The h Bar

0celo7

5:03 AM

@JohnRennie you should read a book

user228700

@JohnRennie :-) Morning sir.

John Rennie

@0celo7 I read lots of books

user228700

@0celo7 Why?

John Rennie

@Kaumudi Morning

0celo7

@Kaumudi because

it's the only logical possibility

user228700

5:04 AM

@0celo7 As opposed to him just being asleep?

0celo7

hmm

no I don't think that makes sense

user228700

@0celo7 Right, 'cause he doesn't need sleep like the rest of us..?

0celo7

no because he wakes up at 5AM

so when he's not here at 5AM, I conclude something horrible has happened

user228700

@0celo7 Riight. What you need is hope; it's the thing with feathers! :-)

John Rennie

I was a bit late today because Amazon asked me personnally to write a review of a mattress I bought a few weeks ago. These important tasks have to be done first thing :-)

user116211

5:08 AM

@JohnRennie Mattress?

user116211

Not a book?

user116211

hmm.

user228700

@MAFIA36790 Lol :-P

0celo7

feathers:

John Rennie

I recommend not sleeping on books or reading mattresses

0celo7

5:08 AM

user116211

@JohnRennie I sometimes make books as my pillow.

user116211

They are really awesome.

John Rennie

Doesn't that make for a rather hard pillow?

user116211

Till your neck pains.

user228700

0celo7

5:10 AM

@JohnRennie no you commie

user116211

^

user228700

^Feathers :-)

John Rennie

I'm a communist because I like soft pillows?

0celo7

no

you're a communist because you hate america

explain that, commie

John Rennie

When did I give the impression I hated America? I've been to several places in the US and I loved them all.

0celo7

5:12 AM

you hate eagles

Since eagle $\cong$ America...

John Rennie

It's not as good as Great Britain of course :-)

0celo7

we won the war

John Rennie

When did I give the impression I hate eagles? Everyone likes raptors don't they? Raptors are such wonderful birds.

→ 1 message moved to Trash

0celo7

Just ban me

If you hate America there's no reason for me to be here

Sniff

John Rennie

I don't hate America. I'm at a bit of a loss as to how I've given that impression.

0celo7

5:15 AM

You said eagle feathers wouldn't be soft

user228700

@0celo7 Are u high? :-P

0celo7

I was

user228700

@JohnRennie: Quick question...sort of.

user228700

@0celo7 (Y)

John Rennie

@Kaumudi ask away ...

user228700

5:17 AM

@JohnRennie Will u please watch a 1 minute-long video?

user228700

When Is A Bungee Jumper's Acceleration Max?

►

John Rennie

Groan. Oh OK then ...

user228700

@JohnRennie Eek. Sorry :-( It would be much quicker than me explaining, sir, that's why...

John Rennie

Ok, watched it. Have you studied simple harmonic motion?

Ramanujan

@Kaumudi d?

user228700

5:20 AM

@JohnRennie Yep. Where's the fastest point sir? (I'll try to figure it out on my own...)

user228700

@Ramanujan d? :/

0celo7

Ok I'm going to sleep.

Some of us aren't vampires

user228700

@0celo7 Good decision (Y)

Ramanujan

@0celo7 it's morning in India

@Kaumudi this might help you m.youtube.com/watch?v=vKsof8ax_Ns

John Rennie

@Kaumudi so you know that in SHM $$\frac{d^2x}{dt^2} = -kx$$ so the acceleration is greatest when $x$ is greatest. Yes?

user228700

5:22 AM

@Ramanujan I know, I watched it; my question is different from the one being asked in the video...sort of.

user228700

@JohnRennie Yes.

John Rennie

And that's going to happen at the lowest point in the jump.

user228700

@JohnRennie Yessir, I understand that. My question is, where exactly is the point at which the velocity is max? ie. point c?

user116211

Hey @MikeMiller.

Mike Miller

Are these the sort of comments that get upvoted on your site?...

user116211

5:26 AM

Welcome at Physics.

John Rennie

@Kaumudi The velocity increases when the force due to gravity is greater than the force due to the bungee cord, and it decelerates when the force due to the bungee cord is greater than the force due to gravity. Yes?

user116211

@MikeMiller What do you mean? the comment that recommended Math.SE?

user228700

@JohnRennie Yes.

Mike Miller

Both of them. The first is wrong, because bad questions don't belong on any site. The second is just nonsense.

John Rennie

So the point where the velocity stops increasing and starts decreasing, i.e. the maximum of $v$, is when the force due to the bungee cable equals the force due to gravity.

user228700

5:28 AM

@JohnRennie Ah! But hang on, if the tension in the rope equals gravity at that point, won't the rope become taut?

user116211

While I agree with the second one, first one is just an advice; not a rigid order.

user116211

But yeh, 3 upvotes is exaggerating.

Mike Miller

So it is.

John Rennie

@Kaumudi No, the rope becomes taut when the distance the jumper has fallen is equal to the natural (unstretched) length of the rope. At this moment the tension in the rope is zero because the extension of the rope is zero.

user228700

@JohnRennie Oh, and then the rope begins to stretch, yeah..?

John Rennie

5:32 AM

@Kaumudi Yes.

The motion has two different parts. At first it's just parabolic motion under gravity and the rope plays no part. Then once the rope becomes taut the motion is a simple harmonic oscillation.

user228700

@JohnRennie So even after he reaches the point at which the force on him due to gravity is equal in magnitude to the tension in the rope, he keeps going down...and the tension keeps increasing until the velocity becomes zero at the extreme point and then he starts to rebound?

John Rennie

Yes

The point where gravity and the tension are equal is the maximum velocity.

Below that point the jumper is being decelerated, but it takes time to decelerate that maximum velocity to zero so the rebound can start.

user228700

Okay, two questions: under the action of which force does he keep going down past the poin at which his velocity is maximum..? And secondly, how is there a difference between the point at which the rebound starts and the point at which velocity is zero?

John Rennie

Remember that force is proportional to the rate of change of velocity.

Ramanujan

@Kaumudi (1) under gravity ?

John Rennie

5:39 AM

So given that the initial velocity is non-zero at the maximum $v$ point the jumper keeps going below that point while their velocity is decreasing.

user228700

@JohnRennie It's a lot similar to S.H.M, right..?

John Rennie

The net force on the jumper is $$F_\text{net} = mg - kx$$ where $k$ is the elastic ocnstant of the rope and $x$ is the displacement from the point where the forces balance.

Once the rope is taut the motion is SHM.

But above that point te motion is just free fall under gravity because the rope can't exert a force on the jumper when it's slack.

user228700

Maybe I should replace the word "rope" with spring in my mind...

John Rennie

To analyse the motion we need to split it into the two parts and analyse each part separately.

user228700

@JohnRennie Yes, riight...

John Rennie

5:42 AM

@Kaumudi actually that's not a good analogy because unlike a spring the rope can't be compressed.

user228700

@JohnRennie Oh snap, correct...

user228700

Okay, I'm gonna try to analyze the two parts now, with all the points that u told me about...

John Rennie

The analysis is straightforward but a bit messy.

I'd be inclined to start at the lowest point of the jump and work upwards, which by symmetry is the same as starting at the highest point and working downwards.

user228700

@JohnRennie Hm. It didn't even occur to me to split the whole motion into two different parts. Sigh.

John Rennie

You can do it by solving the equations of motion, but you can also calculate the velocity by conservation of energy.

user228700

5:47 AM

@JohnRennie I'm trying to do it just qualitatively and make sure I believe everything...

John Rennie

Suppose we take the downward direction to be positive, so the gravitational force is positive and the rope tension negative, and take the origin to be the unstrtetched length of the rope.

user228700

@JohnRennie No, no, no! I'll do it on my own! :-P

John Rennie

OK :-) But I recommend using the conventions I've described above.

That way the motion splits neatly into $x > 0$ and $x < 0$ parts.

user228700

@JohnRennie OK, on it :-) Thanks sir!

DanielSank

6:10 AM

@JohnRennie you've been "sir"'d.

John Rennie

@DanielSank a lot of Hindustani students use the honorific sir. I suspect it's just a standard way to approach any teacher rather than the way it's used in UK or US English.

DanielSank

@JohnRennie I prefer to imagine that @Kaumudi has implicitly knighted you.

user228700

6:39 AM

@JohnRennie Yes, u are correct! :-) How is it used in the US/UK?

user116211

7:04 AM

@Kaumudi knighthood

user116211

Don't know about US though.

John Rennie

7:15 AM

@Kaumudi In the UK the honorific sir is used when schoolchildren are addressing a teacher in school, but outside of school it's only used in exceptional circumstances. People who have been knighted are entitled to be addressed as sir or maam, but few of them insist in it.

It feels very odd to have anyone address me as sir.

Swapnil Das

Sir in India is used to address an elder citizen/ teacher/A Learned person

John Rennie

Oh well, I meet two of those three criteria :-)

Swapnil Das

Of course, Sir :)

I'm feeling drowsy and tired after spending like hours pondering over an optics concept, bleh..

If I won't get any image of my answer, I'll put my doubt here. I hope a kind somebody would help me. TEs are hard 😛

user228700

7:35 AM

@JohnRennie Oh, I see!

user228700

I can't help it :-P I hope that's OK.

John Rennie

@Kaumudi it's not a problem, it just feels odd :-)

user228700

@JohnRennie OK :-)

user116211

@yuggib o/

yuggib

7:56 AM

@MAFIA36790 \o

user228700

8:14 AM

@JohnRennie: Are u familiar w/the valencies of atoms and exceptions to the octet rule..?

John Rennie

Yes, sort of.

user228700

Okay. May I ask u a very small question?

user228700

..?

John Rennie

Yes, go ahead

user228700

Okay. Boron. It has only 6 valence electrons at most, even when bonded to other atoms.

user228700

8:24 AM

This is an explanation that I found:

user228700

*"The principle of full octet is that most reactivity patterns are described well if there is a driving force / energetic gain for full octets in main-group atoms."*

*"However, the only facet of an atom that dictates anything about the number of valence electrons is its nuclear charge. Boron has a charge of 5. This is balanced by 5 electrons. Two of them are core electrons and the remaining 3 are valence electrons. The valence electrons may participate in bonding through sharing with other atoms, to make three bonds. Three bonds = six electrons"*

John Rennie

ok

user228700

No, actually, never mind. The periodic Table usually takes too long to respond but Rubisco has responded there :-) Thank you anyway!

John Rennie

8:40 AM

@Kaumudi I had a quick look in the periodic table, and I would have answered the way that Rubisco did. The octet rule is ony a guideline and breaks down even for such common elements as transition metals. That's why those elements have many different oxidation states.

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