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Homework (conceptual?)

  1. Feb 7, 2009 #1
    Hey guys, I have a few questions about my physics homework. They are mainly conceptual and I have attempted them. I will write down the questions AND my attempts and please correct me if I am wrong.

    1. In a situation of tug-of-war with a friend who is the same size and weight. You both pull hard but neither are winning. Compare the magnitude of the force exerted by you on your friend, with the magnitude of the force exerted by your friend on you. Are they the same or different? If not, which is larger? why or why not?

    Attempt 1: F(a on b)=F(b on a) same magnitude because of Newton's third Law. Since the system is in equilibrium, this implies that the net force = 0 and thus, the magnitude of the forces are equal since both people are around the same weight. (not sure o_O)

    2. Same problem as number 1, but now you (bigger guy) vs. a kid (smaller guy). But both pull and still are in a standoff.

    Attempt 2: The Force applied by the smaller guy is larger in magnitude than the bigger guy. Since the bigger guy will have a bigger friction and thus more tension on the rope, the only way for the smaller guy to keep this system in equilibrium is to apply the same force. (Maybe the surface that the kid is standing on has more friction?, really not sure here >.<)

    3. Same problem as number 2 but now the kid is on a skateboard. Bigger guy is winning.

    Attempt 3: The force applied by the bigger guy is larger in magnitude since the smaller guy is on a skateboard thus reducing friction and thus reducing the force applied/tension in the rope?

    4. Same problem again but now bigger guy is pulling the kid across the floor.

    Attempt 4: The force applied by the bigger guy is larger in magnitude since the smaller guy is actually getting pulled towards the bigger guy? (again, not so sure)

    5. Two cards, a truck and a small car collide at 5 mph. Compare magnitude of forces, which vehicle would you rather be in? why?

    Ftruck>Fcar, because truck has higher mass. Rather be in truck becase______ no idea :(

    I am thinking about all these in a logical sense + the lecture that I went to yesterday about forces. I am still kind of new at this soooo I am kind of paranoid on my answers, I think I got a lot of them wrong. Please help and fix my thinking before tests >.< Thanks
     
  2. jcsd
  3. Feb 7, 2009 #2

    LowlyPion

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    For your pulling questions, if you have a net force in a direction then it will accelerate the mass that is resisting it.

    For your 2 cars, what is momentum? Mass*Velocity. That means in a 2 body collision the heavier mass will experience less change in velocity.

    But you weigh the same whichever car you are in, hence if you are in the heavier car you will experience less change in velocity (acceleration) which times mass is force. The forces on you will be less.
     
  4. Feb 7, 2009 #3
    maybe i stated the car problem wrong. it said that the 5 mph is the initial speed that they are at moving towards each other. I haven't learned momentum yet so I do not know if my teacher wants me to put that as an answer. this homework is the introduction of forces and motion diagrams and such
     
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