the frictional force by the plate
3. gravity.
Since here, it is both in translational as well as rotational equilibrium, the downward gravitational pull must be nullified by an upward friction. But since the friction and the gravitation should have opposite torques, friction should be downwards?
What's going on!

I don't know what you mean by "upward friction", but if I understand correctly, this isn't nearly as strange as you think. An similar experiment would be to hold a board so that a small part of it is on resting on a table and the rest is hanging over the ledge. If you glue the end to the table, and if the glue is strong enough, the board will be stable, and common sense would agree with this.

In your experiment, the balance of forces is as follows: The overhaning end of the rod exerts a torque about the edge of the plate which results in an upward force on the other end of the rod. The downward force on this end is the combination of gravity and the magnetic force. If this is greater than the force from the torque, the board will be at rest, with the balance of the downward force being countered by the normal force.

 

[Creator]

Yep! Grandpa remembers them, delivered twice a week to the milkbox on the porch along with the bread.

I'm certain I saw the experiment before I reached 6th grade.

...

Yep, sometimes older folk can really fill in the blanks.
I'm glad to see that someone else remembers these bottles (and the demonstration).

I'm sure I was in grammar school at the time I first saw it also.
So what was the physics lesson from the egg getting sucked into the bottle?

Creator

 

[Norseman]

i was surprised that it is true that water will stop bullets fast enough that diving into water while being shot at could save your life. i always doubted it, but Mythbusters says i was wrong.

What I find the weirdest about that episode was that the high power guns were less effective than the low power ones. Too fast and the bullet just gets ripped apart.

Edit: Okay, so here's my contribution. The Leidenfrost effect. Take a clean (non-teflon) pan and set it on the stove. Let it heat up (it will need to be well past the boiling point of water). While it's heating, let a drop of water fall on the pan. Notice that it sizzles away. As it gets hotter, the water should vaporize faster and faster. But, when the pan gets hot enough, the water no longer sizzles immediately and now begins to float around on the pan (yes float, the water doesn't stick to the pan anymore and if it's round you can spin the pan around to make the water droplets do laps, sometimes dozens of them) lasting for quite a while.

What has happened is the water vaporized so fast that a cloud of steam formed. The steam insulates the water and makes it take considerably longer to vaporize.

Alright, now for just fun. Take a candle. With a small bit drill, or a needle, make a hole near the wick. Insert a toothpick (make sure the candle doesn't have any metal embedded in the bottom). Place the candle in the microwave. Notice the candle starts releasing small clouds of plasma. I'm actually not sure what happens here, but I know it doesn't work without a toothpick. A plain wick won't do it.

 

[yourdadonapogostick]

you can do that with liquid nitrogen and a surface that is hotter than the boiling point of liquid nitrogen. that is how you can put liquid nitrogen in your mouth without freezing(warning: if you try this, DO NOT SWALLOW!)

 

[CarlB]

Here's a link to the data from a nice experiment from a data acquistion company that demonstrated the Mpemba effect:

http://www.picotech.com/experiments/mpemba_effect/results.html

The hot 42C water froze much faster than the colder 18C water sample.

Just from the slope of the curves, I'm going to guess that it has something to do with the amont of air entrapped in the water. That is, you have to heat the hot water up enough to just below a boil so that the air goes out of it.

A way of verifying that this is the solution would be to examine the icecubes. The cubes made from hot water should be clear throughout, while the cubes made from cold water should have all those little air bubbles.

Either the air bubbles have the effect of insulating the center of the cube, or they increase the heat of solidification due to the increased surface tension involved with all those air bubble surfaces.

If it is entrapped air, then rerun the experiment with cold water that has had its air removed.

Carl

 

[DaTario]

The forces acting on it are :-
1. the magnetic force by the plate which gets canceled by the normal force.
2. the frictional force by the plate
3. gravity.
Since here, it is both in translational as well as rotational equilibrium, the downward gravitational pull must be nullified by an upward friction. But since the friction and the gravitation should have opposite torques, friction should be downwards?
What's going on!

If you have a seesaw with two pivots instead of one, you will be able to balance it with several different weights, lying in a range. In a similar manner, when the rod stuck by magnetic force on the plate, it creates some pivots (i.e. more than one), and so it is able to maintain itself static for a range of forces.

 

[DaTario]

there was a beautiful problem in Physics Teacher of May I guess in which a ball is lauched horizontally in between two vertical and parallel walls. The ball bounce in the walls while the gravity pull it down. After 1 second the ball will be 4,9 meter below. This is Ok. But now consider the walls are substituted by mirrors, with 100% reflectivity. Now, instead of a ball, we send a laser beam horizontally. Of course the beam will reflect back and forth, but the question is: what will happen with the light 1 second later? (Hint: GR)

 

[Creator]

there was a beautiful problem in Physics Teacher of May I guess in which a ball is lauched horizontally in between two vertical and parallel walls. The ball bounce in the walls while the gravity pull it down. After 1 second the ball will be 4,9 meter below. This is Ok. But now consider the walls are substituted by mirrors, with 100% reflectivity. Now, instead of a ball, we send a laser beam horizontally. Of course the beam will reflect back and forth, but the question is: what will happen with the light 1 second later? (Hint: GR)

The light beam will be 4.9 meters lower after 1 sec. (provided the mirrors are perfectly parallel!) Not likely, however, that you'll be able to do a table top experiment since even the best mirrors would not have sufficiently perfect reflectivity to bounce back and forth the required 3 x 10^8 times required for a mirror separation of 1 meter! Good gedanken experiment though.

Another one I like to do ( for H.S. demo) is to use a plastic ruler (bent backwards) to strike two marbles simultaneously, launching them from off the top of a table. Since each marble is struck with a different force one goes much farther horizontally than the other. The question to the class is: "Which one will hit the ground first?" Pretty simple, but it gets the gravitational point across.

 

[DaTario]

That's is it!

I also found it very interesting in deed.

 

[pallidin]

The light beam will be 4.9 meters lower after 1 sec. (provided the mirrors are perfectly parallel!) Not likely, however, that you'll be able to do a table top experiment since even the best mirrors would not have sufficiently perfect reflectivity to bounce back and forth the required 3 x 10^8 times required for a mirror separation of 1 meter! Good gedanken experiment though.

Another one I like to do ( for H.S. demo) is to use a plastic ruler (bent backwards) to strike two marbles simultaneously, launching them from off the top of a table. Since each marble is struck with a different force one goes much farther horizontally than the other. The question to the class is: "Which one will hit the ground first?" Pretty simple, but it gets the gravitational point across.

I would imagine that in far less than 1 second, the light will be completely absorbed by the less than perfect reflections.
Even if the mirrored walls were 1000 feet apart, in one second each photon would have "potentially" reflected about 1 MILLION times ! I don't think(could be wrong) that a photon can reflect that many times without total loss.

 

[DaTario]

I guess, the best superconducting cavities can have the photon bouncing inside for still less than 0.5 seconds. There are perspectives of arriving at 1 second.

The experimental group dealing with such cavities is the French, Ecole Normale Superieur at Paris, Mr. Serge Haroche may be the lead researcher.

 

[Lyuokdea]

Although everybody here has probably heard of this, I've always enjoyed the three polarizer experiment. Basically, you set up two polarizers at 90 degree angles to each other so that there is no transmitted light through both polarizers. Then you set up a third polarizer in between the first two polarizers and spin it around changing the transmittted light from 1/8 of the original intensity to zero intensity and everywhere in between.

Without the mathematical idea of projections, it doesn't make any sense whatsoever, but then with the idea of projections, it shows just how important mathematics is in the physical world.

~Lyuokdea