1. Not finding help here? Sign up for a free 30min tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Object sliding down a block on a scale

  1. May 3, 2015 #1
    1. The problem statement, all variables and given/known data
    A 200g hamster sits on an 800 g wedge shaped block which in turn rests on a spring scale. An extra fine lubricating oil is sprayed on the top surface of the block, rendering it entirely frictionless, causing the hamster to slide down. Friction between the block and the scale is large enough that the block does not slip on the scale. What does the scale read as the hamster slides down?

    2. Relevant equations


    3. The attempt at a solution
    The hamster accelerates down the block with a = gsin##\theta##
    The vertical component of this acceleration is:

    ay = gsin2##\theta##

    So the downward force exerted by the hamster on the block and thus onto the scale is

    Fhamster = may

    I added this to the weight of the block and ended up with

    Reading = Mg + mgsin2##\theta##
    Reading = (0.800)(9.8) + (0.200)(9.8)sin240o
    Reading = 8.650N

    Which is incorrect. I feel that I'm on the right track but I've missed some critical detail. Where did I go wrong?
     
  2. jcsd
  3. May 3, 2015 #2

    haruspex

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    Try a special case. Suppose theta is zero. What do you get for the force the hamster exerts on the block?
     
  4. May 3, 2015 #3
    In that case it would simply be ##mg##
     
  5. May 3, 2015 #4

    haruspex

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    Yes, of course it should be, but is that what you get from your mg sin2θ formula?
     
  6. May 3, 2015 #5
    Indeed not. I can see that mgcos2##\theta## is correct. What I don't see is how to arrive at that.
     
  7. May 3, 2015 #6
    Wait.
     
  8. May 3, 2015 #7
    The vertical component of the force the hamster exerts on the block is

    F = mgcos2##\theta##

    Add this to the weight of the block and there we go.
     
  9. May 3, 2015 #8

    haruspex

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    Ok. All good now?
     
  10. May 3, 2015 #9
    Almost. I would like to look at this a little closer. My initial approach seemed sensible, and I'm not quite sure why it did not work. Logically, that is.
     
  11. May 3, 2015 #10

    haruspex

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    It's because you took the downward acceleration as indicative of the downward force exerted. If you write out the usual ##\Sigma F = ma## equation you'll find you should have negated it.
     
  12. May 4, 2015 #11
    Yes, I see that now. I noticed that subtracting that force from the total weight yielded the correct answer but the reason behind it never clicked. Thanks a whole lot.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted



Similar Discussions: Object sliding down a block on a scale
Loading...