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Member (Idle past 477 days) Posts: 3645 From: Indianapolis, IN Joined: |
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Author | Topic: Using your common sense to solve a physics problem. | |||||||||||||||||||||||||||
DrJones* Member Posts: 2284 From: Edmonton, Alberta, Canada Joined: Member Rating: 6.8 |
I can't remember the equations for rotational momentum or energy rotational energy has for some reason slipped out of my brain but rotational momentum (about a point O) is: HO= r(cross)mvr= position vector from point O to the object m= mass of object v= velocity vector of the object. (cross) is of course the vector cross product operation.
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Ooook! Member (Idle past 5815 days) Posts: 340 From: London, UK Joined: |
...I sat down during my lunch break yesterday and I think I've figured it out . The first time I stuck the numbers into the equations I got a rather silly answer, but common sense told me that the car wasn't trying to break the land speed record so I had another look and noticed something in the equations that I'd forgotten to do.
Like I said, I think I've got it (although of course I'm not sure). In contrast to DrJones (Jerk Engineer), this Jerk Cell Biologist took all of his lunch hour to do it I had unpleasant flashbacks to my physics A levels ("Sir! I can't do it!!) Do you want to see my working? Or are you waiting for rRat to come up with the goods? On a slightly different note : Does anyone have any non-physics/maths problems that could be set ?
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Dr Jack Member Posts: 3514 From: Immigrant in the land of Deutsch Joined: Member Rating: 8.7 |
You play chess? I would love to play you a game. Haven't played in a while, but I would be up for it. Check out ItsYourTurn.com - Play online games: chess, checkers, backgammon, Battleship, Othello, Connect4, and more! FREE turn-based multiplayer Internet board games for an excellent (and can be free) on-line chess site. One it's great strengths is that you don't need to both be on-line at the same time. (You can find me as 'Mr Jack' on there)
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riVeRraT Member (Idle past 416 days) Posts: 5788 From: NY USA Joined: |
Nope. Acceleration due to gravity is a constant 9.81 m/s2 here on earth no matter what angle the object is at.
What????? You mean I will role down a hill at the same speed regrdless of the slope? you obviously didn't understand how I was applying the grade to the formula. From you correction you would understand that the Uk will change on a slope, because gravity will have a lateral affect. Thats why you slide further going down hill. And V=(2Uk*g*d) is correct.That would yield a speed 53.40MPH on level ground from a 30m skid mark I was having difficulty apply the slope to the formula.
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riVeRraT Member (Idle past 416 days) Posts: 5788 From: NY USA Joined: |
You can't put equations together if you don't understand them. Thats why I wanted to start from scratch. Then that would lead me to understand the equations.
I wanted to understand how the Uk was created.Then I wasn't sure what about a few other things. I almost got it from the equations, but it wouldn't have been a demostration of common sense to me.
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riVeRraT Member (Idle past 416 days) Posts: 5788 From: NY USA Joined: |
I even remeber reading about that.
But you really didn't prove anything to me. First off, I would never use the wrong size nut. We didn't discuss nut sizes, and you didn't mention anything about the size of the rod. You only said it was a rod. So it was a trick question with not enough information to give the answer you were looking for. If you used the right size rods, and nuts, then it would have worked exactly the way I said. Next question, try again.
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riVeRraT Member (Idle past 416 days) Posts: 5788 From: NY USA Joined: |
riverrat was taken already, so I am Scripture Police.
I challenged you to a 2 game match. Good luck, have fun.
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Dr Jack Member Posts: 3514 From: Immigrant in the land of Deutsch Joined: Member Rating: 8.7 |
Hmm... your challenge doesn't seem to have come through, so I've challenged you instead.
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coffee_addict Member (Idle past 477 days) Posts: 3645 From: Indianapolis, IN Joined: |
Go ahead and show your work if you want. We're done as far as I'm concern.
Oh yeah, and don't feel bad about it. I'd take a whole hour too if I were doing something in you field. We all have aour specialties This message has been edited by Lam, 09-24-2004 10:29 AM The Laminator B ULLS HIT For goodness's sake, please vote Democrat this November!
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Ooook! Member (Idle past 5815 days) Posts: 340 From: London, UK Joined: |
Okay, here’s how I think it might be done (he says preparing to look rather silly) :
Use tan(d) = a/b to calculate d where a/b = 0.08 (8%) Use d in the formula: Ftotal = uk m g cos(d) — m g sin(d)= 4.397m — 0.781m Because F = ma then a must equal 3.616 Next : 2a(x2-x1) = (V22-V12) Or to put it another way: 2a (Distance breaked) = (Starting velocity) 2 Soooo (he says hesitantly): The answer is 32.86 mph If this is wrong but the method is OK then it is probably to do with my inability to use a calculator and a blind spot for minus numbers. If it is wrong and I’ve cocked up somewhere — give us another hint (its been driving me nuts!) Apologies to anybody else who thinks this is a bit mundane but its been doing me noodle because I know I should be able to do it, and I just have to know!!
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Rei Member (Idle past 7012 days) Posts: 1546 From: Iowa City, IA Joined: |
It wasn't the wrong sized nut. It was the right sized nut; however, it was an invalid design modification that doubled the stress on the nut. The nut was sized right to hold up the original catwalk. The "common sense" modification, however, ruined this. I asked you for the rammifications of the design change. Using your common sense, the fact that you'd have to double the strength of the nut completely slipped past you; you simply stated that you'd only have to bear the weight of a few more inches of rod and an extra nut.
Do you honestly think that if we built buildings without structural analyses, that they wouldn't be collapsing all over the place? This message has been edited by Rei, 09-24-2004 12:22 PM "Illuminant light, illuminate me."
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Percy Member Posts: 22391 From: New Hampshire Joined: Member Rating: 5.2 |
I'll let Lam be the final judge, but it looks right to me. Are you a jerk scientist or a common sensist?
--Percy
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Ooook! Member (Idle past 5815 days) Posts: 340 From: London, UK Joined: |
I'm a jerk scientist who hasn't touched Physics since my A levels (10 years ago), and who could never really get to grips with the subject (angular momentum used to really do my head in).
I don't really know what that says about the point of the thread though. Although I tried to approach the problem with a kind of 'common sensical' method I would have really struggled if Lam hadn't provided the basic equations and I didn't have a basic knowledge of how to manipulate equations.
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DrJones* Member Posts: 2284 From: Edmonton, Alberta, Canada Joined: Member Rating: 6.8 |
That's they way I did it. 33 mph, he was speeding before he hit the brakes.
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Melchior Inactive Member |
quote: Acceleration due to gravity is constant. However, you will roll faster down a hill the steeper it is. This is because while the strenght of gravity is constant, it has a direction (toward the center of the earth). In a slope, it points partially into the ground and partially 'down' the slope. Note that regardless of how steep the slope is, gravity can only contribute from 0 to 9.8m/s^2 acceleration. If the slope is vertical, it's basically a free fall. I think you got the right idea but expressed it strangely. Same with the Uk bit. Uk itself is constant (given in the problem). It's the force between the car and the road that can change. Also, your initial speed formula is off. You need to take the square root of that to get v. The formula starts as v^2 = 2*g*d*Uk. You should always double-check the units (v must be m/s). g*d is in m^2/s^2. This is a perfectly good formula for flat ground, but it's probably better to start from scratch instead of modifying it when you have a slope. This message has been edited by Melchior, 09-24-2004 02:24 PM
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