## Does my physics teacher know what she's doing?

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### Does my physics teacher know what she's doing?

Here was a problem on my physics test

A hunter wishes to crorss a river that is 1.5 km wide and flows iwth a velocity of 5.0 km/h parallel to the banks. The hunter uses a small powerboat that moves at a maximum speed of 12 km/h with respect to the water. What is the minimum time for crossing.

Okay, so here's what I did to solve the problem. I drew a right triangle with 1.5 km and 5.0 km/h * t for the bases and 12km/h * t for the hypotenuse. Then I simply used the Pythagorean theorem to solve for t.

I got t = 0.1375 h or 8.25 minutes for the boat to cross but my physics professor marked it wrong. Is my reasoning sound, is my answer right? Why did she mark it wrong.

She's been wrong a couple times in her lectures. For instance, she told the class that the tagential acceleration can never be zero and thus the acceleration vector can never be perpendicular to the velocity vector and when I remarked that it was so in uniform circular motion she merely snapped and called me the confused one.

So who's right, her or me?
Last edited by Aradae on Mon Sep 14, 2009 6:30 pm UTC, edited 1 time in total.
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roderik
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### Re: Does my physics teacher know what she's doing?

isn't the velocity of the water sort of irrelevant in answering the question?
because in the direction we are interested in, the contributing of that vector is 0 m/s
so the answer would simply be how much time it takes for the boat to travel 1.5 km so: 1.5 / 12 = 0.125 hours or 7.5 minutes.
sure the boat would move a certain distance downstream in this situation, but it would not have an effect on how long it takes to cross the river.

lulzfish
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### Re: Does my physics teacher know what she's doing?

It's a trick question. As long as the hunter doesn't care WHERE he lands, he can cross in 1.5 / 12 hours.

Aradae wrote:For instance, she told the class that the tagential acceleration can never be zero and thus the acceleration vector can never be parallel to the velocity vector and when I remarked that it was so in uniform circular motion she merely snapped and called me the confused one.

In uniform circular motion, the velocity and acceleration are perpendicular.
In normal linear acceleration with no external forces, they're parallel, if you start from rest.

Don't get perpendicular and parallel mixed up.

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### Re: Does my physics teacher know what she's doing?

lulzfish wrote:It's a trick question. As long as the hunter doesn't care WHERE he lands, he can cross in 1.5 / 12 hours.

Dammit!

lulzfish wrote:In uniform circular motion, the velocity and acceleration are perpendicular.
In normal linear acceleration with no external forces, they're parallel, if you start from rest

Don't get perpendicular and parallel mixed up.

roderick wrote:isn't the velocity of the water sort of irrelevant in answering the question?
because in the direction we are interested in, the contributing of that vector is 0 m/s
so the answer would simply be how much time it takes for the boat to travel 1.5 km so: 1.5 / 12 = 0.125 hours or 7.5 minutes.
sure the boat would move a certain distance downstream in this situation, but it would not have an effect on how long it takes to cross the river.
isn't the velocity of the water sort of irrelevant in answering the question?
because in the direction we are interested in, the contributing of that vector is 0 m/s
so the answer would simply be how much time it takes for the boat to travel 1.5 km so: 1.5 / 12 = 0.125 hours or 7.5 minutes.
sure the boat would move a certain distance downstream in this situation, but it would not have an effect on how long it takes to cross the river.

It would have mattered had he been moving to a specific point across the river.
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mmmcannibalism
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### Re: Does my physics teacher know what she's doing?

I don't get what your teacher could be thinking of

One theory--is there any way going with the current could get you across faster? It certainly seems like it shouldn't, but I would ask for an explanation.
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Charlie!
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### Re: Does my physics teacher know what she's doing?

mmmcannibalism wrote:I don't get what your teacher could be thinking of

One theory--is there any way going with the current could get you across faster? It certainly seems like it shouldn't, but I would ask for an explanation.

And yea, an explaination has already been given.
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telcontar42
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### Re: Does my physics teacher know what she's doing?

Just to clarify what ha already been said, The boat is traveling from one side of the river to the other, call this the x-direction. The flow of water is perpendicular to this, the y-direction. The only thing that effects the time that it takes to cross the river in the velocity of the boat in the x-direction. The current only effects the y velocity. The x-velocity is only determined by the speed of the boat. The correct equation is (distance across river)/(speed of boat). The y position of the boat is irrelevant so the current is irrelevant.

doogly
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### Re: Does my physics teacher know what she's doing?

Also, you seem to be filled with an adversarial form of doubt. I would dispel this poison.
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### Re: Does my physics teacher know what she's doing?

Is it not doubt that has advanced science?
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PM 2Ring
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### Re: Does my physics teacher know what she's doing?

Well, yes. It's not the doubt part that's the problem. It's the adversarial bit. It can easily get out of hand. Ref: Galileo.

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### Re: Does my physics teacher know what she's doing?

My relationship with my Physics professor has been pretty rough so far. I always feel a bit of hostility coming from her every time I answer a question in a way that she doesn't like. She seems to always focus on me in asking questions. It feels as if she's trying to destroy me.
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Zamfir
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### Re: Does my physics teacher know what she's doing?

Aradae wrote:My relationship with my Physics professor has been pretty rough so far. I always feel a bit of hostility coming from her every time I answer a question in a way that she doesn't like. She seems to always focus on me in asking questions. It feels as if she's trying to destroy me.

Yes, but make sure it doesn't come from both ways. Teaching a class isn't as easy as it looks. There are kids in the class with different levels of ability and different personalities, and you have to teach to all of them. And you won't like all of your students, no matter how nice a person you are.

Sometimes you simplify things a bit too much for the moment, and plan to explain the more complicated aspects later on. If someone then asks a question about those complicated aspects, they reintroduce the confusion you were trying to avoid for the other kids . The person asking the question didn't do anything wrong, but in some situations it makes teaching a lot harder.

Sometimes you make a mistake, and someone in class picks it up. If you don't deal very careful with such a situation, you can lose authority, and you need that authority to keep your class in hand. Again, the person noticing the mistake didn't do anything wrong, but they still make it harder to teach.

Sometimes, there is someone in your class who thinks they know something better than the teacher, while they actually don't. Usually these are the smarter students, and they have as much a right to make mistakes as others. But correcting these mistakes is difficult, because most of the class will not understand the argument, and if you are not careful you will only confuse them more.

From you description, it sounds as if you have become a bit of a confusing factor in this particular class. That doesn't mean you are really doing something wrong, just that your way of asking and answering doesn't fit with her way of teaching. She has a rest of the class to care about, so she can't change her methods just for you. And perhaps she just isn't a great teacher. That happens.

In such a situation, everyone is better off if you keep a bit quiet in class. Don't be a smartass if you can avoid it, and if you do have a serious question like the one above, ask it outside of class. If you think you are right and she is wrong, be polite, and assume at first that she knows what she is talking about. Without a class watching, it's much easier for her to admit she was wrong, and odds are you were not in fact as right as you thought.

Tass
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### Re: Does my physics teacher know what she's doing?

Great diplomatiic and very true post Zamfir

In moderation. It is also worth considering that people who have studied a particular field for decades might know something about what the are talking about.

doogly
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### Re: Does my physics teacher know what she's doing?

Tass wrote:Great diplomatic and very true post Zamfir

Yeah, that is about exactly what I meant.
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Bobbias
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### Re: Does my physics teacher know what she's doing?

Zamfir has hit the nail on the head. I've had my share of teachers who I've either had a problem with, or haven't fit in with their particular teaching style. So I can definitely vouch for how much of a pain that can be (for yourself, and the teacher, and [if your not careful] the class).

In fact, one of my teachers this semester is someone who I've had various issues with. I had him for a basic CAD class, where his teaching was somewhat lacking. It wasn't terrible, but he glossed over many things, didn't explain things very well at times, and generally confused much of the class by leaving stuff out, or putting in things that weren't really necessary. I had him for a basic computer programming class, where he basically didn't explain anything at all, his code was broken half the time, and focused much more on doing things exactly the way his code was set up (I asked about going about one program in a different way that followed OOP principles, and he wouldn't let me) and answers. He didn't teach any sort of good coding practice (like using comments to explain the more involved sections, cover what a function does, etc.) and focused very heavily on getting results, rather than how we got them (which, considering much of the field of programming is dedicated to doing things better than before, or at the very least, providing an elegant solution to the problem at hand, is just stupid).

But I'm beginning to really digress from the main topic here, I'd like to say that yes, your teacher probably knows what she'd doing, but maybe her teaching doesn't work well for you. If you find yourself having trouble understanding what she's saying, or think she's wrong about something, talk to her about it after class or something. I've had a number of teachers who I sometimes feel don't know what they're doing, but in pretty much every case, they do in fact know, and are either teaching in a style that doesn't work for me (being someone who likes to get the full picture right away) or they are occasionally actually wrong about something (quite rare, actually).