Science Buddies: "Ask an Expert"

I am very lost and struggling with my Physics experiment, so I would really appreciate any help my way!!

A can partially filled with water rolls down an inclined plane. Investigate its motion.
(What am I actually investigating!?)

For this experiment, I will be changing the masses of water in each trial to investigate whether mass water affects velocity of can:
*Independent: mass of water in can (g) using a scale (total mass of water in can MINUS mass of empty can)
*Dependent: Final velocity of can (m/s forwards) using a motion sensor

- My teacher also mentioned something to do with water oscillation/jiggling inside the can when rolling down the slope. Will this aspect/issue affect the results?

- ALSO, I am not sure if I would need to use an equation when processing my data into a table in excel? Will I need to calculate the momentum using p=mv? or acceleration? (which requires time, but I am not using a stopwatch)


Thank you SO much!!

Hello, MinnieMouseInTheHouse! This sounds like an exciting project. And challenging!

As for your question about what you are actually investigating, you will be investigating the physics of the system and how changing aspects of the system affects the behavior. In your case, the system is the can filled with water and the inclined plane, and you will be changing the amount of water/mass of the system. In physics terminology, this is called a "rolling without slipping" problem on an "inclined plane." The "without slipping" part means that you can assume there is enough friction between the cylinder and the inclined plane that the cylinder rolls uniformly down the plane.

The velocity of the can rolling down the inclined plane is affected by the mass and radius of the cylinder, the height of the starting point, the angle of the plane, the gravitational force, and the friction between the cylinder and the plane. As long as you use the same can and the same inclined plane, the coefficient of friction will be constant, so you can ignore the effect of friction in your experiment. Presumably, you will also hold the height and angle of the plane constant, so this effect will also be the same for each trial.

Because you will be changing the amount of water in the can, you will also be changing something called "moment of inertia." Moment of inertia (shown as the variable I) is sometimes also called "rotational inertia." The moment of inertia of an object describes how the object resists rotational acceleration down the ramp. There is a good overview of this concept at these websites:

http://www.batesville.k12.in.us/physics ... e_roll.htm.
http://hyperphysics.phy-astr.gsu.edu/hb ... l.html#hc1 (Concept map: http://hyperphysics.phy-astr.gsu.edu/hbase/inecon.html)

Khan Academy also has a section on rotational motion: https://www.khanacademy.org/science/hig ... r-momentum

I also found this really cool simulator that illustrates how mass, radius, incline angle, and the moment of inertia affect the kinetic energies of various round objects: https://www.geogebra.org/m/zzCuXxBn.

So... After diving into the physics, here are some articles about similar experiments that might give you some ideas for how to design your own experiments:

https://www.scientificamerican.com/arti ... ling-race/
https://iopscience.iop.org/article/10.1 ... /1/F15/pdf
https://www.researchgate.net/publicatio ... lined_Ramp
https://www.researchgate.net/publicatio ... xplanation

I know that is a lot to read, but those resources should be really helpful in answering your questions. Feel free to write back with additional questions as you review the background literature. And good luck!

Best regards,
Dr. Christina Payne

Thank you very much Dr. Christina Payne

This is really helpful information! Thank you very much, I really appreciate it!

I was also just wondering if I were to change the angle of elevation of the plane for each trial with the same mass, what are your thoughts of the possible trends that will happen? (Independent: angle, Dependent: acceleration)

Thank you!

Kind regards,
MinnieMouseInTheHouse

Happy to help, MinnieMouseInTheHouse!

As for changing the angle of the plane... This would need to be a separate set of experiments from those in which the mass is the independent variable, although you could also do a set of experiments where you change the angle of the plane for each of the mass values. For example, let's assume the mass of the full can is 300 grams (g), and you have chosen to determine rotational acceleration (aka moment of inertia) for mass = 0 g, 50 g, 100 g, 150 g, 200 g, 250 g, and 300 g. For a given angle (constant) of the inclined plane, you will have to collect at least 21 measurements (for each of the 7 masses * three trials minimum). If you want to also consider how the angle of the plane affects rotational acceleration, you will need to conduct that same set of 21 measurements for each angle value.

Regarding the expected trend in acceleration as a function of changing incline angle, well... This should be your hypothesis. Keep in mind that our hypotheses can be incorrect, and they frequently are. If we knew how a particular system behaved before conducting the experiment, we would not need to do the experiment. So start by thinking about what you would expect to happen based on what you know about the world around you. For example, pretend you are at a playground, and there are two slides of exactly the same material and length (the hypotenuse of your triangle). The only difference is that one is much taller than the other. If you were to place your can at the top of each slide, which do you expect will reach the bottom first? How does reaching the bottom first correspond to rotational acceleration? Now, consider if you were to fill each can with more or less water. What do you think that does to the rate the can rolls down the slide?

If you are still having trouble visualizing it, the simulator at GeoGebra (https://www.geogebra.org/m/zzCuXxBn) can help you think through the various scenarios. You can zoom out to see more of the inclined plane, and you can change the angle of the plane or the mass and radius of the cylinder to see what happens. The simulator shows both translational and rotational kinetic energy at the top right of the simulator, which is related to rotational acceleration.

Let me know if you have additional questions.

Best regards,
Dr. Payne