Physics Problems (reward)

[The Truth]

Our class average on our last physics 242 course was very low. So, our teacher is letting us rework the problems we missed for 1/2 credit of the problem.

If we miss any part of the problem, (sig fig, math error) we miss the whole problem and dont get 1/2 credit.

I really want to make sure I get these correct, so I am calling in the big guns, 2+2.

Anyone who takes the time to work these 3 problems will recieve a reward on party, stars or via neteller. Other rewards are possible, pm me for details.

The problems aren't too difficult. I will be working the problems myself as well, and I do well in physics, so I will probably get them right.

So, don't just give me some wildly incorrect answers cause i'll know and withhold the reward. /images/graemlins/smile.gif

Also, the teacher said we could us the internet as a source, he wants us to get these right. He mentione places we might could find the problems worked out for us. So, I am not cheating. /images/graemlins/wink.gif

Here are the 3 problems verbatim. The notes at the end in quotations are my hints....

1: Determine the escape speed of a rocket on the far side of Ganymede, the largest of Jupiter's moons. The radius of Ganymede is 2.64 X 10^6m, and its mass is 1.495 X 10^23 kg. The mass of jupiter is 1.90 x 10^27 kg, and the distance between Jupiter and Ganymede is 1.071 X 10^9m. Be sure to include the gravitational effect due to jupiter, but you may ignore the motion of Jupiter and Ganymede as they revolve about their center of mass. (U = -GMm/r)

2: In 1772, the famed Italian-French mathematician Joseph Louis Lagrange was working on the infamous three-body problem when he discovered an intersting quirk in the results. If one mass is much smaller than the other two then there will exist points where this object can be stationary with respect to one of the two masses. These points are known as Lagrange points in his honor. In our treatment we could consider these points to be equilibrium points for a system. If we wanted to find Lagrange point for the Earth-Sun system located between the Earth and the Sun how far from the earth is this point and what is the significance of the other solution? The mass of the Earth is 5.98 X 10^24 kg, the mass of the Sun is 1.991 x 10^30 kg and the radius of the Earth's orbit is 1.496 x 10^11 m. (solve using quadratic eq.)

3: A climber and her gear have a combined mass of 85.0kg. If she uses a 48.0m legnth of nylon rope with a 1.00cm diameter to climb the cliff face, how much is the rope lengthened when she is at the bottom of the rope and what is the stress and strain on the rope? Now find the same things when she is halfway up the length of the rope? Neglect the mass of the rope in this problem. Young's modulus for nylon is 5.00 x 10^9 N/m^2.

Also, remember this is a 242 physics class (the 2nd half of basic phsycis). So, try not to use crazy tricks I don't even know about yet if you can help it /images/graemlins/smile.gif

The rework is due monday, so, I need these by sunday night ~1am at the latest.

I appreciate the help of the collective.

thanks
blake

[ffredd]

I'm not going to try to solve these problems for you, but I'd like to recommend www.physicsforums.com. (http://www.physicsforums.com.) If you post your questions there, in the homework section, they will give you hints on how you can solve them yourself. You probably shouldn't mention the reward.

If you just post the questions, you can expect the first answer to be "show us what you've got so far".

[Trantor]

[ QUOTE ]
1: Determine the escape speed of a rocket on the far side of Ganymede, the largest of Jupiter's moons. The radius of Ganymede is 2.64 X 10^6m, and its mass is 1.495 X 10^23 kg. The mass of jupiter is 1.90 x 10^27 kg, and the distance between Jupiter and Ganymede is 1.071 X 10^9m. Be sure to include the gravitational effect due to jupiter, but you may ignore the motion of Jupiter and Ganymede as they revolve about their center of mass. (U = -GMm/r)

[/ QUOTE ]

This is my take on this one.

Kinetic energy of the body at "take off" equals the sum of gravtational binding energies due to both bodies.

1/2 mv2 = (GMm/R)(J) + (GMm/R)(G)



Where m is the mass of the object, M mass of the body, G is the gravitational constant, R is the distance from the cetre of the body, and v is the escape velocity.

Not using subscripts so you will understand the righthand side of the equation has different values of R and M appropropriate to Jupiter and Ganymede parts of the equation.

For Jupiter part R equals distance from the far side of Ganymede to the centre of Jupiter, ie distance between their centres plus radius of Ganymede [1.071 X 10^9m + 2.64 X 10^6m]. M is Jupiter's mass.


For the Ganymede part R is Ganymede radius and M is Ganymede mass.

m cancels out from both sides of the equation. Plug in the Ms and Rs as above and G to get 1/2v2 (ie v squared). Solve for v.

[Borodog]

[ QUOTE ]
If we miss any part of the problem, (sig fig, math error) we miss the whole problem . . .

[/ QUOTE ]

Your instructor is a dick.

[_brady_]

[ QUOTE ]
3: A climber and her gear have a combined mass of 85.0kg. If she uses a 48.0m legnth of nylon rope with a 1.00cm diameter to climb the cliff face, how much is the rope lengthened when she is at the bottom of the rope and what is the stress and strain on the rope? Now find the same things when she is halfway up the length of the rope? Neglect the mass of the rope in this problem. Young's modulus for nylon is 5.00 x 10^9 N/m^2.


[/ QUOTE ]

d=PL/AE

P=(85kg)(9.8m/s^2)=833.9N
L=48m or 24m
A=pi*((1/100)(1/2))^2 = 7.85*10^-5 m^2
E=5*10^9N/m^2

Plug this in and you'll get:

For 48m: .1020m or 10.20 cm
For 24m: .05099m or 5.10 cm


Stress:

sigma=P/A, = 833.9N/7.85*10^-5 m^2
=1.062*10^7 Pa or 10.62 MPa

Strain:

epsilon=d/L

For 48m: epsilon=.1020/48 = .00213
For 24m: Will be the same


Note: I didn't really check over my sig figs, I just use 4 if the result starts with a 1, and 3 sig figs if the result doesn't start with a 1.

[Metric]

[ QUOTE ]
[ QUOTE ]
If we miss any part of the problem, (sig fig, math error) we miss the whole problem . . .

[/ QUOTE ]

Your instructor is a dick.

[/ QUOTE ]
Grading is a laborious task if you have to examine every scribble to look for signs of intelligent thought that could potentially be worth partial credit points. The instructor probably just threw this out there to make it easier on himself the second time around -- I doubt he's just trying to be an A-hole.

[Borodog]

[ QUOTE ]
Grading is a laborious task if you have to examine every scribble to look for signs of intelligent thought that could potentially be worth partial credit points. The instructor probably just threw this out there to make it easier on himself the second time around -- I doubt he's just trying to be an A-hole.

[/ QUOTE ]

If you don't want to grade physics problems you shouldn't be teaching physics. /images/graemlins/tongue.gif

Seriously, it's not that hard to grade, particularly if you take the time to teach correct problem solving and organization skills, require them to box the appropriate portions of their solution, etc.

Also in my personal opinion, numerical solutions are only interesting in certain circumstances where an actual number will be illustrative of something important, and plugging in numbers is usually a waste of time. The symbolic solution is the important part.

[BruceZ]

[ QUOTE ]
Also in my personal opinion, numerical solutions are only interesting in certain circumstances where an actual number will be illustrative of something important, and plugging in numbers is usually a waste of time. The symbolic solution is the important part.

[/ QUOTE ]

That's how it is in physics, but in engineering partial credit works like this according to one professor. "Say a problem is worth 10 points, and you don't get it exactly right. Then we take the 10 and divide it like this: 1 | 0. That's 1 for me and 0 for you".

[Meromorphic]

[ QUOTE ]
If you don't want to grade physics problems you should get a graduate student to grade them for you.

[/ QUOTE ]

/images/graemlins/wink.gif

[roundmound]

[ QUOTE ]
[ QUOTE ]
Grading is a laborious task if you have to examine every scribble to look for signs of intelligent thought that could potentially be worth partial credit points. The instructor probably just threw this out there to make it easier on himself the second time around -- I doubt he's just trying to be an A-hole.

[/ QUOTE ]

If you don't want to grade physics problems you shouldn't be teaching physics. /images/graemlins/tongue.gif

Seriously, it's not that hard to grade, particularly if you take the time to teach correct problem solving and organization skills, require them to box the appropriate portions of their solution, etc.

Also in my personal opinion, numerical solutions are only interesting in certain circumstances where an actual number will be illustrative of something important, and plugging in numbers is usually a waste of time. The symbolic solution is the important part.

[/ QUOTE ]

The teacher is taking the time to allow the class to redo test questions to get a better grade. I think the teacher is anything but a dick. He doesn't have to let them get a better grade on the test.

[Nielsio]

Remember the teacher is supposedly working for the students? Oh wait, nevermind. He's isn't working for them, what was I thinking.

[Borodog]

[ QUOTE ]
[ QUOTE ]
[ QUOTE ]
Grading is a laborious task if you have to examine every scribble to look for signs of intelligent thought that could potentially be worth partial credit points. The instructor probably just threw this out there to make it easier on himself the second time around -- I doubt he's just trying to be an A-hole.

[/ QUOTE ]

If you don't want to grade physics problems you shouldn't be teaching physics. /images/graemlins/tongue.gif

Seriously, it's not that hard to grade, particularly if you take the time to teach correct problem solving and organization skills, require them to box the appropriate portions of their solution, etc.

Also in my personal opinion, numerical solutions are only interesting in certain circumstances where an actual number will be illustrative of something important, and plugging in numbers is usually a waste of time. The symbolic solution is the important part.

[/ QUOTE ]

The teacher is taking the time to allow the class to redo test questions to get a better grade. I think the teacher is anything but a dick. He doesn't have to let them get a better grade on the test.

[/ QUOTE ]

So he did his job so poorly that most of the class flunked, so now he's giving them the chance to use google to pass his class.

How magnanimous.

[The Truth]

Ok, well I have 2 winners and an honorable mention.

NoamChomsky gets honorable mention for his hard work.

I deleted the pms from the 2 winners. So, I don't remember their log in names. EXCELLENT!

So, One guy was the first to PM me problems 1&3. And Encule helped me out with problem 2.

You guys PM me your names on stars or party and we'll do something /images/graemlins/smile.gif

thanks for you help everyone,
blake