hello

First question - If only force acting on an object is gravitational force then total mechanical energy TME of system is conserved. What about when two objects, each with it's own kinetic energy, collide in an isolated system?
Is total mechanical energy of a system conserved (the way the momentum is conserved)?



Second question - Moving ball has two types of kinetic energy. One is from the fact that its center of mass is moving (I will call it external kinetic energy) and other type comes from the fact that its molecules are in termal motion. If ball comes to a sudden stop we say that its external kinetic energy is now zero (cchange of kinetic energy is equal to work F*s ). But its internal kinetic energy is still the same.

Do the molecules of a ball receive the energy we call external energy and each molecule gains extra kinetic energy (besides already having its own internal kinetic energy we call thermal energy)?
When the ball comes to a stop then the same amount of external kinetic energy that each molecule gained is now again lost?



And third question - If temperature is a measure of the average kinetic energy, then why are the effects of large kinetic energy of molecules in a body so different from external kinetic energy?

If ball receives a large amount of kinetic energy then this ball gains lots of speed (assuming there is no friction and air resistance). But if internal energy of the ball, as in kinetic energy of molecules ( thermal energy), gains lot of kinetic energy, then besides molecules gaining speed, the ball gets hotter.

My question: Both external and internal kinetic energy (thermal energy) cause an object (external) or molecule (internal) to move faster. But since the type of energy is the same (kinetic), why does internal kinetic energy also cause an object to get hotter while external doesn't ?

thank you

Ok, first of all: YES. Energy is conserved - no-one would give a rat's ass about energy if it wasn't. It changes "forms" though, so you can't conclude that the kinetic energy of an object associated with an object's centre of mass (what you've called the "external" energy) is conserved in all collisions, though it is sometimes. Collisions where the total "external energy" IS conserved are called "elastic" collisions.

Now, to see if we can clear some issues about kinetic energy - you seem to have got it backwards, since its usually taught that way.

In general, you have an object, which is basically a bunch of particles (with some structure to them). You can look at all the particles individually and look at how much kinetic energy each one has. If you sum up all these energies you get the total kinetic energy of the object, right? But we've lost some information, since energy is a scalar quantity, not a vector quantity. These particles could all be travelling in the same direction, flying around in every direction, or something in between. So you might want to ask how "organized" all this kinetic energy is.

It turns out to be convenient to split kinetic energy up into "components" if you like. You can measure the average velocity of the molecules, weighed by their masses (the velocity of the centre of mass), and "associate" a certain amount of the total kinetic energy by calculating it using (1/2) * (total mass) * (velocity of centre of mass)^2. This gives the "translational" kinetic energy, which you can think of as the degree to which the kinetic energy is "organized." There's also rotational kinetic energy (for solids), and the rest is heat - a measure of how disorganized the kinetic energy is, i.e. to what extent the molecules are flying arond in every direction (and temperature is proportional to this heat energy). Summing up the translational, rotational, and heat energy just gives you back the total kinetic energy.

Most collisions (except the "perfect elastic" collisions) have a tendency to shake things up a bit. The total kinetic energy of the system stays the same, but the collision causes a bit of havoc at the molecular level. The disorder increases, which means that the center of mass slows down and the translational kinetic energy drops (same goes for the rotational kinetic energy). Since the total kinetic energy stays the same, the heat component goes up. Collisions usually heat things up.

Hope that helps.
Hope that wasn't too daunting!
Welcome to sciforums, by the way... :)

First question - If only force acting on an object is gravitational force then total mechanical energy TME of system is conserved. What about when two objects, each with it's own kinetic energy, collide in an isolated system?
Is total mechanical energy of a system conserved (the way the momentum is conserved)?

Only in elastic collisions. Otherwise, some mechanical energy can be converted to heat.


Second question - Moving ball has two types of kinetic energy. One is from the fact that its center of mass is moving (I will call it external kinetic energy) and other type comes from the fact that its molecules are in termal motion. If ball comes to a sudden stop we say that its external kinetic energy is now zero (cchange of kinetic energy is equal to work F*s ). But its internal kinetic energy is still the same.

Do the molecules of a ball receive the energy we call external energy and each molecule gains extra kinetic energy (besides already having its own internal kinetic energy we call thermal energy)?

It depends on what stops it. Usually, there will be heat tranfer to some other object, but the ball might heat up too.


And third question - If temperature is a measure of the average kinetic energy, then why are the effects of large kinetic energy of molecules in a body so different from external kinetic energy?

Because the thermal motions of molecules are random. Different molecules vibrate in different directions, giving the overall object no net motion. The kinetic energy of the object as a whole is related to the movement of all parts in the same direction at the same time.


My question: Both external and internal kinetic energy (thermal energy) cause an object (external) or molecule (internal) to move faster. But since the type of energy is the same (kinetic), why does internal kinetic energy also cause an object to get hotter while external doesn't ?

What do you mean by "hotter"? If you mean the temperature of the object increases, well it must, by definition, since temperature is directly related to the average internal kinetic energy.

But perhaps you're asking why you would feel the object to be hotter. That has more to do with the efficiency of the heat transfer from the object to your hand.

and the rest is heat - a measure of how disorganized the kinetic energy is, i.e. to what extent the molecules are flying arond in every direction (and temperature is proportional to this heat energy).

And that is the reason we only define temperature as a property for a large number of particles that are moving randomly,but can't define it for single molecule?

thanx to both of you for nice explanations :)