Science Buddies: "Ask an Expert"

Hello, I'm doing my science fair project on the "Roller Coaster Marbles: Does kinetic energy ever equal or exceed gravitational potential energy?" I have a couple of questions about the procedure: https://www.sciencebuddies.org/science- ... M&from=TSW.

In part 5a) it states that: "In order to measure the velocity of the marble, you'll need a way to measure how much distance the marble travels during a measured time interval. A good way to do this is to interrupt the foam track and direct the marble along a smooth, level surface (e.g., two long pieces of Masonite glued in a V-shape). Support the Masonite V (with cardboard, beanbags, etc.) so that it is level with the end of the foam track."

By "interrupt" does it mean to put the Masonite on the foam track or is the Masonite supposed to be placed at the end of the foam track?
If I were to put the Masonite at several points on the track to measure the velocity of the marble let's say as it is going down the first slope, would it still work?

I was also wondering what the independent variable in this experiment is? Would the independent variable be for me to change to HEIGHT of the track? I was wondering if the experiment would still work that way. Or would the independent variable be the LENGTH of the track, as in I make a shorter roller coaster and then keep adding more foam pipe?

And my last question is, in the procedure #12 says: "Does the marble's kinetic energy ever equal or exceed its initial gravitational potential energy?"
Does this question refer to the marble's gravitational potential energy at the start vs. the marble's velocity after the marble travels the whole rollercoaster? Or does it refer to comparing AT different points along the track?

I ask this question because at http://adventure.howstuffworks.com/roller-coaster3.htm it shows an animation where it compares the rollercoaster's potential and kinetic energy at certain moments. Is that what I'm supposed to do in this project? If so, how does the Masonite in a "V" shape/measuring stick help me?

Thank you for your help!

The masonite V has to be level, because this is a constant velocity section. The foam is slightly depressed by the marble, so it slows the marble slightly. The idea is to join the masonite to the foam so that the ball track on the foam is the same level as the ball track on the masonite. adjust the heights until you can roll the ball back and forth without feeling a lump. What this experiment is supposed to demonstrate is that kinetic energy and gravitational potential energy are always conserved, but there are frictional losses that have to be taken into consideration. In other words, if the balll starts off 5 ft above the floor it can never climb higher than 5 ft.

Thank you for your response Paulsdecarli!

So that means the Masonite has to continue the foam track, as in it is joined at the end right?

And, I was also wondering what the independent variable in this experiment is? Would the independent variable be for me to change to HEIGHT of the track? I was wondering if the experiment would still work that way. Or would the independent variable be the LENGTH of the track, as in I make a shorter roller coaster and then keep adding more foam pipe? Can those be used as independent variables that I can change?

Hi,

I think the independent variable would be changes in the HEIGHT of the track. In the conservation of energy, converting all the potential to kinetic looks like this:

mgh = (1/2) m v^2

m = mass
g = gravity
h = height above ground
v = velocity

This means that v = sqrt (2gh)
Mass has no effect, but HEIGHT does.

However, seeing this is in fact an experiment, there's going to be some deviation from the theoretical model (friction, air resistance, etc.)