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Homework Questions? Am I right or the Professor?

  1. Nov 2, 2006 #1
    Here's some questions from a friends homework that we submitted online. I think the answers that the professor marked are wrong. If they are right please explain it to me.

    1) A golf ball moving forward with 1 unit of momentum strikes and bounces backward off a heavy bowling ball that is initially at rest and free to move. The bowling ball is set in motion with a momentum of

    My answer: less than 1 unit of momentum

    Correct answer: more than 1 unit of momentum

    2) A jumbo jet has a mass of 100,000 kg. The thrust for each of its four engines is 50,000 N. What is the jet's acceleration in meters per second per second when taking off?

    I'm not sure if I'm right or he is, but i used F = ma and got 1/2 m/s^2.

    Correct answer: 2 m/s^2

    3)A 1-kg glider and a 2-kg glider both slide toward each other at 1 m/s on an air track. They collide and stick. The combined mass moves at

    I said 1/6 m/s.

    Correct answer 1/3 m/s.

    I guess it depends on how you interpret the phrase "slide toward each other at 1 m/s"


    4) A 5000-kg freight car moving at 2 m/s runs into a 10,000-kg freight car at rest. They couple upon collision and move away as one body at

    My answer: 2/3 m/s

    Correct answer: 2 m/s

    Someone please explain to me how a small object hitting a stationary bigger object can make them both move at the same speed the smaller object was moving before the collision.


    5)Your friend says that the heavyweight champion of the world cannot exert a force of 50 N on a piece of tissue paper with his best punch. (50N is a force equal to roughly a 10 pound weight.) The tissue paper is held in midair - no wall, no tricks. You agree or disagree?

    Why can't you exert a force of 50N on a piece of tissue paper? Is it because the paper rips? What if the paper is not held securely and the paper could move? Could you then exert 50N of force on it?
     
  2. jcsd
  3. Nov 2, 2006 #2
    OOPs didnt read the sticky. Sorry. Perhaps an admin can move it?
     
  4. Nov 2, 2006 #3

    quasar987

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    The quantity that is conserved is not the absolute momentum but the vectorial momentum. Initially, the total momentum of the system is +1. After the interaction, the golf ball bounces back, so it's momentum is negative (speed in the negative x direction). Hence, for the total momentum to be 1, the momentum of the big ball has to be greater than unity.
     
  5. Nov 2, 2006 #4
    That makes sense. It's been awhile since I had physics class.
     
  6. Nov 2, 2006 #5

    quasar987

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    Is that all the information you are given? Because from what I understand, the plane will take off when the vertical force on it counter balances the force of gravity. The vertical force on the plane as a function of the horizontal speed of the plane is a function of the geometry of the plane (roughly, it depends on the angle of inclination and area of its wings) and of the coefficient of friction for air at sea level.
     
  7. Nov 3, 2006 #6
    I think your getting into to much detail. It's an intro course. I think he just wants to know if 50,000N force is appled to a 100,000kg object, how fast will it accelerate?
     
  8. Nov 3, 2006 #7

    quasar987

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    Initial momentum = (1kg)(2m/s)-(1kg)(1m/s)=2-1=1kgm/s

    Final momentum = (2kg)v. It must equal initial momentum, so
    2v=1 ==> v=1/2.
     
  9. Nov 3, 2006 #8

    quasar987

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    Right, he asks for acceleration, not speed. Sorry. Ok...hum

    Total force = 4*50,000=200,000. Resulting acceleration=F/m = 200,000/100,000=2
     
  10. Nov 3, 2006 #9

    quasar987

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    4) is 2/3 in my part of the world as well.
     
  11. Nov 3, 2006 #10
    Ok, that makes sense. Misread that. EACH engine has 50000N of force.
     
  12. Nov 3, 2006 #11

    Shouldn't it be something like. p = m1(mass of object 1)v1(velocity of object 1) + m2v2 which would be p = 2(1) - 1(1) = 1 Then take p = mv, 1 = 3v, v = 1/3

    Unless you interpret that as they are approaching each other at 1 m/s. which would give you 1/6 m/s.
     
  13. Nov 3, 2006 #12
    OK nvm, thats the same thing you had just the mass in the final one is 3kg.
     
  14. Nov 3, 2006 #13

    quasar987

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    and for 5) I'd say he can too. If we assume that the tissue is floating in midair and the guy hits it. The tissue will very rapidly pass from null speed to the same velocity as the punch (the two objects will stick).
     
  15. Nov 3, 2006 #14

    quasar987

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    Yes that's more like it.
     
  16. Nov 3, 2006 #15
    Thanks for your help. Now I can go back and explain them to my friend and make her argue for the other two.
     
  17. Nov 3, 2006 #16

    OlderDan

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    Estimate the mass of a piece of tissue paper. What acceleration would it have if you applied a force of 50N? What force would the fist have acting on it if the fist applied 50N to the paper?

    Actually, there is no definitive answer to this question. One could argue that the force on the tissue paper is enormous, but of such short duration that the fighter would not even notice. But when you throw in stuff like the "give" in the boxing glove and all that you are hard pressed to defend a 50N force.
     
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