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!

Physics question doesn't make sense to me

  1. Sep 27, 2005 #1
    Here is the question:

    A wheel of radius R rolls along the ground with velocity V. A pebble is carefully released on top of the wheel so that it is instantaneously at rest on the wheel. Show that the pebble will immediately fly of the wheel if V > sqrt(Rg).

    What does this mean. In particular, what does instantaneously at rest mean??
  2. jcsd
  3. Sep 27, 2005 #2

    Doc Al

    User Avatar

    Staff: Mentor

    It means that when the pebble is placed on the wheel, its speed is exactly the same as the speed of the top of the wheel. (So, with respect to the wheel, it's at rest.)
  4. Sep 28, 2005 #3
    Like Doc Al says, it means that the wheel and pebble are happily in motion together. Carefully explore this problem. Remember that the velocity at the top of the wheel is not V.

    also try to graph the motion of the pebble vs the ground, vs the wheel, and the wheel vs the ground. thats how my teacher started me to understand this problem. we did the problem with a meter stick and pennies, sliding off of a giant snowball...

    what you are showing is that the velocity of the pebble is so high that the pebble doesnt say on the wheel at all, and that the velocity at this point can be characterized by the velocity of the center of the wheel relative to the ground, which is a function only of R and g.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?