Jedo's School Help Thread

[CrystyB]

Yikes. It finally happened to me. (that guest above was me) Seems that i got careless with the lifespans of the cookies or something...

[Jedo]

Alright, I've got another problem that I don't quite understand. It's just a bonus question, so I don't need an answer immediately, but I do have a test coming up on Thursday that it would be beneficial to know the answer for.

Alright, a penguin is sitting on a sled that is being pulled horizontally across the snow. The penguin has a Force weight of 70 N and the sled has a Force weight of 60 N. The coefficient of friction between the penguin and sled is 0.7 and the coefficient of friction between the sled and snow is 0.1. What is the maximum horizontal Force that can be exerted on the sled before the penguin begins to slide off?

[The Ragin' South Asian]

if i was going to guess i'd say 10.9N, but physics isn't really my strong suite.

[CrystyB]

Mine neither. But here are my thoughts anyway:



Traction-Force -- Max-Friction-between-Sledge-and-Snow = Mass-of-System * Max-Acceleration-of-Movement

Acceleration-of-Movement transfers to the Penguin into Inertial-Acceleration (with its direction reversed). In order for the penguin to not slide, we need:

Max-Friction-between-Penguin-and-Sledge = Mass-of-Penguin * Max-Inertial-Acceleration

Answer: 120 N

[Neo]

Looks like a Sum of the Forces type problem.

I'll work on this later if I can.
_________________
_{Ad Astra}

[guest_austinap]

[Jedo the Jedi]

[guest_austinap]

Oh ok, fair enough. Make sure you dont use sigfigs for intermediate steps or you can end up with some HUGE errors (big roundoff errors), just use them in your final answers. A lot of people mess that up often, especially beginning physics and chemistry students. I just got done grading some physics I papers, and that was probably the #1 error made, after that came mistakes in sigfigs, then major things (like, using wrong formulas, incorrect dimensional analysis, or just plain bad [read: wrong] algebra). If you're using a calculator, just keep the numbers that you get from previous results and THEN round the final answer to the correct sigfigs.

[Jedo the Jedi]

Well, my phyics teacher said that the correct answer is 104 N. I don't know how, but that's apparently the correct answer. (That's invised in case Neo still wants to do the problem.)

[CrystyB]

Have you asked him what's wrong with my approach?

[mith]

Here's how it works, if you don't want to wait on Neo:

The friction between the sled and the snow is 13 N (130*.1). The friction between the penguin and the sled is 49 N (70*.7). In order for the penguin to not slide, the force acting on the penguin must be less than this; however, this is not F - 13 N, as some of that force is acting on the sled; instead it is 7/13 (F - 13). So F - 13 = 49*13/7 = 91, and F = 104 N.

Even though the frictional force is only dependent on the weight of the penguin, since the applied force is also acting in part on the sled, the answer only depends on the combined weight. Maximum applied force = Total weight * Sum of Friction Coefficients.

[CrystyB]

Ahhh... Geez! I can't believe i've been through three years of a minor in Mathematics and 70 + 60 is still 150...

[Jedo the Jedi]

[Jedo the Jedi]

Well, I was the first person to finish my Physics test, and the only one to do so before the bell. I hope that means that I did well rather than poorly.

[Quag]

I could use some help on CHapter 11, considering I don't know what the hell I'm doing.

A tire has an initial volume of 12.5 liters and pressure of 2.0 Atm., at a temperature of -35 degrees Celsius. What is the pressure in the timre if the temperature rists to 45 degrees Celsius and the colume increases to only 12.55 liters?

And also,

An aluminum pan with m=400g has a 600g piece of ice placed in it. How much energy would have to be put into the pan to change all the ice to steam?

[CrystyB]

The first one should be easy for me, considering i used to love Chemistry. It should go like this: pV/nT = constant, and since no gas is supposed to be leaking, n = constant too. So 2atm*12.5L/(273-35)K = ?atm*12.55L/(273+45)K, so answer = 2.66 atm.

[Guest]

It's been a long time since I've had chemistry, but it seems like you need a little bit more to solve #2 (specifically the temperature of the ice and the pan when you first start adding energy).

If you make the assumption that the ice is exactly at 0 degrees celsius, the amount of energy needed to turn just the ice into steam would be the sum of:

The amount of energy needed to melt the ice (334 Joules/g for water *600 g)

The amount of energy required to raise the now melted water from 0 to 100 degrees celsius (4.180 joules per gram per degree times 600 grams times 100 degrees change in temperature).

The amount of energy needed to turn the now 100 degree water into steam (2250 joules/gram *600 grams).

However, some of the energy also needs to go into heating the aluminum pan, which has a specific heat of 0.900 joules/gram/degree . The mass is 400 grams, but I don't see any way to calculate the temperature change without knowing more information.

[Jedo]

Alright, two questions, for now.

1) You and your friend throw ballons filled with water from the roof of a several story apartment house. You simply drop a balloon from rest. A second balloon is thrown downward by your friend 2.3 s later with an initial speed of 45.08 m/s. They hit the ground simultaneously. The acceleration fo gravity is 9.8 m/s ² . You can neglect air resistance. How high is the apartment house? Answer in units of meters (m).

2) A camera falls from a blimp that is 289 m above the ground and rising at a speed of 16.9 m/s. The acceleration of gravity is 9.8 m/s ² .
a) Find the maximum height reached by the camera with respect to the ground. Answer in in units of meters (m). b) Find the speed at which it hits the ground. Answer in units of cm/s.

For the first question, I'm just unsure how to go about finding it. For the second question, I have no idea what it's asking, much less how to find it. Thanks for the help.

[Samadhi]

1. You need to find the +t where distance is equal.
2. The camera has an initial acceleration of (9.8m/s ² -16.9m/s ² )

[Dread Pirate Westley]

1.) Samadhi got it.

2.) Samadhi didn't get it. When you let go of the camera, it's going up at the same speed as you (i.e. 16.9m/s). The only acceleration is due to gravity (32ft/s ² ). Find its max height and the time before it hits the ground.

[Jedo the Jedi]

So, the maximum height is 289 m? And the initial velocity is just 0 m/s?

[Dread Pirate Westley]

Well, its initial velocity relative to you is 0, but relative to the ground, it's 16.9 m/s.

[Samadhi]

Misread it.

[Courk]

I need help thinking of a word. I tried the reverse dictionary, but it didn't help. I need a word that means that something is similar to one thing, while at the same time also being different from it. I could use ambiguous, but I'm hoping there's a different word.

[Chuck]

quasi-similar?

[extropalopakettle]

[Coyote]

'analogous' might work too. Plus it's a cool looking word. But extro may have the right idea here--if a regular word like similar fits the bill, just go ahead and use it.
_________________
Hard work may pay off in the long run, but laziness pays off right now.

[Chuck]

Maybe 'similar' is too strong because the differences are large and that fact needs to be made clear by whatever word is chosen.

[Courk]

I have three sections to my paper. One is for similarities, one is for differences, and the other is for things I couldn't really put into either category. I need a label for that third category.

[Chuck]

miscellaneous?

[extropalopakettle]

[Courk]

My paper is about South America and how it can be thought of as part of European civilization and also how it can be thought of as separate from Western civilization.

Some things, such as land distribution, are very similar for the time period I'm looking at. Other things, such as economy, are very different. And yet other things are similar and differnet - such as the revolutionary slogans.
Mexico: Tierra, Libertad y Pan (Land, Liberty and Bread).
US: Life, Liberty, and the Pursuit of Happiness.
Both want liberty, but the US says nothing about bread and Mexico says nothing about pursuing happiness. Some goals are the same, but some are different.

[tigerbalm]

I think perhaps you miscorrealated there, Courk. Bread = Life. It is Land and Pursuit of Happiness that do not quite match up.

Both modify the original phrasing (by Locke) of "Life, Liberty, and Property." The implication seems rather clear - the US founders deliberately did not want to mention property as an indivisible right, and Mexico not only mentioned it, but defined it very clearly.

I would say the US founders felt that happiness was the core idea, and that property was not the core means to achieve it. The Mexicans, I can only guess, saw owning land as the only real way to have happiness. How else can one feed oneself? What else can one pass on to one's children? What stake does one have in a nation if one doesn't own a part of it?

[tigerbalm]

Also, I agree with extro. There is no third category. Similar and Different are a one-dimensional spectrum. You can fall between them by being similar in some ways and different in others, but there's nothing outside the line they define.

[Samadhi]

[Courk]

I'm going more for a tangible vs. intangible approach. You can see the land you want, you can hold a loaf of bread, but you can't get a loaf of pursuit of happiness. And please don't discuss my paper. If I get any ideas from you I'd have to source it, and sourcing things is not fun.

[Samadhi]

I'll have a slice of liberty and some freedom fries, to go!

[Jedo the Jedi]

Just a quick question, nothing that really needs to be answered.

We were practicing for the new SAT today, and one of our practice essay questions was 'Is art ever made for just beauty, or does it always have a function/purpose/etc.?' (In a nutshell.)

WTF?! How can it be made to have a specific function/purpose/etc. when beauty is in the eye of the beholder? For instance, a piece of abstract art (like a statue) could look like an elephant rolling around in the mud to the artist, but to me it looks like two monkeys fighting over a coconut. In this instance (and there are many like it) the author's function/purpose/etc. was not what I saw it to be. However, we both saw it as beautiful. Does that make sense?

I think it was a stupid question anyway, but as my teacher always says,

[Guest]

[extro...]