This is topic Physics question - help my kid with homework (it's not cheating) in forum Books, Films, Food and Culture at Hatrack River Forum.

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Posted by Belle (Member # 2314) on :
So my daughter has been given a physics problem and the teacher gave instruction to go home and think about it. they can use their notes and any other resource, so I assume that Hatrack qualifies as "any other resource."

Any rate, I found a link to the exact same lab online:

Predicting the landing spot of a projectile

What she needs to do tonight is figure out how she is going to go about setting up her equations and which equations to use. She is allowed to use her notes, her books, and any resources as I said.

If you're not comfortable just giving the answer, some nudges in the right direction would help. She's been behind because of the days she missed due to the death in our family, and has had a lot to do for this class (including building a matchstick bridge, and hers - a really nice K-truss design - held 42 pounds!) She's been up every night until almost 11 trying to get caught up on physics work and I think she is just exhausted. She doesn't know where to start with this and my physics knowledge is too weak to be muhc help right now.
Posted by King of Men (Member # 6684) on :
Well, obviously this is an impossible assignment. To predict it with real accuracy you need to take general relativity into account, and the initial position and velocity cannot both be known at the same time, so you need to unify that with QM. Which wins you a trip to Stockholm. So I suggest that your daughter go to bed and not worry about winning the Nobel Prize just yet.

...Oh, one significant digit is good enough? Oh well then. Use Newton, who is not forgotten. Let's see.

The problem has two parts. One is to determine the kinetic energy of the bob at the moment when the string is cut. This is pretty simple, just take the difference in potential energies between the initial position and the rest position of the pendulum.

Next, how far does the bob travel? Again this is fairly easy: From the kinetic energy you found above, get its horizontal speed. Then find how long it takes it to fall the distance h2. Multiply speed into time to get a distance; voila, that's your answer. Trivial!
Posted by Belle (Member # 2314) on :
King of Men, my daughter says your name is well deserved. She says super duper thanks! (an exact quote) She understands exactly what to do now.

Funny, when she was telling me her problem I told her I knew several people on hatrack who could answer it right away. [Smile]
Posted by C3PO the Dragon Slayer (Member # 10416) on :
Originally posted by King of Men:

...Oh, one significant digit is good enough? Oh well then. Use Newton, who is not forgotten. Let's see.

[Roll Eyes]
Posted by King of Men (Member # 6684) on :
Belle, you're welcome. Incidentally, how old is your daughter?

C3PO, why are you eye-rolling? Newton is unforgotten for good reasons, but at some accuracy the answer is going to be off.
Posted by Belle (Member # 2314) on :
I think for a high school physics class, it will be sufficient. Don't worry, my husband also stated something very similar to what you did...and something I didn't understand about whether or not the razor was cutting the string at what point in the swing....I don't know. I'm sick and very weary and still very sad so I can't process anything complicated right now (and I recognize this was not complicated for King of Men or many other hatrackers, but it's just not in my wheelhouse.)

My daughter is a high school senior.
Posted by King of Men (Member # 6684) on :
The exact position of the razor is indeed very important. If it's not placed exactly so the cut is at the lowest point in the swing, then the bob won't move horizontally to begin with and even a small amount of that can throw off the calculation. I suspect that if they actually run the experiment, they'll get a range of values around the prediction for exactly this reason. Then, of course, there's the question of how effective the razor is at cutting the string. It is surely going to bend the bob's trajectory at least a little bit (upward, I think) and thus throw off the calculation again, but this time in a systematic way.

Interesting lab for a high-school class; is it AP?
Posted by Belle (Member # 2314) on :
They don't offer AP classes at our school, just dual enrollment. This is Honors senior physics, so they are honor students. They've done a lot of neat labs in there, she has really enjoyed it. She liked it a lot better than anatomy and physiology, which she took last year.

I remember really enjoying physics too. My husband really likes it. If he had been home last night, we wouldn't have needed to ask you for help but he was working, so I turned to the Hatrack knowledge base. [Smile]

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