Challenging My Professor's Grade: Proving My Answer Is Right!

[holly]

*sigh* I got my grade back for my test today...I missed these questions and I think *I* am right and my professor is WRONG.

Q. When an object is partly or wholly immersed in a liquid, it is buoyed up:
His answer: By a force equal to the weight of liquid displaced.
My answer: By a force equal to its own weight.

It's its own weight! Because that's the principle of flotation! If it is BUOYED UP, then it is FLOATING. Otherwise, it would be SINKING. If it is PARTLY IMMERSED, then it MUST be floating, and to be floating, the B.F. is equal to its weight. Am I right? PLUS, you can float UNDER the water, I've seen logs doing so. FLOATING is FLOATING.

The other question I got wrong was the one I tried to get answered last night...the one about "Place a 1-kg block of iron at 40 C in a kg of water at 20 C & final temp of the two becomes:"
He says the "at or about 30 C" and I went with Less than 30 C.

I think I'm right! Because of what someone else on the boards noted, the diff between the spec. heat capacities...you can't go mix and match different substances like that, can you?

I'm depressed. I want to get some mental ammo and fight for those points.

 

[chroot]

Your answer is not correct. Your professor's answer is correct.

Think about this scenario: you dive to the bottom of a swimming pool holding a balloon full of air, and release it.

If, as you contend, the balloon feels an upward force equal to its own weight, it won't move an inch. Why not? Because it has its weight pulling it down, and an exactly equal force pushing it up. The net force would be zero, and it wouldn't move at all. You know that can't be right.

On the other hand, if the balloon feels an upward force equal to the weight of the water it displaces, that means it feels quite a strong upward push, since water weighs a lot more than a balloon. THAT would make the balloon go up.

The case of the log floating underwater (a situation called "neutral buoyancy" by scuba divers) requires that the object have exactly the same density as the water itself.

As for your other problem: the heat lost by the block of iron is gained by the water. Use Q = m C \Delta t for both the heat lost and the heat gained, and set them equal. You'll wind up with:

C_{steel} \Delta t_{steel} = C_{water} \Delta t_{water}

from which you can see that the temperature change is inversely proportional to the heat capacity. Since water has a much higher heat capacity than steel, you know its temperature won't change as much as the steel's.

- Warren

 

[Doc Al]

It's its own weight! Because that's the principle of flotation! If it is BUOYED UP, then it is FLOATING. Otherwise, it would be SINKING. If it is PARTLY IMMERSED, then it MUST be floating, and to be floating, the B.F. is equal to its weight. Am I right? PLUS, you can float UNDER the water, I've seen logs doing so. FLOATING is FLOATING.

Ouch! Holly... remember those delightful discussions we had about bouyant force? Time to reread those posts!

 

[holly]

Doc Al, that's not fair! I *did* read those posts! I printed out everything you said and studied it for the test! I *pored* over your remarks! Those objects in the question were floating. It doesn't say neutral density. It says partly or wholly submerged. Partly HAS to mean part is sticking up. Therefore, it's floating. When floating, the B.F. exactly equals the object's weight, right? I had to put that definition right on the test itself! "A 100-N boat floating displaces how much weight?" The answer was "100-N." See? Can you have it both ways? Right one question, wrong the next? Is that how physics works? Bah humbug on physics. Fie, physics. Poo.

Okay, chroot, why don't I see any NUMBERS in your answer...? This is the question about the kg of metal at 40 C and the kg of water at 20 C. Now that I double-think on it, I think the water gets hotter than 30 C, that's what I think, because we are talking a large piece of hot metal and only 1 liter of water. So I think the professor and I were both wrong on that one. So I ought to get the points.

Hmph. Have a nice weekend you brainiacs.

 

[chroot]

Well, actually... now that you mention it, you're on to something. I did not notice the "partially submerged" part of the question. When an object is partly submerged, like a boat floating on the surface of a lake, it displaces just enough water so that the weight of that water is equal to its own weight.

Therefore, in the case of a partially submerged object, the buoyant force is equal to both the object's own weight and the weight of the displaced water. Both you and your professor are correct -- in that one specific case. In the other case mentioned in the problem, the case of wholly submerged object, only the professor's answer is correct, and yours is not.

In other words, the professor's answer is correct in both cases (and in fact in ALL cases), but yours is only correct for one case.

The reason I didn't include numbers in my answer to the second part is simple: I didn't feel like looking up the specific heat of steel. Did your professor provide that number for you on the test?

- Warren

 

[NateTG]

Regarding the first question:

You interpreted 'bouyed up' to mean floating. The prof. did not.

Regarding the second question:

The specific heat of steel is 452 Joules per Kilogram per degree C
The specific heat of water is 4186 Joules per Kilogram per degree C

(These aren't exact, but certainly close enough)

That means that when the two reach equilibrium, the temperature will be just under 22 degrees.

BTW the density of steel is roughly 8 grams per cc, so you're looking at piece of steel that's less than half the size of a soda can.

 

[Doc Al]

floating? What floating?

I'm afraid I'm going to have to agree with the prof on this one. No where does the problem mention floating!

Q. When an object is partly or wholly immersed in a liquid, it is buoyed up:
His answer: By a force equal to the weight of liquid displaced.
My answer: By a force equal to its own weight.

The prof's answer is always correct. Holly's is only correct if the object is floating.

Perhaps there is a slight ambiquity about the meaning of bouyed up.

 

[cookiemonster]

Woohoo! Fight! Fight! Fight! Fight! Fight!

cookiemonster

 

[chroot]

I agree with NateTG; I get 21.94 degrees C. Your professor probably did make a mistake on this one -- are you sure you copied the problem correctly?

- Warren

 

[holly]

Hmm, yes, I am sure I copied the question about the metal/water final temp right. He grades the tests really fast, really offhandedly; oh, how he hates having to deal with us little fluff-brains...I doubt he got it wrong, too...he's really smart...maybe he just marked it wrong accidentally. I haven't seen the test paper yet; I hounded him via email until he graded it & then bugged him until he told me the ones I got wrong, and what the correct answers were. I wanted to have the whole weekend to plot how to get those points back.

I still say, if an object's nose is sticking up out of the water, and it's not a living object, then it's floating in some manner. Maybe I was supposed to pick the answer that was the MOST MOSTLY correct.

Well, I'll go for those points Monday. He is very intimidating, btw...not one to ask for clarification...you can ASK, and he complains no one asks, but then he's snide & shakes his head and says kind of hurtful things "in jest."

Thanks for the help. If anyone knows how I can get my ferret's title of Radio Wave back, let me know. It was ABOVE the ferret, not below.

 

[enigma]

Thanks for the help. If anyone knows how I can get my ferret's title of Radio Wave back, let me know. It was ABOVE the ferret, not below.

You can go into your user CP and change it to anything you'd like. Good luck talking to the prof

 

[ShawnD]

You really shouldn't hound the prof. He'll be less willing to do stuff for you if you're annoying.

Good luck on that thermal energy question. I'm confident he will give you the marks if you give him the calculation. Make sure you show him a calculated equlibrium temperature, don't just say "oh well the heat capacity of water is higher so it will be closer to 20". Numbers are the best backup you can have.

 

[Doc Al]

I still say, if an object's nose is sticking up out of the water, and it's not a living object, then it's floating in some manner. Maybe I was supposed to pick the answer that was the MOST MOSTLY correct.

Well... I'm so used to these tricky questions that I tend to be a real stickler. To me, "wholly or partly submerged" just means that all or part of the object is in the water. For all I know, the object could have sunk like a rock or someone could have just dunked it in part way (like a donut in coffee).

Well, I'll go for those points Monday. He is very intimidating, btw...not one to ask for clarification...you can ASK, and he complains no one asks, but then he's snide & shakes his head and says kind of hurtful things "in jest."

You definitely deserve the points on the steel & water problem. Your prof is just plain wrong about that one. Just keep calm and present the facts, like ShawnD says.

And as long as you are asking for points, you may as well explain how you were interpreting "bouyed up"! Good luck.

 

[holly]

Geez, lots of good advice...THANX...I'm not so hot to bug the prof now...I never even considered that the "partly or wholly submerged" object could just be a rock sitting in the shallows...it sure wouldn't be floating then...I will mention "the facts" about the other problem, the kg steel/water one. But I don't really DESERVE the points for that one, because I didn't think for myself, it was cookiemonster & ShawnD that did the thinking on an earlier thread, I was just repeating without understanding. I'm kind of humbled now.

Obviously, I have to study even more. JUST FIVE MORE WEEKS, BRAINS, THEN I'M GONE FOR GOOD! Please bear with me!

Can't thank you EACH and ALL enough; you're making it possible for me to become a sonographer...actually SAVING my life here...***sniff***truly, I am standing on the shoulders of Giants! (And looking off in the wrong direction, no doubt, but that's not YOUR faults).

 

[cookiemonster]

We're not letting you leave until you start to like physics. Just thought I'd give you a heads up. ;)

cookiemonster