Science Buddies: "Ask an Expert"

Dear Science Buddies, I need help. I have never done this kind of thing before. Can you give me some help or ideas of where I would start in proving weight (not body weight) will effect speed on a skate board. I am a bit overwhelmed and don't know exactly how to prove such a theory. Thank you for your help.

Please help me. I ask a question and have not heard back. How long does it usually take? I am needing help on my science project. I have never done this before. I need help on how to start. I need some information on how weights effects the speed of a skateboard? Thank you for your help.

Hi esis06,

Welcome to the Ask an Expert forums! We're happy to help you. I'll respond here, and then make a post to the other topic you started. We ask that you keep all of your questions about your project on a single topic thread--it helps use Experts help you better.

The first step is to decide what scientific question you will test. It sounds like you already have an idea of what you want to do, which is great. Try turning your idea (how weight affects the speed of a skateboard) into a question. Next, write down your hypothesis, which is an educated guess of what your experiment will show. A hypothesis is often written as an "If . . . then . . . " statement. Once you have your question and hypothesis, you will need to do some background research about skateboards and forces. After that, you will design your experiment.

Science Buddies has a great Project Guide (see link below) with great info about the various "chunks" of a science fair project (question, hypothesis, background research, materials and procedure, etc). The Guide explains what to do, when to do things, and how to do things. Check it out. I think you'll find it very useful.

https://www.sciencebuddies.org/science- ... ndex.shtml

Try writing out your question and hypothesis. You can use the Project Guide to help you (look at the pages about questions and the hypothesis). Also, do some background research about skateboards, wheels, and friction. Friction will be a very important scientific word for your project. Friction is a force that makes it more difficult to move objects past one another.

Post back with your question and hypothesis, and we'll help you with the next steps of your project! It sounds like you have a very interesting idea!

P. S. Experts usually sign in to the Forums once a day; they don't work like a chat room. As a result, most posts are responded to within 24 hours, but not right away.

In addition to the topics above, you might want to look into how gravity affects things.

The science behind how a skateboard moves down a ramp is fairly complicated, as are the things that can happen (it might move, it might not move, it might move only with a gentle push to get it started, and might or might not stop moving before it reaches the bottom).

Friction's very complex for 5th grade, but the basic forces in play are:

1. Gravity - the only reason it wants to go down the ramp. This force is always straight down, but the ramp will prevent the skateboard from going straight down, putting other forces in play.
2. friction between axle and wheel - resists any motion but on a skateboard, they have complex ball-bearing wheels that give near zero friction of this sort.
3. friction between wheel and surface (a very slick surface would have it slide down, not roll down, and sliding resists motion better than rolling)

Adding weight increases the force of gravity on the skateboard, improving #1, and also for reasons really too complex for 5th grade science, also helps with #3, helping the wheel grip the surface better (a heavy weight over an axle makes the wheel less likely to skid, and more likely to just turn). Adding weight also puts more strain on the axle and increases whatever friction is there too, #2, but the wheels on a skateboard are so well designed that this is near zero...generally weight on a skateboard improves its ability to travel on a surface (it is after all designed for a human to stand on it, a mass much, much greater than the weight of the board).

I do not think you will have difficulty setting up a basic experiment ("I predict that more mass will increase speed on an incline") is something fairly straightforward to set up, as long as your incline is not so slick that the wheels skid instead of rolling and you have come up with a reliable way to measure speed.

Understanding the math behind the result you are seeing, doing the background research and applying it to the experiment may be quite challenging at your grade level. The reason wheels work well is surprisingly complex....they let you have the "grip" strength of static friction while not having motion resisted by the static friction. A ball-bearing skateboard wheel is really wheels within wheels, using the ball bearings as little "wheels" to prevent the axle from adding friction to resist motion, and also the larger wheel surface to let it roll instead of skid. If you can find a good article on skateboard design focusing on wheels that can explain it without the math that would be a good place to start, to explain why you don't need to worry about friction much on the axle or wheel as long as you have enough that nothing "skids".

Dear Science Buddy, Thank you so much for your e-mail. It has made me feel like I can do this hard project. I want you to know that I have done my research on what I need to do for my Science Fair Project and what is required. Right now I guess I need your help with what my next step is? The Research Paper? I have already started looking up on the internet information regarding speed, weight, and acceleration. I have not found a lot that helps on my project. I have found different things regarding speed and weight. I have already set up a basic experiment as far as making a ramp with a slight incline and putting different weights on my skateboard and timing each weight. I had to try two different skateboards. The first one, probably because of the wheels, never made it totally down the ramp. It would go off to the side. The second one worked much better and would go straight down, so I was able to least time the speed of the skateboard with different weights. My question is what information do I need do prove or disprove on my Research Paper regarding my Hypothesis, "The heavier the weight on the skateboard will cause it to go faster because of the natural force of gravity". What facts am I looking for on my Research Paper? You say, "Understanding the math", what math would I need? I do plan on putting several different weights and timing each one. I do agree with you that some of this is very hard, so if you could help with what information do I need to start looking for and putting on my Research Paper.Thank you so much for your help. My project is due December 7, 2014.

"The heavier the weight on the skateboard will cause it to go faster because of the natural force of gravity" is not very precise. Precision is important even when working at a high level, without worrying about all of the complexities of friction.

You should be able to find something in your research that says "if mass is greater, gravitational pull is greater".
That lets you then pose the question "Does increasing the mass on the skateboard make it go faster down a ramp"
and your hypothesis is "Because greater mass increased gravitational pull, that increased force should make the skateboard go faster."

The research paper is where you explain how you arrived at those conclusions before you start testing. You need to start with something you know about gravity and mass, and then apply it to your question, and then come up with a prediction.

Your early experiments with testing show that even something this simple in concept can be challenging to measure. Your two skateboards did not behave the same way going down the ramp (wheels might be out of alignment on one, causing it to turn, or your surface might not be very even, causing random changes of direction). Eliminating problems like that is part of your experimental procedure. This can be handled several ways - it looks like one of your skateboards reliably goes straight, so attaching weights to it is likely easier than using two different skateboards of different weight (also this means the axles, wheels, all the other things that might vary speed are the same, which is good from an experimental process standpoint - when you don't understand complex forces, you can eliminate them from your experiment by making everything affected by those forces the same. I strongly recommend using the same skateboard every time)

Part of your plan is to explain all the choices you made in setting up the experiment (eg...it might matter where you put the weights on the skateboard. No matter how you approach the problem, you want to do it the same way every time, no matter how much weight you are adding, and explain how so someone else could duplicate your experiment later).

The math for gravitational forces on a mass and friction is both done properly with Calculus (senior HS or college math), but you can be a bit less rigorous and use just plain algebra and geometry (physics is often taught in HS without quite enough math to really do it right, but it still needs freshman and sophomore HS math - well beyond what is taught in 4-6th grade). At your level, instead of trying to understand the math, what you want to focus on is looking up how gravity and friction work, and just look at the conclusions, trusting that the person who wrote the article/textbook/whatever did the math correctly.

Then focus on what you can actually do. You can measure weight, you can measure how much time it takes to get down the ramp, you can measure the angle of the ramp (if you have the right tool, have your math/science teacher help). You can learn if it goes faster or not without understanding the math. Often in real life this is how you learn the math - you measure things that are happening and later come up with math that explains the result - then prove your math is correct by doing a new experiment, predicting the outcome with your new equations.

Don't let the math intimidate you. Just try to learn something about how wheeled objects roll down hills in your research and try to make a prediction based on what you learned, then try it out and measure the results. You learn just as much by NOT having your prediction come true as you do by having it come true, so don't worry about the experiment proving or disproving your theory. Just do it to the best of your ability, make it as accurate as possible and then it will either show that you grasped enough of these complex concepts to make a valid prediction, or it will show there was a nuance you missed and from the failure you'll learn even more.

(in real life, people got this wrong for thousands of years. The authorities made some predictions based on how they imagined the world worked, without testing it, and were so respected everybody assumed they were right. To be fair, they WERE right on a lot of topics, but often wrong about why things behaved the way they observed, so drew conclusions about things they had not observed that were not correct. The entire scientific method was designed to take common sense and test it, and most of the really interesting things are when the universe doesn't work the way common sense would expect.)

Dear Science Buddy, Thank you so very much for helping me. This project is way above me.You say my Hypothesis is not very precise. I had already turned my hypothesis into my teacher? Is it ok for me to change my hypothesis and bring in "Mass"? Should I then go to the library and research words such as gravity, motion, friction? Do I look up "Mass"? Any other words that would help me in my research for my hypothesis of "Because greater mass increased gravitational pull, that increased force should make the skateboard go faster." Sorry that I need so much help but I have never been involved in science like this. Thank you again for your help.

I would say to talk to your teacher about my response. I suspect your science teacher would have an easier time knowing what is appropriate to your current math and science level than I can. It could be your question is fine, and I'm making it overly complex for what you are trying to do.

Look for a good definition of gravity, mass, friction and also a good description of how skateboard wheels are made and why they are made that way (which will likely introduce some of the other concepts). You might want to look up what "rolling" and "skidding" combined with "friction" means.


====this following is just if you are curious about mass vs weight.

Mass is something every object (or person) has. It depends only on the object itself, not anything around it.

Weight is the combination of gravity and mass. Weight is zero if you are in outer space, but you still have mass. On a planet with less gravity, you would weigh less but have the same mass. (on the moon, a scale would measure you at 1/6 what it would measure if you are on the earth).

People doing physics tend to use mass rather than weight in most situations except where they explicitly want to include the strength of gravity as a variable. Most people use the two terms as the same thing, because the only place they care about either weight or mass is on earth.

Dear Science Buddy, Thank you again for all your help. I have been working on the suggestions that you have given me regarding key words and also on how I will do my experiment. I have two question for you. The first one is what kind of basic graph could I draw to show my results? I am putting different weights and my timing as my skateboard goes down my ramp that I have made. My second question to you at this point is how do I start writing my research paper. What is the first thing that should be on my Research Paper? I understand that I will explain my experiment but how and where do I start on this research paper? I know for my display board I will have my Hypothesis, "Stating the Results and Stating the Conclusion. But how do I start my research paper? Do I start off with stating the facts I have found such as how friction, motion, gravity and force all relate to my project? Do I state my Hypothesis in my research paper? Is my research paper the combination of everything I have done relating to this project. I just am kind of confused as to what and how I start this research paper. It seems like I will be repeating a lot of information? Thank you for all the help you have given me because it has helped me to feel like I can continue on and do a good job on my project.

Science Buddies has learning resources for most of your questions

Writing a Research Paper
Data Analysis and Graphs
(and also this section, which lists the above plus other related project oriented topics: Science Fair Project Guide

The research paper is something you do before experimenting, documenting what you learned about the topics of your experiment. So it is where you describe what you have learned about gravity, friction etc and why or why not you think each element will affect your experiment. Your experimental results do not belong there, rather you refer to the research paper (and bibliography to give credit to sources of the paper) in your experiment to explain your results.

A graph is a way to visually represent your results. People understand what they see better than numbers on a page. The link gives advice on how to orient your graph in a way standard to science fair projects that have a single dependent and independent variable.

Dear Science Buddy, I am sorry but it is me again. I want to thank you for all your help. My science project is almost due but I am still struggling with my Research Paper. I know that you said the research paper should be done before I start my experiment. I have done research on all those elements that I think effect my project. I have already done my basic experiment with testing different weights on my skateboard and plan to do more. I don't know if I am making this harder on me but the more I work on my Research Paper the more confused I become. My research is on motion, friction, gravity, force and speed. Could you please help me to understand in simple terms why each of these does effect how fast my skateboard goes with the different weights on it. When I start to write, it becomes very hard and then I feel like I am so confused and get each one mixed up with the other. I am not that good at science. I am sorry that this has been so hard for me but please could you help me understand a little bit better why, motion, friction, gravity, and force are factors in my experiment. Thank you very much.

Maybe some definitions will help.

Distance: How far the skateboard travels (feet, inches, centimeters - that kind of thing)

Duration: How long it takes the skateboard to travel a given distance (probably in seconds)

Speed: Distance/Duration (distance divided by duration) measured in something like feet/second. This is unfortunately a complex concept, as really your skateboard goes faster as it moves down the ramp, only settling on a given "speed" once it reaches the bottom and gravity no longer acts on it. For the purpose of your experiment, it's "close enough" to call it the distance the skateboard moves divided by the time it takes to move that distnace.

Acceleration: Gravity is what causes the skateboard to move, and how fast it goes from zero is based on how steep your ramp is. As long as you use the same ramp for all tests (and do all your tests on Earth, not on the moon or in outer space), the complexities here all cancel out. The physics behind how an object moves is expressed as "Force divided by Mass = Acceleration" with Gravity = the Force and your skateboard weight = the Mass (again the complexities caused by weight all cancel out if you do all your experiments on Earth, so this is close enough at your grade level). In the end, how much acceleration the skateboard experiences is strictly based on your ramp angle, except for Friction.

Friction: When something is in motion, it will not slow down unless there is Friction. Friction is a catch-all term for anything that slows motion - it is why a feather falls slower than a pebble ("air resistance"), it is why you slide further on ice than on concrete ("dynamic friction coefficient"). In the case of a wheeled vehicle like a skateboard this is extremely complex, but the main thing to consider is that most of the friction is tied with the wheel turning on the axle. If you have good wheels (properly packed with ball bearings) a skateboard will travel further under the same force than one with poor wheels. All of these forces will pretty much cancel out if you use the same skateboard for all tests. If you use a different skateboard, some of what you see will be based on the difference in wheel/axle friction and perhaps air resistance due to different shape of the skateboard.

The most important thing in the research paper is to focus on why the way you set up your experiment accounts for each of these things, and eliminates them as a variable, everything but weight.

Most importantly:

1. Same ramp for all tests eliminates changes to the force Gravity exerts, and any friction effects caused by interaction of ramp and wheels.
2. Same skateboard for all tests minimizes impacts of air resistance and friction between axle and wheel. (adding the weight does have an effect on both, but it should be very small compared to using different skateboards)

Your theory is that adding weight should increase acceleration caused by gravity, and therefore the skateboard should be moving at a higher speed at the bottom of the ramp.
What your experiment shows....it shows. Which might or might not match your theory. If it shows something different, maybe your research paper will help understand what you see (perhaps something you thought was small turned out to be important, or something you thought cancelled out was affected by the increased weight).

Dear Science Buddy, Thank you again for all your help. My project is almost due and I am still working away at it. Thank you for all your help. I have done my testing and found that with each weight I added it did increase the speed. I do see now maybe my testing is not so accurate because of my ramp being only 16 feet long. I am testing all the different weights with the same ramp and same surface, not changing anything. I have researched all the key words such as Speed, friction, acceleration, force and gravity. How do I bring everything together now in my research paper? I have identify all those words in my paper but how do I finnish it off. What exact information do I need to put on my research paper? In my paper is this where I state the steps of of experiment or that goes on my result sheet. Sorry, that I have asked you so many questions. Thanks

The way you tie everything together is to try to relate what you saw in your experiment to the research you did before the experiment.

For example, you detail all the things you did to try to make the weight the only variable that mattered (you did the experiment in the same physical location (on earth, sea level), you used the same ramp, the same skateboard etc - all to minimize considerations like friction, air resistance and variations in gravitic force).

Then you describe how you are measuring speed, and how you attach the weight (this is important if anyone else tries to duplicate your research)

Then you show the different weights applied, and speeds measured as experimental results (this is the graph we touched on earlier)

Then you try to explain what you see.

If the speeds went up with weight, but not very much, the results might be either experimental error (you could prove this by running more tests the same way - if after multiple trials you always see faster speeds with more weight, it isn't experimental error, something is really there. If you see sometimes more, sometimes less, you are seeing only "noise" in your results, small changes caused by differences between trials), or it could be that more weight helps, but only with a smaller effect (doubling the weight increasing speed only a little probably means the weight helps overcome friction but doesn't increase how fast gravity accelerates the skateboard)

Or with less words - if you saw that more weight helps, what do you think it helps most with in the various effects you learned about that modify the motion of a skateboard. This can be different than what you expected before you ran the experiment - most real science is like that. The most interesting things are often unexpected outcomes - success in predicting means you already know something and are just confirming it. Predicting one thing and getting something different teaches you something new about how the universe works...or at least you realize there is something you don't know and need to consider.

Dear Science Buddy, I want to thank you for all the help you have given me. The time for my project to be done is almost here and because of you I think I might be ready. I am getting ready for my Oral Presentation. I am going to speak of those elements which I believe made my skateboard go down the ramp:speed, friction, mass, force, gravity, and acceleration. The one part I am having the hardest part in presenting is "gravity". I am confusing myself when I tried to explain. Please help me help myself to understand "gravity". Could you give me some ideas that will help me in my oral presentation regarding how gravity had the biggest effect on my project. If you could respond soon I would be grateful. I am running out of time. Thank you.

Ok, a quick physics lesson to try to help you understand Gravity better.

First Law of Motion - an object will want to stay at the same speed if nothing else acts on it. (if not moving, it won't move. If it is moving, it won't speed up or slow down)

For your skateboard, this means something must be acting on it to make it want to roll down the hill. Something else must be acting to make it stop rolling eventually.

Gravity is what makes it want to roll down the hill. Friction is what makes it eventually stop once it is on a level surface.

Both Gravity and Friction have limits.

Friction is a force that can only slow motion, it does not ever speed it up. So it can not cause the skateboard to move, but it can cause it to stop.

Gravity only works in the "up and down" direction ("perpendicular to the surface of the earth" is the technical term) It is much stronger than friction forces when a skateboard is on a ramp, so the skateboard goes from not moving to moving.

When the skateboard hits level ground, Gravity no longer matters (except to keep the wheels touching the ground). Friction (wheels to axle, and air friction a little bit) now is the most important force, and will eventually cause the skateboard to stop moving.

Here's one more thing. The force gravity exerts rises with mass, but so does the resistance to motion (a feather falls just as fast as a brick if there is no air to resist the motion). Friction, by contrast, has a complicated relationship with mass that, in a skateboard, will usually result in better "grip" on the surface with the wheels and reduces the impact of air resistance. I was not sure when you did the experiment if it would show a difference, it depended on if the friction effects were large enough to measure. It appears that they were, which to me is pretty cool. Usually I know what will happen in most science fair experiments if performed properly. It is fun to get one where I know the theory, but can't predict what will happen (it's been 30 years since I could do the math on something as complicated as the frictional effects of a skateboard's wheels and air resistance, and I've never had to do a comparable experiment to be able to guess at the outcome)