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Author Topic:   Using your common sense to solve a physics problem.
DrJones*
Member
Posts: 2284
From: Edmonton, Alberta, Canada
Joined: 08-19-2004
Member Rating: 6.8


Message 61 of 188 (144379)
09-24-2004 4:19 AM
Reply to: Message 15 by Percy
09-23-2004 5:21 PM


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)mv
r= 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.

This message is a reply to:
 Message 15 by Percy, posted 09-23-2004 5:21 PM Percy has not replied

Ooook!
Member (Idle past 5815 days)
Posts: 340
From: London, UK
Joined: 09-29-2003


Message 62 of 188 (144385)
09-24-2004 5:39 AM
Reply to: Message 8 by coffee_addict
09-22-2004 7:29 PM


Another example of common sense...
...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 ?

This message is a reply to:
 Message 8 by coffee_addict, posted 09-22-2004 7:29 PM coffee_addict has replied

Replies to this message:
 Message 69 by coffee_addict, posted 09-24-2004 11:27 AM Ooook! has replied
 Message 99 by Rrhain, posted 09-25-2004 12:03 AM Ooook! has replied

Dr Jack
Member
Posts: 3514
From: Immigrant in the land of Deutsch
Joined: 07-14-2003
Member Rating: 8.7


Message 63 of 188 (144391)
09-24-2004 6:26 AM
Reply to: Message 21 by riVeRraT
09-23-2004 6:14 PM


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)

This message is a reply to:
 Message 21 by riVeRraT, posted 09-23-2004 6:14 PM riVeRraT has replied

Replies to this message:
 Message 67 by riVeRraT, posted 09-24-2004 9:08 AM Dr Jack has replied

riVeRraT
Member (Idle past 416 days)
Posts: 5788
From: NY USA
Joined: 05-09-2004


Message 64 of 188 (144397)
09-24-2004 8:52 AM
Reply to: Message 58 by DrJones*
09-24-2004 3:26 AM


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.

This message is a reply to:
 Message 58 by DrJones*, posted 09-24-2004 3:26 AM DrJones* has replied

Replies to this message:
 Message 75 by Melchior, posted 09-24-2004 3:17 PM riVeRraT has replied
 Message 76 by DrJones*, posted 09-24-2004 3:23 PM riVeRraT has replied

riVeRraT
Member (Idle past 416 days)
Posts: 5788
From: NY USA
Joined: 05-09-2004


Message 65 of 188 (144398)
09-24-2004 8:55 AM
Reply to: Message 59 by coffee_addict
09-24-2004 3:38 AM


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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 Message 59 by coffee_addict, posted 09-24-2004 3:38 AM coffee_addict has not replied

riVeRraT
Member (Idle past 416 days)
Posts: 5788
From: NY USA
Joined: 05-09-2004


Message 66 of 188 (144401)
09-24-2004 9:01 AM
Reply to: Message 60 by Rei
09-24-2004 4:03 AM


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.

This message is a reply to:
 Message 60 by Rei, posted 09-24-2004 4:03 AM Rei has replied

Replies to this message:
 Message 71 by Rei, posted 09-24-2004 1:21 PM riVeRraT has replied

riVeRraT
Member (Idle past 416 days)
Posts: 5788
From: NY USA
Joined: 05-09-2004


Message 67 of 188 (144403)
09-24-2004 9:08 AM
Reply to: Message 63 by Dr Jack
09-24-2004 6:26 AM


riverrat was taken already, so I am Scripture Police.
I challenged you to a 2 game match.
Good luck, have fun.

This message is a reply to:
 Message 63 by Dr Jack, posted 09-24-2004 6:26 AM Dr Jack has replied

Replies to this message:
 Message 68 by Dr Jack, posted 09-24-2004 9:49 AM riVeRraT has not replied

Dr Jack
Member
Posts: 3514
From: Immigrant in the land of Deutsch
Joined: 07-14-2003
Member Rating: 8.7


Message 68 of 188 (144408)
09-24-2004 9:49 AM
Reply to: Message 67 by riVeRraT
09-24-2004 9:08 AM


Hmm... your challenge doesn't seem to have come through, so I've challenged you instead.

This message is a reply to:
 Message 67 by riVeRraT, posted 09-24-2004 9:08 AM riVeRraT has not replied

coffee_addict
Member (Idle past 477 days)
Posts: 3645
From: Indianapolis, IN
Joined: 03-29-2004


Message 69 of 188 (144419)
09-24-2004 11:27 AM
Reply to: Message 62 by Ooook!
09-24-2004 5:39 AM


Re: Another example of common sense...
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!

This message is a reply to:
 Message 62 by Ooook!, posted 09-24-2004 5:39 AM Ooook! has replied

Replies to this message:
 Message 70 by Ooook!, posted 09-24-2004 12:46 PM coffee_addict has replied

Ooook!
Member (Idle past 5815 days)
Posts: 340
From: London, UK
Joined: 09-29-2003


Message 70 of 188 (144433)
09-24-2004 12:46 PM
Reply to: Message 69 by coffee_addict
09-24-2004 11:27 AM


Ok here goes
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!!

This message is a reply to:
 Message 69 by coffee_addict, posted 09-24-2004 11:27 AM coffee_addict has replied

Replies to this message:
 Message 72 by Percy, posted 09-24-2004 1:39 PM Ooook! has replied
 Message 74 by DrJones*, posted 09-24-2004 3:11 PM Ooook! has not replied
 Message 77 by coffee_addict, posted 09-24-2004 3:23 PM Ooook! has not replied

Rei
Member (Idle past 7012 days)
Posts: 1546
From: Iowa City, IA
Joined: 09-03-2003


Message 71 of 188 (144443)
09-24-2004 1:21 PM
Reply to: Message 66 by riVeRraT
09-24-2004 9:01 AM


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."

This message is a reply to:
 Message 66 by riVeRraT, posted 09-24-2004 9:01 AM riVeRraT has replied

Replies to this message:
 Message 80 by riVeRraT, posted 09-24-2004 5:20 PM Rei has replied
 Message 172 by Dr Jack, posted 09-30-2004 8:19 AM Rei has not replied

Percy
Member
Posts: 22391
From: New Hampshire
Joined: 12-23-2000
Member Rating: 5.2


Message 72 of 188 (144446)
09-24-2004 1:39 PM
Reply to: Message 70 by Ooook!
09-24-2004 12:46 PM


Re: Ok here goes
I'll let Lam be the final judge, but it looks right to me. Are you a jerk scientist or a common sensist?
--Percy

This message is a reply to:
 Message 70 by Ooook!, posted 09-24-2004 12:46 PM Ooook! has replied

Replies to this message:
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Ooook!
Member (Idle past 5815 days)
Posts: 340
From: London, UK
Joined: 09-29-2003


Message 73 of 188 (144450)
09-24-2004 1:51 PM
Reply to: Message 72 by Percy
09-24-2004 1:39 PM


Woo-hoo
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.

This message is a reply to:
 Message 72 by Percy, posted 09-24-2004 1:39 PM Percy has not replied

DrJones*
Member
Posts: 2284
From: Edmonton, Alberta, Canada
Joined: 08-19-2004
Member Rating: 6.8


Message 74 of 188 (144463)
09-24-2004 3:11 PM
Reply to: Message 70 by Ooook!
09-24-2004 12:46 PM


Re: Ok here goes
That's they way I did it. 33 mph, he was speeding before he hit the brakes.

This message is a reply to:
 Message 70 by Ooook!, posted 09-24-2004 12:46 PM Ooook! has not replied

Melchior
Inactive Member


Message 75 of 188 (144465)
09-24-2004 3:17 PM
Reply to: Message 64 by riVeRraT
09-24-2004 8:52 AM


quote:
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.
  —RiVeRraT
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

This message is a reply to:
 Message 64 by riVeRraT, posted 09-24-2004 8:52 AM riVeRraT has replied

Replies to this message:
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