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Homework Help: Momentum Conceptual Questions help!

  1. Nov 29, 2012 #1
    1.A bug and the windshield of a fast moving car collide. Tell whether the following statements are true or false:
    1) The forces of impact on the bug and on the car are the same size. False
    2) The impulses on the bug and on the car are the same size. False
    3) The changes in speed of the bug and of the car are the same. False
    4) The changes in momentum of the bug and of the car are the same size. True

    2. Does a larger force always produce a larger impulse on an object than a smaller force? Explain.

    3. A fully dressed person is at rest in the middle of a pond on perfectly frictionless ice and must go to shore. How can this be accomplished?

    4. If 2 bodies collide and one is initially at rest, is it possible for both to be at rest after the collision? is it possible for one to be at rest after the collision? explain.

  2. jcsd
  3. Nov 29, 2012 #2


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    Hello Mrclean214. Welcome to PF !

    According to the Rules of this Forum:
    Give your reasoning for your 4 answers to problem #1.

    For 2, 3, and 4: What have you tried ? Where are you stuck ?
  4. Nov 29, 2012 #3
    My teacher taught me this today but I honestly have no idea how to answer it using scientific reasoning, but my explanation will be as follows:
    1. 1) False, because the car will obviously apply more momentum to the bug than the bug did to the car due to larger momentum.
    2) False, because the car have more velocity and mass, therefore the car will have more impulse than the bug
    3) False, again because the lack of momentum from the bug will really change the speed of the car, the bug on the other hand will change speed dramatically due to the massive transfer of momentum energy from the car to the bug
    4) True, because according to Newton's Third Law, when an object collide with another, it will apply equal and opposite energy to the second object.

    2. I honestly dont know how to answer this one...

    3. If he/she takes off his/her clothes and throw it hard in the opposite direction of his/her destination, it will slide him/her smoothly across the frozen lake.

    4. No, once collide, the accelerating object have transferred from of the momentum energy to the object at rest, which will cus the object at rest to move, on the otherhand, the object at rest will reflect those energy back to the accelerating object and force it to move the other direction. It could be possible when the accelerating object's mass is significantly smaller than the object at rest.

    Please correct me if I stated anything wrong, and sorry I didn't know the rules, thanks!
  5. Nov 29, 2012 #4


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    This question is about force, which will vary over the time the two are in contact. So think about some instant during that contact. Is the car pressing harder on the bug than the bug is pressing on the car? What does Newton's 3rd law say about that?
    Certainly the car had a lot more momentum available to it, but that is not the question. What does the law of conservation of momentum imply here?
    I presume you meant "the bug will not really change the speed of the car".
    Right answer but wrong reasoning. Suppose the changes in momentum are equal. Can you think of another reason the bug's speed change will be the greater?
    This is effectively the same as question 2. Newton's 3rd Law doesn't mention energy.
    Do you have a formula relating momentum, force and time? (Impulse is change in momentum.)
    Sounds good. I must try it.
    Sort of right, but your terminology is all wrong. You mean the moving object, not the 'accelerating' one.
    It's not 'momentum energy'. Momentum and energy are different things. For this question, energy is not useful since it might not be conserved by the collision. Just use conservation of momentum.
    Again, it's not a matter of 'reflecting energy back'. There is an equal and opposite change in momentum. That could result in the originally moving object having no momentum. But it would not necessarily be a lot lighter for this to happen. In a perfectly elastic collision (i.e. energy conserved), the first object would stop if the masses are the same. If the first object is lighter it will bounce back.
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