Science Buddies: "Ask an Expert"

I did an experiment with my students last week in which we let a light, plastic ball roll down an incline to determine the effects of changing the height from which it rolls on the distance it rolls after reaching the bottom of the ramp. My students had some excellent questions. They wondered what would happen if we tried heavier balls, such as marbles. My questions is this: will marbles go the same distances when started at the same heights on the ramp as the plastic balls? Does the falling bodies principle apply to objects on a ramp as well or only to free-falling objects? Why or why not? Thanks for your help!

The balls may or may not go the same distance. The principle for the objects falling through the air does not apply because the balls are also rolling. Some of the energy from the height of the ramp goes into the forward motion of the ball and the rest goes into the rotational energy of the ball spinning about its axis as it rolls down the hill.

The Force (momentum) equation for a moving object involves 0.5 * m * v * v where m = mass, v = velocity.
The Force (friction) equation involves m but the multiplier will be extremely small.
The Force (gravitational) equation that causes the acceleration down the ramp is proportional to m.

Given this, a large difference in m will likely cause a significant difference in your expected results.