Science Quiz

[The Last Conformist]

I think a quiz is suboptimally suited for that, because the format somewhat discourages one from mentioning an idea unless one thinks it amounts to a complete answer.

By now, I'm very curious as to the answer to you question, but I'll be damned if I can think of anything.

 

[col]

All good examples indeed. But youre right I have something else in mind that seems paradoxical. I asked a similar question to a student at a university interview once. Sadly they couldnt come up with anything at all.

How can friction increase kinetic energy?

(No atomic or heat based answers either please )

 

[betazed]

Ok. I came up with another scenario where friction can increase kinetic energy (not directly but eventually).

Imagine throwing an object towards a planet with atmosphere. because of atmospheric friction it will slow down and remain longer under the gravitational influence of the planet. Hence the kinetic energy gain owing to gravity slingshot is more.

I am not saying this will always happen for all trajectories. But I am sure given a particular planet and atmosphere we can devise an initial trajectory that can have greater final velocity than if the planet did not have atmosphere.

And I am sure you did not have this one on your mind either.

 

[col]

Not quite the right scenario but you're getting warm. (pun intended).

 

[betazed]

Originally posted by col
Not quite the right scenario but you're getting warm. (pun intended).

Ok. I will try again.

A Ice block moving on an ice bed, will create friction and generate heat melting a little layer of water beneath the moving ice block which will act as a lubricant and reduce friction and increase speed.

I can also imagine that friction can raise the temperature of a moving object and change its surface properties and thereby reduce the coefficient of friction. But I know of no example of this.

 

[The Last Conformist]

It seems to me the kinetic energy wouldn't increase, merely decrease more slowly, in those scenarios?

 

[col]

Nope cold again. I agree with TLC - I dont think you can gain KE with a lubricant.

Gravitational fields are involved.

 

[Pirate]

Hmm the only example I've heard was the bowling ball slipping and rolling down an alley. Friction acts against the direction of sliding which is in the direction of rotation. So friction increases the ball's rolling velocity.

Does the friction directly increase kinetic energy? Or can it be some Rube-Goldbergian way to harness the waste heat of friction to boil water and shoot a jet of steam out the back of the vessel to add thrust? Hey, I'm just spitballing here...

 

[col]

Ok a hint then.

Why is docking spacecraft more difficult than docking aircraft?

 

[The Last Conformist]

Naively, I'd say because relative velocities are typically greater ...

 

[betazed]

Originally posted by col
Why is docking spacecraft more difficult than docking aircraft?

Is it? I always thought it was the other way around. While docking spacecraft you can calculate the entire glide path beforehand and fire your thrusters appropriately etc.

For docking aricraft you would need a feedback loop control system because of all the turbulence etc. will not allow to calculate a path before hand.

I am sure I am missing some point here big time!

 

[Guildenstern]

One would think that if the solution "involves gravitational fields" then it is the gravitational field and not the force of friction which is responsible for the increase in kinetic energy.

Considering that the force of friction is defined to be opposite the direction of motion, I don't see how frictional forces can be anything but dissipative.

Also, there is kinetic energy associated with rotation. Thus, an object could begin with more rotational kinetic energy, and transfer some of that rotation into lateral motion, perhaps through the intervention of friction, but if friction is involved then the overall energy must decrease (meaning the sum of rotational kinetic energy, linear kinetic energy, and potential in whatever field is causing the motion).

If I drop an object in air, it experiences friction, but its kinetic energy increases. This does not mean the friction has caused the object to accelerate; the gravitational field is responsible for this.

 

[col]

Ok Here's the solution. Consider an object in circular orbit. It has KE and PE. Its PE is negative and always twice its KE which is positive. Its total energy is thus negative if its bound. When it gains energy it moves to higher orbit and slows down. When it loses energy it moves to a lower energy and speeds up.

If we try to close on an object in front of us by firing our rockets, we move to a higher orbit and slow down. If we want to speed up we have to fire our rockets forward. This is why docking is so much harder when youre in orbit.

When a satellite hits the top of out atmosphere, friction causes it lose total energy. If you do the sums for an object on orbit, when it loses energy it speeds up - it gains KE but loses twice as much PE. Thats why satellites burn up - they go faster and get more friction going into a positive feedback loop.

So for an object in orbit, friction makes it gain Kinetic energy.

 

[The Last Conformist]

*Slaps myself*

Really, I should've recalled that.

Who's to go now?

 

[col]

Floor is open for anyone to ask a question.

 

[betazed]

Ok. Seeing that I was closest to Col's answer I will take the liberty to ask the next one. An easy one.

Why does lightning strike sharp pointy objects (like the lightning rods that are placed atop tall buildings)?

 

[The Last Conformist]

Because an electric field around something with a constant surface charge density is strongest near sharp points.

But I guess you want an explanation why that is so. Hm, I should recall my EM courses better ...

That a stronger field helps, btw, is because it's closer to break lose electrons from air molecules to create a conducting path.

 

[betazed]

Originally posted by The Last Conformist
But I guess you want an explanation why that is so. Hm, I should recall my EM courses better ...

You answered the how. As you pointed out a why answer wil be nice. It is pretty obvious anyway.

 

[The Last Conformist]

Lemme see. The displacement field strength immediately above a conducting surface is proportional to the surface charge density. Since we're speaking about a conducting surface, the charges are free to move to minimize their potential energy. At a perfectly even charge distribution, charges near a sharp point will feel repulsion from the charges on the main part of the object, but nothing much from the direction of the point, and will therefore move towards the point. This continues till there's enough excess charge at the point to counterbalance the charge of the bigger surfaces on the main part, at which point the minimum potential energy distribution is reached. There's now a higher charge density near the point, and therefore a stronger field next to it.

 

[betazed]

Originally posted by The Last Conformist
Lemme see. The displacement field strength immediately above a conducting surface is proportional to the surface charge density. Since we're speaking about a conducting surface, the charges are free to move to minimize their potential energy. At a perfectly even charge distribution, charges near a sharp point will feel repulsion from the charges on the main part of the object, but nothing much from the direction of the point, and will therefore move towards the point. This continues till there's enough excess charge at the point to counterbalance the charge of the bigger surfaces on the main part, at which point the minimum potential energy distribution is reached. There's now a higher charge density near the point, and therefore a stronger field next to it.

Now you contradicted yourself. In your last post you said that the entire surface is in a constant charge density. And that is correct. Otherwise there will be surface current till it is at same charge density. The entire surface of a conductor is always at a equipotential.

So no the explanation is not owing to anything about charge density.