1. Limited time only! Sign up for a free 30min personal 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!

Elementary science project - HELP!

  1. Feb 26, 2010 #1
    I am looking for an easy way to explain to my 2nd grade son which ball will roll down a ramp faster. I have a ramp (more like a trough) set at about 30 deg angle incline. He is going to roll a baseball, pool ball, golf ball, and basket ball down the rramp. He is only 2nd grade so I can't get into the math since he hasn't had multiplication yet. Any suggestions? - This is his science fair project.
     
  2. jcsd
  3. Feb 26, 2010 #2
    This might be too advanced for a second grader. The acceleration down the ramp is g·sin(incline angle) minus retardation due to the moment of inertia. Solid spheres have a moment of inertia 2mR2/5, while hollow spheres have moment of inertia 2mR2/3. See

    http://en.wikipedia.org/wiki/List_of_moments_of_inertia

    The hollow basketball will (should) roll slowest.

    Bob S
     
  4. Feb 27, 2010 #3
    Pretty complex for a second grader....

    Maybe you can contrast the difference relative to free fall???...free fall acceleration due to just gravititational inertia (mass)...versus and the combined rotational inertia and gravitational acceleration [g sin (angle)].....

    Think about your trough in a vertical position (free fall objects) and then, the other extreme, horizontal, to contrast the different behavior...and an angle in between....

    equal acceleration in the first free fall, no acceleration in horizontal, different acceleration at the "angle". You can illustrate in an angle fall some objects fall slower than others....the slowing is what's called Bob's rotational interia.

    I can't think of a simple way of contrasting rotational inertia with free fall inertia....maybe check on gyroscopes??
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook