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Homework Help: Moving object acquiring mass on a frictionless surface

  1. Oct 29, 2007 #1
    Hey all. My professor gave us this question in class the other day but I'm having a hell of a time reasoning out how the answer can be true. Firstly, the problem:

    There's a bucket on wheels moving at a constant velocity on a frictionless surface. It's raining out so the bucket is gaining mass. Will it slow down, speed up, or remain at that velocity as it gains mass?

    My gut tells me that on a frictionless surface that it'd remain constant, but he told the class that it will slow down and that it doesn't matter if the surface is frictionless or not.

    It makes perfect sense to me that it'd slow down with friction (becuase as it gains mass, its normal force increases which increases the frictional force).

    If anybody could help me reconcile this it'd be appreciated. Thanks a lot!
  2. jcsd
  3. Oct 29, 2007 #2


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    The energy of the moving bucket will not change. If the kinetic energy is constant, what happens to the velocity if the mass increases?
  4. Oct 29, 2007 #3
    Oh, I meant to add this to the post above. We have not yet discussed energy in the class so I'm guessing there's an explanation somewhere that has to deal with Newton's laws of motion...

    Thanks for the reply :).
  5. Oct 30, 2007 #4
    I guess, even kinetic energy would decrease. And yes, explanation lies in some chapter before energy!
    As the rain drops get into the bucket, eventually we are looking at a case of inelastic collision. This is bound to decrease it's speed, ain't it? (Since the mass is increasing.)
  6. Oct 30, 2007 #5
    so, you see, your professor had something on his mind.. when he made that statement!
  7. Oct 30, 2007 #6
    Hey, Terp!
    Do you go to the university of maryland? If you do, so do I, let me know.

    Anyways, to answer your question...

    So, the object is moving at a constant speed. It needs to force to keep it moving it just keeps going on this frictionless surface. Well, the raindrops are falling directly into the cart. The raindrops previously had no horizontal(x) velocity, and now the cart must accelerate them to have a horizontal(x) velocity. Well, the cart must exert a force on each raindrop in the horizontal direction to have it move that way. If the cart exerts a force on the raindrop, the raindrop exerts a force on the cart, the cart slows down.

    Basically, since the cart has to push each raindrop from 0 velocity to some x velocity, it has to exert an extra force and slows down.

    Did that help?
  8. Oct 30, 2007 #7
    I haven't got all that far in class yet, but it makes sense to me like this:
    An object moving across a frictionless surface will maintain the same velocity due to its inertia. Inertia is dependent on an object's mass so it follows that if the mass increases, the degree at which the object resists change in velocity also changes. Since the amount of force necessary to get an object of a certain mass moving, it wouldn't be enough to move an object a certain mass +x. Thus it will have a negative acceleration!
  9. Oct 30, 2007 #8
    Hey, you seem to be really physics savvy, could you check out my post--maybe you could help me too? Its 'Analyzing Force vs. Time graphs'
  10. Oct 30, 2007 #9

    Ahh, that makes a lot of sense. And yeah, I do go to UMD I'm a junior in the civil engineering dept. I'm taking these general courses becuase I wasn't sure what I wanted to major in until this year, which was when I had to declare.
  11. Oct 30, 2007 #10
    Nowhere in the question does it say that the rain is coming vertically down. Your answer is dependent on this assumption. If the rain was coming in at an angle the answer would be different.
  12. Oct 31, 2007 #11
    Terp -- What class are you in and when? You might be in my class.

    Tyco -- You're wrong. If the rain was coming down at an angle, it still has LESS horizontal velocity than the cart, so the cart will have to give it more horizontal velocity, or, the cart will still exert a hnorizontal force on the raindrop (the only force we care about in this problem). Hence, the cart will slow down. No matter what (within the bounds of this question).
    The coefficient or proportionality that the cart would slow down compared to how many raindrops fall, may chage slightly due to an angle - but that has nothing to do with this question.
  13. Oct 31, 2007 #12
    It doesn't necessarily have less horizontal velocity than the cart.
  14. Oct 31, 2007 #13
    u are moving on a chair from above some one drops a cannon ball in ur lap. the cannon ball is moving in 1 direction u are moving in another to change the direction of the cannon ball it needs energy the only energy there is is your's so it takes ur energy and u slow down
  15. Nov 2, 2007 #14
    tyco. lol. For some reason I knew you'd say that just to try and prove a point. OK. you win, the cart might have to be moving 0.1 m/s or more... Or maybe it's a typhoon and the raindrops are shooting through your "cart". But then it wouldn't matter because the wind would pick up and since you were smart enough to put a sail on the back of your cart, your defying 12 laws at once. It is implied that the rain is falling vertically. ^_^
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