# Can i post a list of my homework problems

1. Nov 24, 2003

### Security

Can i post a list of my homework problems (not to get answers or anything) but to give me pointers on problems?

2. Nov 24, 2003

### Staff: Mentor

The thing to do is post the problems one at a time, showing what you've done so far in solving them. If you can't even begin, just tell us what you know and you may get some hints.

3. Nov 24, 2003

### Security

well...

well in my physics exams, i am on the last 3 subjects and then a finish high school. Right now im am dealing with eletric current, flow of charge, ohm's law etc.. In my future exams, my last two, deal with eletric circuits, quantum physics, nuclear fission and fusion. and its giving me a headache (but keep in mind that this is conceptual physics, so they dont go too in-depth. but any hints or help, or even study habits that can help would be appreciated. And i really need to turn in these exams to finish high school. Thank You for reading.

4. Nov 24, 2003

### ShawnD

For the basic electricity stuff, just make sure that the units work out. Volts is always J/C, Amperes is always C/s, Watts is always J/s, Ohm is always V/A. If you can just remember the units, it's easy as hell.

For the nuclear reaction part, make sure the mass and charge work out. If you seem to have too much mass, try adding negative mass like positrons.

5. Nov 24, 2003

### Security

can i do this...

how about i try and finish all my exams, and the ones im not sure about, ill post the questions and tell you what i put down as the answer. Is that o.k.

6. Nov 24, 2003

### Staff: Mentor

Of course. Until you give us specific questions, we won't be able to help much.

7. Nov 25, 2003

### Security

I have a couple of question with one exam. But the first one goes like this:

A "deep" is the name given to any ocean area with a depth of more than 5,490 meters. What is the minimum water pressure (ignoring atmospheric pressure) at the bottom of an ocean deep? Show your calculations. (Note that the weight density of sea water is 10,100 N/m3.)

i put down pressure due to: liquid = weight density x depth
water pressure = 1031kg/m3 x 5,490 meters
meter pressure = 5,660,190

But that wasnt correct. Am I using the correct equation for this problem. Are there any hints that i can use?

8. Nov 25, 2003

### Staff: Mentor

You stated the equation correctly, but didn't plug in the right numbers:

Pressure = Weight density X depth

Well, what's the weight density? And what did you plug in?

9. Nov 25, 2003

### Security

10,1000 N/m3 x 5,490 meters = 55,449,000 ?

10. Nov 25, 2003

### Staff: Mentor

Looks OK to me. (Don't leave out the units when giving the answer.)

11. Nov 25, 2003

### Security

what units would be proper for this equation?

12. Nov 25, 2003

### Security

I got it. N/m2. correct?

13. Nov 25, 2003

### Security

Well moving on.
The Next problem is kind of hard for me, because i dont really understand volume and how it relates to mass, weight, etc.. But it goes like this:

An ant 2 cm long weighs .0003 Kg and can lift 10 times its weight, in addition to the weight of its own body (in other words, it can lift a total of .0033 kg.) Show your calculations for A, B, and C.

for the A question it asks -- If the ant were 100 times as large, how long would it be? I wrote : 2 cm x 100 = 200 cm, which was correct.

The second question asks -- How much would the giant ant weigh? I wrote .0003 kg x 100= .03 kg, which was incorrect

The second question asks -- How much could the giant ant lift? I wrote .03 x 100 = 3 kg
3 kg + .03 = 3.03kg, which was incorrect.

Teachers notes say: The ant would have an increase in weight proportional to its increase in volume for the 2nd question / Teacher says for the third question that The ant would have an increase in strength proportional to its increase in surface in surface area.

I don't understand volume or scaling enough to understand these hints or to do the problem, can you help?

14. Nov 25, 2003

### Staff: Mentor

Let's use an example to make it clearer. Consider three things: a stick of length L, a square of length L, and a box shaped like a cube with all sides with length L. Here are the relationships that matter:

Length (of stick) = L; Area (of square) = LxL; Volume (of cube) = LxLxL.
So what happens if I double the length---instead of L, I put 2L?
Length = 2L (it doubles); Area = 2L x 2L = 4L (it goes up by 2-squared); Volume = 2L x 2L x 2L = 8L (it goes up by 2-cubed).

These can be tricky. If someone asked me: "if the ant is 100 times larger, how long would it be?" I would have to ask what do you mean "larger"? Do you mean longer? (apparently they meant longer)

For example: if you say a guy is twice your size, do you mean twice as tall? or twice as heavy? Depends! (And it obviously makes a difference: a guy twice your height would most likely weigh a lot more that twice your weight.)

The weight and volume go together. So if the ant increased it's length by 100x; the volume would go up by 100x100x100= 1,000,000--and so would its weight. If the normal ant weighs 0.0003Kg; the giant ant would weigh 1,000,000 x 0.0003 = 300Kg!

Strength goes up with the area. (Imagine the cross-section of the muscles getting bigger.) So the ant would be 100x100 = 10,000x stronger. Thus if a normal ant could lift 0.003Kg, the super size ant could lift 10,000 x 0.003 = 30 Kg. (But his weight went up by a million times! The giant ant probably wouldn't be able to move, as he now weighs 300Kg!!)

Make sense?

15. Nov 25, 2003

### Security

it makes sense, but why do you have to multiple 100 three times?

16. Nov 25, 2003

### Security

I get it. Thanks alot. I have 3morequestions on this exam. The next one goes like this:

Archimedes was an ancient Greek philosopher. According to tradition, he discovered what has come to be called Archimedes' Principle in the course of trying to solve a dilemma posed to him by King Hiero of Syracuse. King Hiero had provided pure gold to a goldsmith to fashion him a crown. When the crown was finished, the king suspected that the goldsmith had substituted less costly and less dense silver for some of the gold and thus kept some of the gold for himself. The King asked Archimedes to analyze the crown and determine if this was indeed the case.

Archimedes asked for a mass of gold equal in weight to the mass of gold King Hiero had supplied the goldsmith. He then put the gold in a container, filled the container to the brim with water, and removed the gold.

'The first question says: What happened to the water level in the container when the gold was removed? Why?' I answered, "The water level in the container decreased because of displaced water when the gold was put into the container." which was correct.
Secondquestion says: Archimedes then took the crown fashioned by the goldsmith and carefully lowered it into the container of water. What should have happened to the water level if the crown had been made of pure gold.' I answered on my second try, "To the point that water would be displaced and would be equal to the mass of Gold" which was correct.

The third question says: When Archimedes lowered the crown into the container, the water overflowed. What did this indicate about the volume of the crown compared to the volume of the mass of gold? Did this prove or disprove that the goldsmith had cheated the King?Why?
Again I tried the second time and failed by saying, "This proved that the goldsmith was not cheating the king. The teacher put a hint asking to consider weight density = weight/volume. Is their a way that i can post a more questions the one at a time or is this the standard for doing this here?

17. Nov 25, 2003

Archimedes then took the crown fashioned by the goldsmith and carefully lowered it into the container of water. What should have happened to the water level if the crown had been made of pure gold.' I answered on my second try, "To the point that water would be displaced and would be equal to the mass of Gold" which was correct.

It seems to me that since the crown will be denser than water and therefore does not float, regardless of whether it is made of silver or gold, then the amount of water displaced would be equal in volume to the crown, but not equal in mass.

The third question says: When Archimedes lowered the crown into the container, the water overflowed. What did this indicate about the volume of the crown compared to the volume of the mass of gold? Did this prove or disprove that the goldsmith had cheated the King?Why?

If the crown and the piece of gold are of equivalent mass, then this proves that the crown is not made of pure gold. The fact that the water overflowed with the crown, and did not with the equivalent mass of gold, indicates that the crown is of larger volume and therefore lower in density. Gold is of higher density than silver, so it must be that it was made, in part, with silver.

18. Nov 25, 2003

### Security

But how does it being of higher density show that the goldsmith didnt cheat the king. my weak point in physics are volume and density.

19. Nov 25, 2003

But how does it being of higher density show that the goldsmith didnt cheat the king. my weak point in physics are volume and density.

Well, if it was of equivalent density it would show that the goldsmith didn't cheat the king.

Density is mass per unit volume.

&rho; = m/V

For a given mass, m, as the density increases, the volume decreases.

Now, assuming that the crown and the lump of gold are the same mass, which the goldsmith could have made sure of with a balance, if the crown has a significant silver content then it will be of larger volume than the lump of gold because silver is less dense than gold i.e. it needs to occupy a larger volume to be of the same mass.

So, both the crown and the gold will displace an amount of water equivalent to their respective volumes. The gold will be less voluminous than the crown if the crown is not pure gold. Therefore, when the crown is placed in the water, the water level will rise higher than the water level with the gold in the water if the crown is not pure gold.

Am I making sense?

20. Nov 25, 2003

### ShawnD

Here is how I would solve this:

N/m^3 is the same as pascals/m
10100 N/m^3 = 10100 Pa/m

Since I have a rate for perssure and I have a depth, just multiply.
10100 Pa/m * 5490 m = 55449000 Pa

Huge pressure :D