Science Buddies: "Ask an Expert"

hello my name is salma and im in the 11th grade...my question for my project is this:
Does the type of surface of an inclined plane affect how fast and far an object rolls?
surfaces: uncovered plane, carpet covered, aluminum foil, (small bubbled) bubble wrap
object: marble

this is my hypothesis, i think its pretty decent for now since my project isn't due yet:
I believe that the marble will roll farther and faster on the inclined plane covered with aluminum foil than the uncovered and bubble wrap, carpet covered planes.

i have a few questions
1. is the marble also a controlled variable?
2. is the distance and time of the marble my dependent variables
3. is the surface type my independent variable
(i always get my dependent and independent variable confused because i know one depends on the other)
4. what would be the best inclined plane, if it was to be raised 5 inches above the ground? could i use a shoe box or bleep in the house or would i have to buy wood from home depot.

umm and it is possible that i use the speed formula distance over time to show understanding of acceleration?
i want to use centimeters over seconds and then just convert my centimeters into meters so it'll be m/s

You've got a great start.

Controlled variables are things you attempt to keep the same between trials. By using the same marble in all trials it falls into an experimental class of "self controls" so the marble's physical properties (mass, diameter, density, etc.) are controlled variables (a subtle difference from the marble as a whole being a controlled variable).

Assuming the marble doesn't sink into the carpet and stay in place (small marble, long shaggy carpet, low angle of incline, high probability of staying put), then the distance the marble has traveled from the time it is released will be a function of several things including the elapsed time. This relationship is going to be fairly complex and hard to measure. You probably need to think about ways of making part of this relationship a controlled variable. For instance, if you used a video camera to measure the distance traveled in some number of frames, then time would be a controlled variable and distance would be the dependent variable.

Yes, the surface type is your independent variable because you are changing the surface type in your experiment independent of what ever else happens.

Finding stuff to use around the house is great starting point. Until you try out some things to figure out how fast things are going happen and what distances you need to accomodate your various surface materials, you won't know what you can make work consistently and last long enough to be able measure them accurately enough to detect differences.

I believe that the marble will roll farther and faster on the inclined plane covered with aluminum foil than the uncovered and bubble wrap, carpet covered planes.

From a practical measurement perspective, your compound (farther and faster) hypothesis creates some problems. Are you only covering the inclined plane with the different surfaces? What happens after the marble leaves the inclined plane? The physics certainly undergoes a boundary change and conditions at the boundary become what is known as boundary conditions which results in some complicated math.

If you refine your hypothesis to only consider the time it takes to travel from the starting point to the boundary, you have a much simpler problem.

Forgot to address the acceleration question...

Acceleration is difficult to measure. If you use a video camera, you can determine how far the marble went in each frame. Unfortunately, you can't take the limit as the frame interval goes to zero so you are only going to get an approximation of the rate of change of velocity over time (e.g. approximatation of the average acceleration over the interval). For uniform surfaces like aluminum foil over some smooth surface, you can do some curve fitting and get a fairly accurate approximation; however, for your non-uniform surfaces (bubble wrap in particular), you are going to have upward and sideways accelerations as the marble rolls around and/or over the bubbles.

Objects rolling down an inclined plane is, in part, how Galileo determined the laws of motion.

One of the ways that Galileo, who had neither clocks nor video cameras, did his measurements of the speed at the end of the inclined plane, was to measure how far they went afterwards. For this to work, the inclined plane has to be nearly horizontal so you don't impart much velocity in the vertical direction. The way to do this might be to have a little horizontal lip at the bottom so the ball starts off with only horizontal motion when it comes off the plane. A sand trap below will let you know how far it went.

To clarify what Dr Weaver said....

I believe what he's describing is bleep like a ski-jump, except with the "jump" being flat rather than angling up again.

\_ <-- first a ramp, then a flat bit, and measure how far the marble shoots off to the right by having it land in a bed of sand.

This will work best if that ramp is curved a bit at the bottom, so the marble rolls smoothly and does not "bounce" off the bottom. Also as Dr Weaver said, the lower the incline the less chance that transition from "rolling down" to "rolling sideways" will absorb the energy or cause a random motion sideways because of a "bounce".

It will also work better if instead of a ramp you use a channel. Ie, bleep like a pipe cut in half, or a "V" shaped piece of wood instead of a flat one. That will keep the marble from rolling left or right, which can distort the results.

The reason measuring distance works to measure accelleration is because distance, velocity (how fast it moves relative to you), and accelleration are related. Unfortunately proving how they are related involves calculus, but the basic idea is:

Distance = velocity multiplied by time
velocity = accelleration multiplied by time

So if control the other variables (how long your ramp is, the degrees of incline, use the same marble and make sure that it is not moving sideways, bouncing or otherwise behaving unpredictably), you can measure distance and convert that to accelleration (and velocity, for that matter, if you are interested in how fast the ball shoots sideways).

To make the math more complex, it also matters how high your "ski jump" is above the sand, since how far the ball goes will depend on how long it takes to fall and hit the sand.

The math for all of this is bleep your instructor should be able to help with. It's high school physics, sometimes not taught till college. But if you get some help with the equations, you should be able to literally translate how far the marble travels directly into what you did to the accelleration of the marble on the surface of the ramp.

Assuming you get the math squared away, I would recommend against using cardboard. It is too flimsy and may buckle or otherwise change from experiment to experiment. If you go to a hardware store you can probably find a lot of interesting things to use as a ramp that will "channel" the marble. Bring your marble to the store. There is wood trim, there are metal angle-iron pieces, there are also plastic plumbing and molding pieces to look at. Some of these might be interesting surfaces all by themselves. If you pick a time when the store isn't busy you may be able to interest a sales attendant to help you and give you more ideas. (carpet samples are also likely easy to find, sometimes rubber surfacing samples too). What you are looking for is anything you can roll a marble down that won't cause the marble to stray sideways. bleep with an angle, or that is flexible enough that you can curve it a little, or has a channel molded into it for the marble to roll.

If you are using a variety of materials, Dr Weaver's suggestion about using a shallow incline is important, because 'rounding' the transition from downward to sideways will be difficult.

Hi salma,

if you provide “Does the type of surface of an inclined plane affect how fast and far an object rolls?â€