Bet ya can't answer these...

chump

Well I hope ya can...

Question 1.
If 25Joules of energy is created in the form of heat at 300K, how much energy is required to extract this heat and maintain an enviromental temperature of 75K?
Basically it's a closed system you wish to maintain at 75K, heat is produced inside this system at 300K (25Joules worth of heat energy), how much energy will be wasted in eliminating this heat and restoring the systems desired temperate of 75K.

Question 2.
A small boat in a pond has a 50kg box of chocolates in it. If you throw the 50kg box of chocolates into the pond and it travels to the bottom, where it rests, which scenario ...
a) chocolates in the boat or
b) chocolates at the very bottom of the pond (touching the ground)
leaves the water level in the pond at a higher level?

If you can answer these badboys I'll throw you a few more later...

DonkeyStyle \o/

50kg?! cripes that's some box of chocolates

chump

Think I might have the 1st one...

efficiency carnot fridge = Tc / (Th - Tc) = 75 / 225 = 1 / 3

So It takes 75Joules worth of work to eliminate the 25Joules created at 300K?

chump

Found the 2nd answer:
http://physics.about.com/cs/puzzles/a/160603.htm

Sev

Here's a good one, got this from the site above, takes a real physicist to get this. Managed this myself actually pretty quickly due to a lot of practice with this kind of thing this year.

Anyway... youre given a wooden plank and two metal spheres of equal size and equal weight. However, one is hollow and one is solid, and you're asked to determine which is which.. how?

*Angel*

Could you weigh one in water and the difference between its weight in air and in water is its buoyancy, B = W/S. Do the same for the other one and find out which has the greater density, thereby having your answer.

Can you whack each one with the wood, and see which one has a bigger dent?

*Angel*

oops I forgot about the wood...

Sev

*Angel* wrote:

Could you weigh one in water and the difference between its weight in air and in water is its buoyancy, B = W/S. Do the same for the other one and find out which has the greater density, thereby having your answer.

You'll find they both have the same density that way, because they both have the same volume, and both have the same mass, so they will behave identically when put in water.

Kevin_rc_ie

strike them together and see which makes the hollow sound,=

Kevin_rc_ie

also will one vibrate more because it's hollow?

*Angel*

um hit them both with the plank of wood and the hollow one will make more noise...

Sev

Can you whack each one with the wood, and see which one has a bigger dent?

Well they both weigh the same, so there should be no difference.

strike them together and see which makes the hollow sound

Thats a good one actually, somebody just brought that to my attention and I was about to post that.. but not the answer I was hoping for.. lets just say for the sake of argument, that they both make the same noise when you hit them.

Kevin_rc_ie

If I remember physics well enough, Jutt's Law "a hollow object when struck with a non solid metalic object plays the tune to family guy backwards."

Sev

Kevin_rc_ie wrote:

If I remember physics well enough, Jutt's Law "a hollow object when struck with a non solid metalic object plays the tune to family guy backwards."

Ok, well that may be true.. but lets just say that in this problem, the solid ball is rigged with some device so that when it is struck it also plays the tune to family guy backwards, so you cannot differentiate by that method.

LeperKing

Lever the wood submerged in water, and place the two spheres at either end. The solid sphere will go down.

Kevin_rc_ie

i assume cutting them in half is out of order.

Sev

LeperKing wrote:

Lever the wood submerged in water, and place the two spheres at either end. The solid sphere will go down.

They both weigh the same, and have the same volume, so theres nothing that floating them in water will tell you about them.

Kevin_rc_ie wrote:

i assume cutting them in half is out of order.

Yeah.. I dunno about your experiences, but I think you'll have a hard time trying to cut a solid steel ball with a plank of wood.

Kevin_rc_ie

I've just cheated so I'm giving a wee clue

Think of Potential Energy and Kinetic Energy and a ramp

Would the hollow sphere go down the ramp slower, because most of its weight resides on the surface of it?

Sev

Yeah, it takes more energy to rotate a hollow sphere, than to rotate a solid sphere of the same weight (the hollow one has a greater moment of intertia.

It's like if you had a rod balanced on a point, and all its mass was concentrated in the centre over the point, it would be very easy to rotate it. And would have much less energy of rotation when you got it spinning than a rod that had its mass concentrated at its end.

This is just a generalisation of that idea to a sphere. More of the energy that the sphere is gaining at a steady rate from gravity goes into rotation for the hollow ball than for the solid ball, so the hollow one has less translational kinetic energy as a result.

Kevin_rc_ie

Sev wrote:

They both weigh the same, and have the same volume, so theres nothing that floating them in water will tell you about them.



Yeah.. I dunno about your experiences, but I think you'll have a hard time trying to cut a solid steel ball with a plank of wood.

But the balls are metal not steel. They could be steel, but they also could be lead. Chopping them in half isn't completely impossible.

ISAW

Sev wrote:

Here's a good one, got this from the site above, takes a real physicist to get this. Managed this myself actually pretty quickly due to a lot of practice with this kind of thing this year.

Anyway... youre given a wooden plank and two metal spheres of equal size and equal weight. However, one is hollow and one is solid, and you're asked to determine which is which.. how?

Roll them down the plank.
Angular momentum must be different if they are the same size and weight i.e. the hollow one has more mass near the outside.

derek27

if you were to take the two metal spheres, and charge both of them equally using the charge by induction technique, you would be able to determine which of them was the hollow one because it is known that all charge resides on the outermost surfaces of a hollow metal conductor, and it could be determined which of the spheres has all the charge on its surface.

Capt'n Midnight

Dolphins can tell the difference between hollow and solid balls.

The answer is to do with rotational energy. The hollow sphere will have less of it's mass at the centre so will have less energy if both are spinning at the same speed. Or if you apply the same amount of energy to both the the hollow one won't spin as fast. Potential energy is mgh so..

Incline the plank and roll both balls down at the same time. The solid one should get there first.

freakydeadgirl

yeah the solid sphere would reach the bottom first because the hollow sphere has the greater rotational inertia relative to its mass, which makes it "harder" for it to start rolling

MrAbc

My version of the rotation test would have been styled for use in Caveman Physics 101:
* set the two spheres rotating on a smooth surface at the same angular speed(more massive the better...heh heh)
* and then, bringing the wood into equally forceful contact with each in turn, see which rotating sphere exerts the more prolonged friction and so more easily sets fire to the wood :-D LOL ....Ugh Ugh Fire.... and, naturally, the quicker to flare up is hollow.


Alternatively, you could first set fire to the wood(good practice for cavemen)... then heat the surface of each sphere in turn with same flame/same distance/etc and the difference in specific heat capacities (hollow sphere must be made of higher density material) would result in different temperatures in the initial heating stage.
A slower rising temperature is likely to indicate the (higher outer layer density of the) hollow sphere... not a purely guaranteed method, but almost so, and doesn't involve measuring any velocities... although you might burn some fingers checking on the surface temps :-)