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Is this work?

  1. May 2, 2015 #1
    I have a question about work. In a lil debate over something related to work.. and my question is, if a car runs over a quarter laying flat on the ground, will the quarter being doing work on the tire? I say yes.. because when the tire rolls over the quarter, it is putting force on the quarter and the quarter is putting force on the tire. As that happens, the quarter displaces the rubber where there is contact.. so wouldn't that be work? My friend says no.. the quarter isn't doing work on the tire because the tire returns to it's original position after it rolls off the quarter.. and since there is no net displacement, there is no work being done. Who is right?

    Thank you!
  2. jcsd
  3. May 2, 2015 #2


    Staff: Mentor

    Review the definition of work and apply it to your situation:


    What level of detail do you want to get into?

    If the quarter doesn't move then what work is being done on it? If the quarter doesn't get compressed then what work is being done on it?

    If we decide that the quarter doesn't move and the quarter doesn't get compressed then you have to conclude that no work is being done on the quarter.

    What do you think?
  4. May 2, 2015 #3


    User Avatar

    Staff: Mentor

    Yes, but he's asking about work being done on the tire, not the quarter.
  5. May 2, 2015 #4
    Honestly, as much detail as you see fit.. but I am more curious about the tire itself. For the quarter itself, assuming it doesn't move.. wouldn't there still be a minute amount of compression from the weight of the car? If the quarter were secured to one location and you drove over it again and again and again.. you'd wear off the face of it, so wouldn't that mean (even if it is on a micro scale) that the quarter is being changed each time you ran it over? Since there is force coming from the tire and a minute change, couldn't it said that there is still work from tire to quarter and quarter to tire?
  6. May 2, 2015 #5
    Sorry if I sounded like I wanted you to do all my work.. on work. It's something that I've been pondering on for a lil bit now and have thought it out as much as I could.. and I am certain that there is work. I kind of feel dumb for not thinking about it sooner, but I am just going to do an experiment that will only cost me 25 cents. I'm going to put a quarter on the road tomorrow and see what happens. I think a few things will happen. The quarter will move, even if it's just a little bit.. might even be a few times where it gets kicked up. Then might try to get away with super gluing it somewhere where it will get ran over by regular traffic.. but don't know if I really should. I'm going to find an empty road and run over it myself.. but I can just see someone get caught up in trying to pick it up if I glue it down and something dumb happens. If I do glue one down, I am pretty sure that over time the face will wear off. If I do both and see movement or the face wearing down, that will tell me that there is some work being done on the quarter. I can also see the side touching the ground getting all dented up..

    As for the tire.. am I wrong in thinking there is work being done on it? For example, when you stretch a rubber band out.. aren't you doing work on it while you're stretching it? Or is that actually not considered work since it would return to it's original state when you let go? I kind of feel like since (if) there is work being done on the quarter.. there should be work being done on the tire too. The tire is exerting a force onto the quarter, so the quarter is exerting a force that is equal and opposing on the tire.. and that force is displacing the tire when being applied.
  7. May 2, 2015 #6


    Staff: Mentor

    I wanted to set the stage.
  8. May 2, 2015 #7


    User Avatar
    Education Advisor
    Gold Member

    It sounds like you're both right, but talking about different things. If you look at it from a net displacement point of view from t0 is just before transiting over the quarter, and t1 is right after, your friend in right. If you look at during a shorter time frame of the tire transiting over the quarter. Then obviously there is work being done to the tire. Work is strongly dependent on your frame of reference.

    I don't think you need to do your experiment. =P
  9. May 3, 2015 #8
    I might still do it.. lol, cause I looked on Google and YouTube and couldn't find any information or videos of a car running over a coin. I know that I've found coins on the road that were all banged up, but never saw a car hit a coin and am now curious as to what will happen. I want to see how far it will move if it does.. maybe try a couple different speeds.. see if it gets kicked up.

    This was a weird random debate, but this specific question started when I said that during the collision with the quarter.. the tire applies a force and transfers energy to the quarter while the quarter applies an opposing force and transfers energy to the tire. He said that energy doesn't get transferred to the tire, and that it doesn't because if energy were transferred to the tire.. the quarter will then be in a different energy state. But it's not in a different energy state.. it's the same as it was before running it over and you could run it over and over and over and nothing will change. Yet I am sure that you would see a noticeable change in everything about the quarter, especially if you ran it over numerous times. It's position will change, it's mass will change as the face wears off, and it's shape will change as it gets all dented up.. pretty much everything about it changes.
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