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4:28 AM
Hello @satan29
 
hi
@PrateekMourya whatsup?
 
I am bit confused with worl
Work
Is it force times displacement of point of application
@KnightadmiresChappo is the answer $(2m((omega)^2)r)$ vector+(mg)vector?
This is actually a jee question
In terms of time is it (2m((omega)^2)e^(omegat))?
Sorry i miscalculated
It is same from jee advanced 2016 pt 2
 
5:01 AM
@PrateekMourya yes
@KnightadmiresChappo this is very easy in the rotating frame: The only force acting radially is the centrifugal pseudoforce mw^2 *x ( at a doistance x from the center)
thus, You get the differential equation: $m\dfrac{d^2x}{dt^2}=mw^2x$
 
There is coriolis force too which is balanced by normal from wire
In sideways
 
note that the LHS is $dv/dt$= $(dv/dx) * (dx/dt)=v*(dv/dx)$
@PrateekMourya correct.
So the DE effectively becomes $vdv= w^2xdx$
you can solve for v as a function of x, and then v=dxt/dt, so you can solve for x as a function of timw
OH Wait lol, I just realised the question asked for force, and not for x(t)
for force, I will work in polar coordinates
@RewCie yo wassup
 
JEEA 2016 gave the equation in the comprehension....so it effectively became a maths question rather than a physics one
 
Ya original equation had -ve sign
@satan29 suppose a block with velocity v is projected on a horizontal table
The work done here will be the displacement of block time friction on table?
For table?
Actually in various book they don't mention work is force component along displacement of point of application Times displacement of point of application so i found in some questions i am confused
 
6:01 AM
Hello @JohnRennie sir
 
@PrateekMourya hi :-)
 
Sir suppose a block with velocity v is projected on a horizontal table
The work done here will be the displacement of block time friction on table?
For table?
Actually in various book they don't mention work is force component along displacement of point of application Times displacement of point of application so i found in some questions i am confused
 
It's always worth taking a step back and considering how the energy is moving. In this case the block starts with some KE and it loses that energy as the KE is turned to heat by the friction. So energy is flowing out of the block, and that means the block is doing work.
 
So the block is doing work on the table, and the work done is just the force the block exerts on the table times the distance the block moves. Yes?
 
6:05 AM
Here the point of application of force will be on the?
 
The point of application of the force is where the block touches the table.
 
It is changing
 
That's OK, the point of application is allowed to move
 
Ok so changing the point of application and moving is same?
 
Yes
 
6:07 AM
@JohnRennie ok
 
By Newton's third law the force the block exerts on the table is the same as the force the table exerts on the block, and that is just the frictional force. Yes?
 
Yes
Ok i got it
 
So the work the block does on the table is the distance the block moves times the frictional force.
 
One more question in joule heat law
 
Yes ... ?
 
6:10 AM
Why do we take the entire kinetic energy of block
And say it is used to change temperature of table
Wait
Every question had some percentage of conversion
Does it indicates that some portion is used to increase temperature of tabel
 
100% of the KE that the block has goes into the work done by the block.
The question is where that energy goes.
If we assume the table is fixed then the velocity and KE of the table cannot change, so all the work done by the block goes into heat.
 
And some to block?
 
Yes, I guess both the table and the block heat up so when the KE turns to heat that heat is shared between the block and the table.
 
If we take block as a system
Can we said mechanical work on it is equal is equivalent to to heat supplied to it to change is temperature
 
The work is always force times distance, or ∫F.dx if the force varies. The work done by the block is equal to the change in its KE i.e. all the KE the block originally had is used to do work.
The block gets back some of that energy as heat, so the total energy of the block decreases by the work done and increases by the amount of heat it gets back from the friction.
The rest of the work goes into heating the table.
 
6:22 AM
I believe I am unclear with the law
I must read it again
 
It's easy to get confused about work, but it is simpler than you think.
 
Work is equal to change in kinetic energy of the block
 
Energy is always conserved. We know the KE of the block decreases to zero so that energy must go somewhere, and it goes somewhere by the block doing work.
@PrateekMourya yes, that's the work-energy theorem.
 
So taking block as a system its changing kinetic energy
Is the work on it
Sau cording to joules law
We can find the the equivalent heat energy associated with the work
Where this goes wrong
 
@PrateekMourya this point is a bit subtle since we mostly are accustomed to point particles. This point comes in handy when we talk aboutt rigid bodies.
for instnace, suppose we apply 10N force at the top of a rolling wheel
and suppose we apply it till the COM of the ring moves 10 metres in front
 
6:27 AM
Please wait satan i have to clear my question on joules law
 
now work is not 100N in this case
 
Before class
As its recorded
So no opportunity to ask doubts
 
@PrateekMourya the change in the KE of the block is certainly equal to the work done on or by it. So far so good.
 
Instantaneous ly
 
since the point of application of force (i.e the TOP point) has moved by 2*10 metres, not 10 meters. So the work done is 200 N.
 
6:28 AM
Are you OK with this so far?
 
@PrateekMourya Oh ok, apologies
 
Ok @JohnRennie sir
 
So where are you running into problems?
 
The joules law saya tge mechanical work is equivalent to in heat energy
 
6:32 AM
Joule's law is a special case that applies when all the work goes into heat and none into KE.
 
Statement above equation 25.3
 
OK, but I don't see how that means all work always has to go into heat.
 
Statement below 25.3
 
You mean "It is clear that ..."
 
6:40 AM
That just says if work is converted to heat then the amount of heat is equal to the amount of work.
NB if work is converted to heat
 
All that for a drop of words
@JohnRennie thanks
 
:-)
 
Can you give the final statement of juice law with the with all the the final points to be kept in mind
Sorry joules law
if a portion of mechanical work is converted to heat energy then the portion of mechanical work can be mathematically related to the heat energy generated by the joules law
 
Yes
 
Thanks sir
I will discuss remaining questions later
 
6:53 AM
OK :-)
 
@JohnRennie Hi! Does the volume remain constant when a metal(say Iron) is squeezed?
 
@YouKnowMe it depends what you mean by "squeezed"
If you take a sphere and apply a force evenly over the whole surface than obviously the sphere shrinks. Yes?
 
@JohnRennie Yes
 
An iron ball won't shrink much because iron is very hard. But it will shrink a bit. The amount its volume decreases under pressure is determined by the bulk modulus.
 
@JohnRennie Volume is different for both the solids?
 
7:08 AM
What is the diagram showing? It looks as if you get from the left side to the right by cutting off the top half of the cone.
 
NO
 
So what is going on here?
 
@JohnRennie
 
It depends what the cone is made of, so the question can't be answered. If the cone is made of a very soft metal like lead it will squish out sideways. The volume will stay constant but it will be some weird shape that's complicated.
 
OK
Thanks
 
7:15 AM
@YouKnowMe If you know what the force is then you can calculate the area because the pressure (force/area) will be equal to the yield stress.
 
7:59 AM
All the videoa on YouTube all that I watched none mentioned if work is converted to heat
 
 
1 hour later…
9:23 AM
@JohnRennie sir Hello
Please clear the following points
 
@PrateekMourya hi :-)
 
How do we assume 100% conversion of mechanical work into to heat energy
 
The original kinetic energy has to go somewhere. It can't just disappear because then energy wouldn't be conserved.
 
Would not be the target in the bullet case get heated
 
And there isn't anywhere else the energy can go.
 
9:29 AM
Would not the floor on which the water strikes get heated
Aur just hear the spherical cow assumption is applied?
 
Oh, OK, yes the book is assuming all the energy goes into the water and the bullet respectively. That is an approximation because some of the heat would go into the stream bed and the target respectively.
 
Spherical cow assumption :)
 
@satan29 Nothing much... Reading Systems Programming... Complicated chapter btw...
 
9:47 AM
I was not aware of this Endianness convention and it was named like that...
 
 
2 hours later…
11:45 AM
@RewCie there is an advantage to little endian notation because it means an int* pointer can be cast to any size of int and the pointer still works. OK this isn't an amazing advantage, but it is nevertheless a real one.
 
 
3 hours later…
2:36 PM
@JohnRennie int *, void *, unsigned char *
 
 
2 hours later…
4:57 PM
@JohnRennie hi sir
 
@PrateekMourya hi :-)
 
I am but confused with heat and energy
I mean what flows when two objects with unequal temperature are kept
Also book said heat is not a form of energy
It is process if flow of energy
 
If you consider a solid then when we talk about it containing some heat what we actually mean is that the atoms in the solid are vibrating around, and the "heat" is actually energy stored in these oscillations.
If we start at absolute zero the atoms are not oscillating at all, so the solid contains no "heat".
Then as we raise the temperature the atoms vibrate more and more, so more and more energy is stored on those vibrations.
If we heat the solid enough the vibrations get so large that the solid shakes itself to bits, and that's exactly what melting is.
OK so far?
 
Now consider what happens if you place a hot solid and a cold solid in contact.
The atoms in the hot solid are vibrating more than the atoms in the cold solid, and those vibrating atoms literally bump into the atoms in the cold solid and make them vibrate more.
So the energy is transferred from the hot solid to the cold solid as kinetic energy when atoms at the junction between the two solids bump into each other.
 
5:04 PM
Ok
 
But when we do thermodynamics we specifically avoid worrying about what happens down at the atomic level.
We know energy gets transferred from the hot to the cold body, but we don't care exactly how this happens.
 
So any form of energy can be equated to each other in conversion due conservation of energy like here kinetic energy is converted to heat energy
 
Yes, though note my comment above.
 
Ok
@JohnRennie this one is important
@JohnRennie this one
@JohnRennie and also this one
I was mainly confused with these
Sir here in these chapters on heat
I am not able to properly digest idea of energy
Will with time i will be ?
I mean the answer you gave are clear
But still
It is sometimes we think we are not still able to know something properly
 
My recommendation is always that if you don't understand the details of something just ignore it for now and move on.
You'll often find that when you return to it later you'll find it a lot easier to understand.
 
5:15 PM
So right now i should just focus on laws and keep them in mind
So that while solving questions it makes sense
 
Yes.
 
Ok sir
I believe you are correct
Good evening
 
At the end of the day your aim is to get a good rank in the JEE. Once you are at college that's when you get a chance to study things in more depth.
 
 
5 hours later…
10:28 PM
i
hi
is anyone on?
 

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