Science Buddies: "Ask an Expert"

I am doing a science fair project to see if weight affects the speed you go down a zipline. In my research so far, it seems like friction will have a lot to do with the speed I go down a zipline. If there is no friction is there a formula that tells how fast I would go down the zipline? Does weight affect the amount of friction? Does weight affect friction or velocity more? Does acceleration increase with greater weight?

Also, are there other factors that I should be considering that have a significant impact on the speed I go down a zipline?

Thanks for your help.

Fascinating idea for a project! I assume (I hope!) that in conducting your experiments, you will not be sending humans down a real zipline, and instead will be using a miniature zipline and sending objects of various weights down it.

It seems to me that most of the questions you asked in your post would be great questions for you to investigate yourself as you do your science project. Obviously you should do some research beforehand to learn about ziplining in general, but you don't want to necessarily find out all the answers to all your questions through research, otherwise you won't have anything left to investigate. So I would suggest you do a bit of research, and then figure out which question(s) you're going to investigate, and start setting up the experiments to gather the data. It sounds like a lot of fun.

Just some thoughts on various aspects of your experiment ...

* After looking at the ziplining article on Wikipedia, it seems that most ziplines are constructed using a pulley that essentially rolls along the top side of the rope (as opposed to something that drags along the rope creating a lot of friction). So, if you're able to construct a similar pulley system, then I think the effects of friction will be less (still not zero of course).

* "If there is no friction, is there a formula that tells how fast I would go down the zipline?". Yes, there is, but it's complicated to the point that you would need to calculus to derive the formula. This is because not only does the speed vary as you travel down the zipline, but also the acceleration will vary as you travel down the zipline. The acceleration will vary because the angle of the rope will continually change as the object moves from one end to the other. At any given point along the zipline (let's just take the center of the zipline as an example), you could figure out the acceleration at that point using basic formulas from motion physics. First figure out the angle that is made by the remainder of the rope when an object is hanging from that point. So, in this example, if an object is hanging from the center point, then the rope segment that stretches from the center point to the end point (destination) will approximate a straight line, and you should be able to compute the angle of that line segment using some basic trigonometry. To start, you'll need to know the length of the rope, the distance between the endpoints, and the height difference between the endpoints. Once you find the angle that the remainder of the rope makes, the acceleration at that point in (m/s^2) will be 9.8 * sin(angle). I'm not sure what grade you're in and if you've had a basic physics class yet. But if you need more help with this formula, ask again and I'll try to give a more detailed answer. Remember that this is all assuming you have a frictionless pulley system, which is of course impossible. To take friction into account, the formula would get even more complicated.

* Does weight affect the amount of friction? Does weight affect friction or velocity more? Does acceleration increase with greater weight? I could tell you the answers to these questions based on what physics tells us, but I think it would be more interesting for you to investigate these as part of your project.

* Also, are there other factors that I should be considering that have a significant impact on the speed I go down a zipline? Sure, there are lots of factors that might impact the speed! 1.The height difference between the beginning point and the end point. 2. The length of the rope. 3. The distance between the endpoints. 4. The type or thickness of the rope (because that would affect the friction).

Good luck!

--Rajeev

This is a great project. I think you will enjoy it a lot!

Rajeev did a great job giving you things to think about. I agree with him, your experimentation will give you a lot of the answers you are seeking.

I did think of a few other things you might consider. The material you use for the zip line, combined with the tension on the line, the relative heights of the end points, and the weight of the mass that is being "zipped" will determine the shape of the line as the mass moves along it. This will affect the speed of the mass along the line.

If your zip line works really well, that is if you can get up some good speed, the aerodynamics of the mass will play a role too. At high enough speeds wind resistance will affect speed.

Have a great time with this project.

Ed Neu
Buffalo, MN

Consider a small coin and a large boulder. If you drop them from the building, will they land at approximately the same time?
What if done in a vacuum with no air (and therefore no drag)? At exactly the same time?
What about a feather and a boulder? Again, what if done in a vacuum?


The most important equation you should know is the drag force equation:
F = (1/2) (ρ) (u^2) (CD) (A)

F is the force of drag, which is by definition the force component in the direction of the flow velocity
ρ is the mass density of the fluid,
u is the velocity of the object relative to the fluid,
A is the reference area, and
CD is the drag coefficient — a dimensionless constant

I'm not sure if you've taken a physics course yet, but you'll learn how Force (Newtons) = mass (kg) x acceleration (m/s/s)
So with a coin, large boulder, and feather, the force will most definitely change because of different masses. But you'll need to think about acceleration and whether it is variable or constant...