What are the effects of oil on a ball rolling up a ramp?

[wetwilly92]

I'm working on a simple question involving finding the initial velocity of a ball as it is launched from a shooter and goes up a ramp. The ball is lathered in oil to reduce friction and create a marking for easily measurable travel distances.

From there, you can gather several trials of data and average your travel distances and the corresponding heights as well as measure the angle of elevation. After that, it's simply a matter of conservation of energy and the work done by the frictional force to solve for the initial velocity.

What I want to know is, what are the consequences of lathering the ball in oil? How much in general would the oil reduce friction? Is that calculable? Can we find the coefficient of kinetic friction? If so, how?

This is just a simple first semester physics problem, so things such as center of mass and air resistance are negligible.

Am I overlooking anything important?

 

[KingNothing]

I'm a bit confused - is the ball rolling on top of a track, or sliding along a track?

 

[DickL]

I think the consequence of lathering the ball with oil will be to increase the ball's rolling resistance. A thick oil layer will create viscose drag as the ball rolls (or skids).

I suspect that using a hard ball (marble, steel ball) on a smooth, hard dry surface (hardboard, steel, enameled wood) will give a lower friction combination. I'm basing this on some experience the soap box derby, buggy racing in college, and some experience with rolling element bearings. In a rolling element bearing the oil (or grease) is largely their to provide cooling of the surfaces not to reduce friction.

As for determining the distance the ball rolls, I would go with a set of scaled lines marked on the rolling surface. Using a pattern of lines (e.g. black lines every cm, a red line at 5 cm, and green at 10 cm spacings) would make for relatively easy spotting and measuring of the distance traveled.

 

[wetwilly92]

I'm a bit confused - is the ball rolling on top of a track, or sliding along a track?

The ball (marble) would start at the bottom of the ramp and get "shot" up towards the top of the ramp.

I think the consequence of lathering the ball with oil will be to increase the ball's rolling resistance. A thick oil layer will create viscose drag as the ball rolls (or skids).

I suspect that using a hard ball (marble, steel ball) on a smooth, hard dry surface (hardboard, steel, enameled wood) will give a lower friction combination. I'm basing this on some experience the soap box derby, buggy racing in college, and some experience with rolling element bearings. In a rolling element bearing the oil (or grease) is largely their to provide cooling of the surfaces not to reduce friction.

As for determining the distance the ball rolls, I would go with a set of scaled lines marked on the rolling surface. Using a pattern of lines (e.g. black lines every cm, a red line at 5 cm, and green at 10 cm spacings) would make for relatively easy spotting and measuring of the distance traveled.

Huh, I had always just assumed that the oil would reduce friction, I didn't think it would actually increase it. This sounds like a solid plan.

One last thing, the goal of the entire question is to acquire the most accurate initial velocity possible (as we want to compare it to an initial velocity found in a lab we did), so would friction be easily calculable? Is it even worth calculating given such a small mass?

 

[DickL]

Computing the initial velocity will be similar to finding the initial velocity of a ball thrown vertically based on the height obtained (ignoring air drag). However, there are 2 added effects that should be considered: 1) the rolling resistance, which is different than standard sliding friction, and 2) the rotational inertia of the ball (to roll up the ramp its rotational speed, rpm, will accelerate then decelerate). The I of the ball is easily looked up and solved for once you know the diameter and mass.

To measure the rolling resistance, I would run some trails of letting the ball freely roll down the ramp and its time to travel a known distance. Then back solve for the rolling resistance. I would anticipate the rolling resistance will be quite low, maybe low enough to be ignored when other error sources are considered.

Rolling resistance, at least as I've worked with it, is composed of a number of effects, all lumped into RR. Some of these effects are: deflection of the wheel (your ball) at the contact point, deformation of the running surface under the wheel, surface roughness (having to roll over bumps), dirt, liquids (rain on the road, oil lather onto the ball), hysteresis or non-elastic deformation of the wheel and/or road as the wheel rolls, etc. Generally on a dry and reasonably clean surface a single value can be used to cover a wide range of loads and speeds. When you think about these effects and their likely values during your experiment, the surface roughness may be the dominate effect.

I've not put any of this into equation form, because that's your job :) Have fun.