Ask a physicist anything. (6)

[Wiccan_Child]

What is your favorite - and least favorite - branch of physics?

Favourite: particle physics
Least favourite: electromagnetism and electronics. Those blasted resistors!

If you held a magnifying glass to the sun, then pointed that light refraction at another magnifying glass just below it at the point where the light is most concentrated, and did this over and over repeatedly...

So say like 8 stacked magnifying glasses where the ray of sun light would be filtered through at the point of highest concentration, what might be the end product of this?

Would it be a small nanoparticle but of pure hellfire or would it be nothing at all since it's been filtered so many times?

This is what would happen:

Jem Melts Rock Using Sunshine - Bang Goes The Theory - Series 3, Episode 5 Preview - BBC One - YouTube

 

[AV1611VET]

If I hold a basketball out at arm's length, with a tennis ball on top, then drop both of them; as I understand it, the basketball will bounce, but the tennis ball will be shot high into the air.

What law of physics is this demonstrating, and what exactly is happening please?

 

[Wiccan_Child]

If I hold a basketball out at arm's length, with a tennis ball on top, then drop both of them; as I understand it, the basketball will bounce, but the tennis ball will be shot high into the air.

What law of physics is this demonstrating, and what exactly is happening please?

Elastic deformation and (ultimately) electromagnetic repulsion. What exactly is happening is that, when the basketball hits the ground, it deforms. Its strong structure pushes against this, so it quickly reforms, pushing the deformation outwards, creating a ripple that rapidly travels up and around the ball to the top (like a ring expanding from the 'south pole' to the 'equator' to the 'north pole'). When this ring hits the top, it basically kicks the tennis ball up.

So you have elastic deformation rippling up the ball, which then imparts a lot of energy to a small object, accelerating it by a large amount. This is the same thing that causes the basketball to bounce: just as the ripple kicks the tennis ball off, the start of the ripple also kicks the basketball off the ground.

 

[Jazer]

So say like 8 stacked magnifying glasses

Telephoto cameras have 8 or more lens. Sometimes glass sometimes plastic.

 

[TheReasoner]

Telephoto cameras have 8 or more lens. Sometimes glass sometimes plastic.

Or other materials. Depending on the requirements of the camera in question. For regular uses you're right. Though I've been working (theoretically) with CaF2 lenses for VUV.
It's quite cool ^^

 

[mzungu]

Telephoto cameras have 8 or more lens. Sometimes glass sometimes plastic.

The only problem with more lenses is light loss due to absorption. So in order to compensate for this one needs larger lenses. But with nanotechnology around the corner I think we may yet see amazing lens technology!

 

[Chalnoth]

If I hold a basketball out at arm's length, with a tennis ball on top, then drop both of them; as I understand it, the basketball will bounce, but the tennis ball will be shot high into the air.

What law of physics is this demonstrating, and what exactly is happening please?

Wiccan_Child already answered this, but I thought I'd provide a more quantitative answer since I do like this particular problem (which often appears in physics classes).

To get a handle on this problem, I'll make three simplifying assumptions:
1. The basketball and the tennis ball are perfectly-elastic (that is, if you bounce them off of the Earth, they bounce as high as you dropped them).
2. The basketball is so much more massive than the tennis ball that we can completely neglect the mass of the tennis ball.
3. We can ignore air resistance.

Okay, with those assumptions out of the way, we can calculate pretty easily how high the tennis ball will bounce. Since the tennis ball is much less massive than the basketball, from the tennis ball's perspective, bouncing off the basketball will be like bouncing off the Earth: it will bounce upward with the same relative velocity as it struck the basketball. But what is the relative velocity?

By the time the tennis ball strikes the basketball, the basketball has already struck the ground and is bouncing upward. So the basketball is going upward just as quickly as the tennis ball is going downward. Their relative velocity, then, is 2v. The tennis ball will go from a relative velocity of 2v downward to a relative velocity of 2v upward, and since the basketball is going upward at v, this means the total velocity of the tennis ball is 3v relative to the ground.

And when you throw an object upward, how high it goes changes as the square of the velocity: throw it twice as fast upward and it goes four times as high. So the tennis ball ends up going nine times as high as the basketball. That means you can drop the two, say, one and a half feet off the ground, and the tennis ball will smack the ceiling (unless you have a particularly high ceiling).

Of course, there were some simplifying assumptions made here, but the end result isn't going to be horribly different in reality. And it's fun to do!

 

[Chalnoth]

The only problem with more lenses is light loss due to absorption. So in order to compensate for this one needs larger lenses. But with nanotechnology around the corner I think we may yet see amazing lens technology!

I think that in practice, it isn't absorption, but rather light loss due to reflection at the boundary of the lens. However, this can be almost entirely eliminated at specific wavelengths with the use of special coatings on the lenses.

 

[AV1611VET]

Elastic deformation and (ultimately) electromagnetic repulsion. What exactly is happening is that, when the basketball hits the ground, it deforms. Its strong structure pushes against this, so it quickly reforms, pushing the deformation outwards, creating a ripple that rapidly travels up and around the ball to the top (like a ring expanding from the 'south pole' to the 'equator' to the 'north pole'). When this ring hits the top, it basically kicks the tennis ball up.

So you have elastic deformation rippling up the ball, which then imparts a lot of energy to a small object, accelerating it by a large amount. This is the same thing that causes the basketball to bounce: just as the ripple kicks the tennis ball off, the start of the ripple also kicks the basketball off the ground.

Thank you, sir!

 

[Tuddrussell]

Depends on how the universe ended. If you could go to another universe, sure. If you can't, no. Besides, the universe trucked along for 13.5 billion years, all of which is interesting and fascinating, and our paltry 70-odd years aren't any more interesting.

What you are describing here is a contradiction of terms, namely objectively subjective. Interesting is an adjective, that requires an observer. Without an observer the universe is merely itself: A noun.

 

[chris4243]

Is there anyway to survive or stop the end of the universe? It would be very boring without us around.

Probably not, though the details will depend on just how the universe fails. If it is heat death, best you can do is hunker down near a black hole. The black hole can last you a good 10^100 years or so, during which time it will give pathetically little energy. Won't last forever, but a long time nonetheless.

If you have a Big Rip or Big Crunch, it might be possible to escape it by clearing the galaxies out of or into a large area to change the density of space. Good luck trying to do so however.

 

[razeontherock]

If you have a Big Rip or Big Crunch, it might be possible to escape it by clearing the galaxies out of or into a large area to change the density of space. Good luck trying to do so however.

How big of a shepherd's crook would one need to be a galaxy-herder?

 

[AV1611VET]

How big of a shepherd's crook would one need to be a galaxy-herder?

Reminds me of Kenneth C. Flemings' book, God's Voice in the Stars, in which he shows that the plan of salvation is spelled out in the names of the stars and constellations.

From page 128, concerning Ursa Minor & Ursa Major:

Many people have noticed that the bears in these two star groups have enormous, raised tails, which is uncharacteristic of bears. The bear idea seems to have come from a confusion of words. An old Persian word for bear is similar to the word for sheepfold, and the Greeks appear to have mistake one word for the other...

Once again we must look at the ancient names and their meanings to find any light on the prophetic meaning which God intended from the beginning. The Greeks called this constellation Arcas or Arctos, from which we get words like arctic in English. Arcas meant Bear and is used as such in Scripture, but the root meaning is The Stronghold of the Saved.

...

It shows us the secure sheepfold where the redeemed are gathered, waiting for the coming of Messiah. The names of the stars confirm this. Kochab means Waiting for the Coming and thus adds its witness. Alkaid, an unidentified star, means The Redeemed Assembly, and another star Alpherkadain, means The Redeemed Assembly. Another name which we have noted in several constellations is found here. It is Algedi, The Kid, and pictures the price by which the redeemed were bought. The place of this safe sheepfold is found in the name Polaris, which the Greeks called Cynosure, meaning High in Rising...

These point to the heavenly and central place where the redeemed are held safely. Abraham looked for such a place, "a city which hath foundations, whose builder and maker is God (Hebrews 11:10)."

 

[Ba'alServer]

Quoting Chalnoth's response to AV1611VET:

"Wiccan_Child already answered this, but I thought I'd provide a more quantitative answer since I do like this particular problem (which often appears in physics classes).

To get a handle on this problem, I'll make three simplifying assumptions:
1. The basketball and the tennis ball are perfectly-elastic (that is, if you bounce them off of the Earth, they bounce as high as you dropped them).
2. The basketball is so much more massive than the tennis ball that we can completely neglect the mass of the tennis ball.
3. We can ignore air resistance.

Okay, with those assumptions out of the way, we can calculate pretty easily how high the tennis ball will bounce. Since the tennis ball is much less massive than the basketball, from the tennis ball's perspective, bouncing off the basketball will be like bouncing off the Earth: it will bounce upward with the same relative velocity as it struck the basketball. But what is the relative velocity?

By the time the tennis ball strikes the basketball, the basketball has already struck the ground and is bouncing upward. So the basketball is going upward just as quickly as the tennis ball is going downward. Their relative velocity, then, is 2v. The tennis ball will go from a relative velocity of 2v downward to a relative velocity of 2v upward, and since the basketball is going upward at v, this means the total velocity of the tennis ball is 3v relative to the ground.

And when you throw an object upward, how high it goes changes as the square of the velocity: throw it twice as fast upward and it goes four times as high. So the tennis ball ends up going nine times as high as the basketball. That means you can drop the two, say, one and a half feet off the ground, and the tennis ball will smack the ceiling (unless you have a particularly high ceiling).

Of course, there were some simplifying assumptions made here, but the end result isn't going to be horribly different in reality. And it's fun to do!"

End Chalnoth's quote



This is why we can't have nice things.

Throwing something up at twice the velocity doesn't make something travel four times as high.

Don't neglect the mass of the tennis ball and basketball. You can't rely on velocity alone - momentum comes into play.

The tennis ball and the basketball are still in contact when the basketball hits the ground.

The basketball bounces, and the force acting on the tennis ball is the basketball's mass times its acceleration, coupled with the tennis ball's mass times its acceleration.

I'm trying to avoid using equations here, so here's a site that explains it without them: science-projects dot com/drop/dropballs dot htm
Replace the dots with periods because I still can't post links here.

Chalnoth - Do you happen to work for Fiat?

 

[Chalnoth]

This is why we can't have nice things.

Throwing something up at twice the velocity doesn't make something travel four times as high.

As long as you can safely neglect air resistance and as long as you don't throw it very high, yes, that's precisely what it means. Gravitational potential energy near the surface of the Earth is linear with height, but kinetic energy goes as the square of velocity. So twice the velocity means four times the energy, which means four times the height.

The tennis ball and the basketball are still in contact when the basketball hits the ground.

The basketball bounces, and the force acting on the tennis ball is the basketball's mass times its acceleration, coupled with the tennis ball's mass times its acceleration.

I'm trying to avoid using equations here, so here's a site that explains it without them: science-projects dot com/drop/dropballs dot htm
Replace the dots with periods because I still can't post links here.

Go ahead and try it, if you have access to a fully-inflated basketball and a tennis ball (or any small object will work, really: a marble, a golf ball, whatever...all you need is a large, bouncy ball and a small object that is much less massive). Hold it about a foot and a half to two feet off the ground, and watch as the smaller object either gets close to or actually smacks the ceiling.

But no, it isn't as you paint it. You'll get the wrong answer because the basketball's mass is irrelevant as long as it is much larger than the tennis ball. If your statement were correct, then as you bounce smaller and smaller things off the basketball, they'd go higher and higher. But the truth is they'll limit out at 9 times the drop height.

Chalnoth - Do you happen to work for Fiat?

Nope. Working in Italy on the Planck satellite.

 

[AV1611VET]

Chalnoth, I'm with BS on this one.

The basketball/tennis ball were not "thrown", they were "dropped".

And the tennis ball did not lose contact with the basketball on the way down, thus the basketball did not bounce up and hit the tennis ball on its way down.

 

[Chalnoth]

Chalnoth, I'm with BS on this one.

The basketball/tennis ball were not "thrown", they were "dropped".

And the tennis ball did not lose contact with the basketball on the way down, thus the basketball did not bounce up and hit the tennis ball on its way down.

Like I said, try it. The height closely matches the solution I gave: 9 times the drop height. It will be a little bit less than that, but close nonetheless.

 

[AV1611VET]

Like I said, try it. The height closely matches the solution I gave: 9 times the drop height. It will be a little bit less than that, but close nonetheless.

-- I'm not allowed to bounce a basketball in the house.

 

[razeontherock]

And by the looks of you, it's a good thing too! Are you really in Guam?

 

[AV1611VET]

And by the looks of you, it's a good thing too!

Are you really in Guam?

No -- but we used to live there.