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Homework Help: Conservation of Energy Post Lab Questions NEED HELP!

  1. Mar 3, 2010 #1
    i have to say physics kicks my butt. i never worked so hard in my life to earn a D in a class. now, i have to take the lab for that course and they are trying to kill us! the TA tore up my lab partners work because he didn't like the way she presented it in her lab book! ok, so that is what i am dealing with here. plus if i don't pass this i can't graduate this semester. i need some help on the following:

    1 . What is the gravitational potential energy of a ball on Mars with a weight of 56.3 N when it is sitting on a shelf 17.64 m above the Martian floor?

    my attempt: PE=mgh gravity on mars according to wiki= 3.69m/s^2

    PE= 5.63 X 17.64 X 3.69 = 3664.7

    2 . Suppose you are performing the downhill run of the experiment you did this week, what is the energy ratio (Ef/Ei) of a system on Ganymede that has a final velocity of 996.9 cm/s, initial velocity of 8.8 m/s, and a height of 0.0748 km. Assume that the gravity on Ganymede is 1.428 m/s2.

    my attempt: no clue where to start

    3 . Two cars have the same mass, but the red car has 4.55 times the velocity of the blue car. What is the kinetic energy ratio of the blue car to the red car? Express as Kr/Kb= ? .

    my attempt: again no clue where to start.

    4 . An astronaunt lands on the dwarf planet Eris and decides to perform experiments. One of these experiments is to measure the acceleration due to gravity. He finds that the acceleration due to gravity is constant at 0.8 m/s2. Then he finds a deep crater and wants to know its depth so he fires a rope to the center of the crater. He then slides down the rope without friction and finds that his velocity at the bottom is 70.97 ± 0.3 m/s. Using this information and the fact that the astronaunt has a weight with his suit of 240 N on Mars, find the average depth of the crater.

    my attempt: a= 0.8m/s2 ; d=? Vo= 0m/s ; Vf= 70.97m/s; m= 240N

    to me the most relivant equation is Vf2= Vo2 + 2ad

    (70.97)2= (0)2+ 2(.8)(d)
    3147.9m = d

    5 . The space shuttle is the most complex machine ever built and the first orbital spacecraft designed for partial reusability. It has a mass of 2,209,203 kg and is capable of taking up to 11 astronaunts and 24,400 kg of payload into orbit. For it to achieve orbit, it takes an average $450 million and a two stage rocket capable of delivering 5,253 kN of thrust. Once in orbit, it must maintain a speed of 17,500 mph. When the space mission is over, the shuttle must dissipate all of the energy in the form of heat (friction) by performing a series of S turns. Assuming the shuttle lands at 246.8 mph, has 4 astronaunts each with a weight of 200 kg, and a payload weight of 22847 kg, find how much energy (in GJ) the shuttle has the second that it lands (does not come to a complete stop).

    my attempt: huh?
  2. jcsd
  3. Mar 3, 2010 #2


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    IF this incident was as extreme or dramatic as you are making it out to be, then maybe you should lodge a complaint. I'm a TA, and I have to say that we are expected to conduct ourselves in a professional manner, especially since we're being paid to TA as a job. There is no reason why the TA should have been touching your friend's lab book in a violent way. He should have merely informed your friend in an authoritative but polite manner that his work was unacceptable and needed to be redone.

    Here, it looks like you assumed that g = 10 m/s2, and divided the weight by that in order to find the mass of the object. However, as you pointed out, in the problem the object is on Mars, where g is NOT equal to 10 m/s2. Do you see the inconsistency?

    The thing is, you don't need the mass OR g. You've already been given the weight, which is equal to mg by definition. Therefore, all you have to do is multiply the given weight by the height in order to find the gravitational potential energy.

    Well, we can't really help you if you don't give us the details of "the experiment you did this week," can we? That having been said, energy is conserved, so why should the final energy be any different from the initial energy? Unless, of course, you're only considering mechanical energy (kinetic + potential) which is not conserved, because it can be converted into other non-mechanical forms of energy (e.g. into heat, by friction).

    This problem doesn't require anything other than knowing the definition of kinetic energy. How does the kinetic energy of an object depend upon its mass and its velocity? Once you know that equation, all you have to do is plug in the numbers.

    I didn't check your numbers, but your method seems right. His mass is not relevant, because all objects in free fall will accelerate at the same rate.

    It looks like there is a LOT of extraneous information in the problem. When the shuttle lands, it has a certain amount of kinetic energy. You can figure out how much kinetic energy this is (at landing) just from the total mass and speed of the shuttle upon landing. You don't even need to use any of the information that was given about the shuttle prior to landing, as far as I can see.
  4. Mar 4, 2010 #3
    thanks for the reply. i will take another shot at working the problems when i get back from school. the TA thing is an honest to god true story. my partner went complain to the dean or department head. the guy in charge of the TA is the old department head who is the most pompous a hole you have ever seen in your life. his aids reflect his personality big time.
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