Recent content by raxp

Parcel theory holds that as air is heated, it expands. Its density hence decreases and the hot air "floats" upwards, pushed by the colder, more dense air surrounding it. It is an experimental fact that hot air rises, but the explanation from buoyancy seems suspect. In a gas, all motions are...

No, that's true. That's a force that is equal to Ma. Including this load will increase the frictional force in the forward direction, such that in the left-most diagram, the frictional force is ##f##?

I don't quite follow. You are saying that taking into account the effect of gravity, there must be a forward frictional force in the left-most force diagram?

No, the block and tackle are stationary and not part of the system.

Well, the paradox must be apparent after all. The thing is, you speak here in terms of energy and work and I agree, it is hard to see any paradox then. When, however, expressed dynamically in terms of the acceleration, you obtain Newton's second law with an effective mass that is smaller when...

Here is what I have been told of the frictional force, when a torque with net linear force is applied to the wheel. The green arrow represents the frictional force. Is this wrong?

I would love for this explanation to be correct (and it was my initial conclusion), but I have it on good authority that the frictional force from the ground for a single wheel when a force is applied parallel to the ground at the top as in my second case is zero, and that the ground friction...

Yes, but the paradox is the following in this case: You have before you a large weight, and even though its underside is lubricated, its inertia is simply too large for you to make an appreciable change to its velocity. You then add a small wheel (small ##m r^2##) to the configuration...

It depends on the reference frame. If we take the reference fram to be comoving with the floor, then yes. See below, however. Yes, thank you. In other words $$E = \frac 1 2 (M + \frac 3 4 m) v^2$$ in the original post. As for the paradox, however, it still remains. In particular, if we let ##r...

OK. And I take it the analogous explanation for the case where ##M \to \infty## and/or ##m \to 0##? Introducing a massless wheel of vanishing radius before the mass ##M## will double the acceleration (essentially double the force)? The result is only counterintuitive, not paradoxical?

This is the third time I try posting, and the first time after having written an Introduction. I hope it will work this time. Imagine we have a cuboid of mass ##M## and height ##2 r## that slides without friction on a horizontal surface. It is accelerated by a line or rod that is connected to...

I tried to post twice, but my posts disappear. Then I read the instructions, and I suppose I will not be allowed to post until I have introduced myself. I am chemist by training with a general interest in physics.