# Homework Help: I have to make a ramp, eek

1. Sep 12, 2006

### Crystal Faye

I have to make a ramp, eek!!

Here is my assignment. I would appreciate help with choosing what formula to use. I am so lost.

Design a Ramp:

The ramp can be made of any material and be any width, length, and/or height. The only rule is that there must be a ball of any material that will consistently roll from a noted point A to a noted point B on this ramp.

The ball much be rolling from point A to point B in 1.8 consistently over a one meter stretch. [ It is also an option to create a senario in which the ball with take .9 seconds to travel over a .5 m ]

2. Sep 13, 2006

### andrevdh

The linear acceleration of an object rolling down an incline, without slipping, with an angle of $\theta$ with respect to the horizontal is given by

$$a = \frac{g\sin(\theta)}{1 + A}$$

where

$$A = \frac{I_{com}}{MR^2}$$

where $I_{com}$ is the rotational inertia of the rolling object around its centre of mass.

3. Sep 13, 2006

### Crystal Faye

Thanks for the help :)

What ball and ramp material do you recommend?

And also, if the ball must travel 1 meter in 1.8 seconds [or .5 meters in .9 seconds], should we get the ball to accelerate BEFORE the "measured meter".

We are only in our first few weeks of physics.

4. Sep 13, 2006

### andrevdh

The ball need to be of a uniform material (not hollow) because you need to evaluate $I_{com}$. It should be rather hard, but it must not slip on the incline while going down.

5. Sep 13, 2006

### Crystal Faye

Do you think that a marble will work? We are considering creating the ramp (the surface) out of lamenated card stock.

We also have bouncy balls at our disposal :)

I am really happy that you are helping, yay!

6. Sep 14, 2006

### andrevdh

I would rather go for a ball that has less chance of slipping, like a superball. If the ball is of such dimensions and mass that the angle becomes too steep the chances of slipping becomes great. So calculate $\theta$ for the particular ball (working backwards from the given time to the acceleration to the angle) and then you will have an idea if the angle is such that the ball will probably slip.

7. Sep 14, 2006

### Crystal Faye

ok! I am going to try and draw a diagram...

8. Sep 14, 2006

### andrevdh

I am not sure why you need a diagram. But I wo'nt so no to nice pictures.

Last edited: Sep 14, 2006
9. Sep 14, 2006

### Crystal Faye

Are we identifying theta correctly?

10. Sep 14, 2006

### andrevdh

That is the danger one faces when one have'nt derived a formula oneself: misinterpreting the meaning of it. $\theta$ is the angle that the ramp makes with the horizontal. So it is the bottom right hand angle of the triangle in your drawing. You can see its meaning by multiplying the formula with the mass of the ball on the left and right. The numerator then becomes the component of the weight of the ball parallel to the ramp.

Last edited: Sep 14, 2006
11. Sep 15, 2006

### andrevdh

The $g$ in the formula is the value of the gravitational acceleration. Its value changes a bit with lattitude. Where are you on this planet?

12. Sep 15, 2006

### Crystal Faye

I live in America, in the south...near the Gulf of Mexico.

13. Sep 15, 2006

### Crystal Faye

Oh, I understand what you meant by sine now. I had looked it up on the internet and I was misled.

14. Sep 15, 2006

### andrevdh

I am not at work now so I don't have the information at hand to look what your g will be at appoximately 30 degrees latitude. Will post it on Monday. When is the project due and how are you going to check the time?

I guess that is now about one o'clock in the afternoon there by you.

Last edited: Sep 15, 2006
15. Sep 15, 2006

### Crystal Faye

Yes, it is 1:54 as I am writing this.

My project is due on Friday.

What do you mean by how will I check the time?

16. Sep 15, 2006

### andrevdh

That it comes to about 1.8 seconds. It is too fast to measure accurately with a stopwatch.

Last edited: Sep 15, 2006
17. Sep 15, 2006

### Crystal Faye

Whoa. It is part of my project to make the ball go 1 meter in 1.8 seconds.

Are you saying that my teacher is asking us to be too accurate?

18. Sep 15, 2006

### Crystal Faye

oh. I think I understand what you are saying. We may possibly measure the ball's time with an electrical timer.

19. Sep 15, 2006

### andrevdh

Sorry, I am a scientist and are use to strive for perfection. If it takes 1.7 or 1.9 seconds I would consider it not good enough. So I was wondering how your teacher was going to evaluate your design, maybe by using photogates? Just by changing the angle of the ramp you could get to a better result.

Do the rest of the class have different projects? Do you work in groups? I myself work with students in the Physics pratical lab all day so I find this project rather interesting.

20. Sep 15, 2006

### Crystal Faye

My teacher told us that it had to be exactly 1.8 seconds. I am really hoping that he does not rely solely on a timer. I can see how that would cause problems.

I find this project rather interesting as well; however it is also extremely challenging for me. I am a senior in high school and so far we have only learned the scientific method and a little about velocity. We are pretty much left to figure this out on our own.

I am in a group with three other people, and the entire class was assigned the same project.

21. Sep 16, 2006

### andrevdh

I was toying with the idea of using some sort of lever system to start and stop a stopwatch so that you could record the time more accurately. Envisage a holder for the stopwatch (fixed to the incline) at the top so that when the ball is released it starts the stopwatch (normally you can hold the button down and it only starts when the button is released). The button rest against the side of the holder and a pin runs through the side wall of the holder. The ball opersates this pin at the release point as I explained. At the bottom is a buffer at the 1 meter mark that connects via a stiff piece of wire to the rear of the stopwatch. This wire runs back up the bottom of the incline and pushes on the other side of the stopwatch to stop it. Sounds good in theory but in practice... Maybe you can design something similar along these lines to operate the stopwatch mechanically.

If you feel even more adventurous you can open up the stopwatch and check whether it is possible to connect two wires to the two contacts that the button short out (crocodile clips, paper clips). This will enable you to duplicate a button press by shorting the ends of the two wires.

Anyway, if you need more help feel free to ask.

Last edited: Sep 16, 2006
22. Sep 16, 2006

### Crystal Faye

I love your idea, it sounds so fun!

I will let you know once I actually figure out the equation [yes, I am still working on it,haha] Then I can move on to the timing aspect.

23. Sep 17, 2006

### andrevdh

So more ideas for your project.

The stopwatch is operated by a lever at the starting position.

The idea behind the threaded bolt is to adjust the angle to obtain the correct time.

What problems dou you have with calculating the angle of the incline?

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24. Sep 17, 2006

### Crystal Faye

I like the bolt idea, my little brother had mentioned something like that to me. I believe that he is going to help me in the actual construction of the ramp, along with my other group members.

Where did you find that image, or did you draw it up?

25. Sep 18, 2006

### andrevdh

I drew it with CorelDraw.

If you operate the stopwatch via some interaction with the ball just check that it does not deliver a substancial force to the ball. Although I think that the rules of the design did not specify that the ball should start out of rest!

You should also check that the surface that the ramp is resting on is level. Maybe by pouring some warm water with a bit of detergent on it. If this will get you into trouble you should maybe put some plastic on it first.

Show us your acceleration calculation thus far so that we can help you. The best appoach is to use the calculated angle just as an initial setting. It is best to then set final adjustment via a calibration curve.

Last edited: Sep 18, 2006
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