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How to interpret the given informations in physics problems

  1. Apr 14, 2015 #1
    1. The problem statement, all variables and given/known data:A skier is being pulled along a horizontal surface at constant speed with a force of 50.0 N. What is the weight of the skier?coefficient of kinetic friction=0.05.
    My main problem is that what "at constant speed" implies here?i.e A skier is being pulled along a horizontal surface going at constant speed or
    A skier ,when pulled along a horizontal surface attains constant speed.

    When I was given this to solve, I thought that
    A skier is being pulled along a horizontal surface going at constant speed
    ,but when viewed the solution the other was true.

    I know how to solve this in this case .I am filling the template .But my real problem is what is wrong in my interpretation.How to avoid such wrong interpretations.



    2. Relevant equations:at constant speed
    force applied=fk



    3. The attempt at a solution:50N=0.05 multiplied by weight
    weight=1000 N.
     
  2. jcsd
  3. Apr 14, 2015 #2

    CWatters

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    I believe you were correct (A skier is being pulled along a horizontal surface going at constant speed).

    Why do you think the two options are different? The skier ends up at a constant speed in both cases.



     
  4. Apr 14, 2015 #3
    Because if I interpret this as A skier is being pulled along a horizontal surface going at constant speed
    I will not come to the below conclusion.
    force applied=fk
    will I?
     
  5. Apr 14, 2015 #4

    CWatters

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    Why not? Constant speed implies no acceleration. Zero acceleration means the net horizontal force acting on the skier is zero. Therefore the two forces friction and tension must be equal (and opposite).
     
  6. Apr 14, 2015 #5
    By skier is being pulled along a horizontal surface going at constant speed I mean
    skier was going at constant speed and then horizontal force acted on it ,What happens after that is not known whether or not it balances friction.
    You are also answering in
    ,when pulled along a horizontal surface attains constant speed this context.
     
  7. Apr 14, 2015 #6
    I would interpret it as the force pulling the skier cause him to travel at a constant speed (because there is a frictional force opposing his motion). So the force is the cause and the effect of it is constant speed, which is the second case you mentioned.
     
  8. Apr 14, 2015 #7

    CWatters

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    Ah ok, that interpretation hadn't occurred to me. The original problem says..

    I think it's pretty clear the 50N force is doing the pulling and it isn't a force thats applied later.

    I suppose it might have been clearer had it said

    Anyway the key to interpreting the problem is to recognise that "constant speed" implies a balance of forces.
     
  9. Apr 14, 2015 #8
    Ok,how would you interpret this one
    A 53. kg block, slowed by friction, has an acceleration of -0.1 m/s2. What is the force of friction on the block?

    Block has acceleration of -0.1 m/s2 after slowing down?Right?
     
  10. Apr 14, 2015 #9

    jbriggs444

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    The block has an acceleration of -0.1 m/s2 while it is slowing down.
     
  11. Apr 14, 2015 #10
    in the process of slowing down?
     
  12. Apr 14, 2015 #11

    jbriggs444

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    Yes.

    After the block has finished slowing and has come to a stop, its acceleration will not be -0.1 m/s2. It will be zero. So it makes no sense to understand the problem as specifying the acceleration after slowing down. In addition, if we know the acceleration now, it does not matter what the acceleration was previously. So again, it makes little sense to understand the problem as specifying the acceleration after slowing down.

    One of the first rules of problem interpretation is that if an interpretation does not make sense, it is not the interpretation that was intended.

    [The tricky part is handling situations where none of the possible interpretations make sense]
     
  13. Apr 14, 2015 #12
    I meant after slowing down to some extent it will have acceleration of -0.1 m/s2 .
     
  14. Apr 14, 2015 #13
    If some object has stopped slowing down,does it necessarily imply that it is at rest or moving with constant speed?cannot it be speeding up?
     
  15. Apr 14, 2015 #14

    jbriggs444

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    A "53 kg block slowed by friction" will not end up moving with a constant non-zero speed and will not speed up. It will come to rest and stay at rest... Barring some unexpected problem setup.

    Yes, if the block were sliding across sandpaper and came to a patch of ice, it could indeed stop slowing and proceed at a constant speed. And if the block were sliding across sandpaper to the rear on a train that is moving forward then it could indeed stop slowing and begin speeding up (as measured against the ground).

    Rule 2 of problem interpretation: If the problem does not mention icy patches or trains, assume that there are no icy patches or trains.
     
  16. Apr 14, 2015 #15
    Block has constant acceleration of -0.1 m/s2 throughout the slowing process,right?
     
  17. Apr 14, 2015 #16

    jbriggs444

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    The only acceleration that we can care about is the acceleration that is given. This pretty much comes under the heading of ice patches and trains. If the acceleration is given as -0.1 m/s2 then you should not concern yourself with the possibility that the acceleration is different from -0.1m/s2.

    Edit: You should look at the problem statement again. It does not ask you for the force throughout some process. It just asks you for the force. Even if the acceleration varies throughout the process of slowing down, the only thing that matters to determine the force at a particular moment is the acceleration at that moment. Any concern about whether the acceleration is constant is irrelevant.
     
    Last edited: Apr 14, 2015
  18. Apr 15, 2015 #17

    CWatters

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    +1 to what jbriggs444 said.

    You can't solve the problem unless you assume that it's the friction causing the deceleration. It's a simple f=ma problem.
     
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