The Board

My edit time elapsed for my previous post so I will add this observation about handling stiff springs in the model here.

However we use a conventional spring in the SHO model, so I don't know how to handle what really happens for a stiff spring. The model says a stiff spring would produce oscillations in the brick velocity, but I do not think that would happen with a one-way spring such as a rubber band.

Those oscillations happen in a normal SHO since the mass has a hard connection to the brick. But since you cannot push a string, there would be no way to impart a retarding acceleration force on the brick in Aitor's experiment. However, with the optimum spring constant value the spring does not go through compression until after the maximum launch velocity it reached, so I do think it will give reasonable values for the weak spring constant values you would want to use to get the maximum marble launch velocity in this experiment.

Gordy
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I keep referring to the relative distance the brick has traveled when the lead hit the floor because that is the impression you gave, and when Lasse answered that:

The brick without spring has covered more table when the weight hits the ground...

You did not come back and say, "That is not the distance I am talking about. I mean the total distance they have traveled when they come to a stop on the table."

I can not understand how I gave that impression. As for you last paragraph above... it is exactly the opposite because I came back asking for the total distance (which is the one that matters).
Lasse wrote:

The brick without spring has covered more table when the weight hits the ground....

And then I wrote:

When both bricks stop, which one has covered more distance?

As you can easily check here:viewtopic.php?f=11&t=36&start=10

As far as the model is concerned the distance they would travel on the table would never end since there is no friction. So at least in terms of validating (or disputing Merlin's model) measuring a stopping distance does not make any sense.

Yes, it makes a lot of sense, because in the real world the marbles will eventually stop, and if the spring has transferred more energy to its marble this one will cover more distance than the other one. And we will see it without any further calculations of velocities, work or energy.
So I expect that, if the spring is a real mechanical advantage, the result of the experiment will be that the marble/spring has covered more distance over the surface it has been moving. If it doesn't go further it is because it didn't get more energy in the first place.

When both bricks stop, which one has covered more distance?

Aitor,
And you also wrote in response to Lasse's report that when the brick was pulled by the string it had traveled a longer distance than when accelerated by the spring at the point the lead mass hit the ground in his runs:

A simple answer to a simple question. Thanks Lasse.

That is why I thought you were talking about the relative distance the brick had traveled when the lead hit the floor. My mistake.

Even though there is no coming to a "stop" in the model, I agree that If you have a very long loading dock the marble may eventually come to a stop because of friction and/or bumps in the floor, but I think you will find it may roll for a very long time with an initial velocity of around 6 m/s.
Gordy

gordonjudd wrote:

When both bricks stop, which one has covered more distance?

Aitor,
Even though there is no coming to a "stop" in the model, I agree that If you have a very long loading dock the marble may eventually come to a stop because of friction and/or bumps in the floor, but I think you will find it may roll for a very long time with an initial velocity of around 6 m/s.
Gordy

Yes, maybe the experiment isn't that easy to implement. The floor is around 7 m long and the available height to the floor is around 2 m (I don't know for sure, I have never measured it). I think that I can play with the height for the marble to stop before the floor ends because there is no space in front for the marble to fly.
I will make some tests but I must wait till next saturday for that. In the meantime I will buy a couple of marbles and will find a suitable weight.

The floor is around 7 m long

Aitor,
I think you will find that is much too short for the marble to stop rolling.

Therefore it will be better to use Tracker to measure the velocity of the marble at the point it has traveled the same distance as the lead mass falls. With a high speed video I would think you will get a very accurate velocity measurement that way, especially if the camera is positioned close to that 2m (or whatever your real drop distance is) so you have a big pixel/meter ratio in your video. Putting a meter stick along that path can also help to get rid of any camera aspect distortions as well if you really want to get accurate distance vs time values.

I would be glad to make some velocity vs time plots with Matlab as well if you send the video to our Dropbox folder.

Gordy

I think you will find that is much too short for the marble to stop rolling.

you're gonna need to get a bigger house, Amigo....

gordonjudd wrote:

The floor is around 7 m long

Aitor,
I think you will find that is much too short for the marble to stop rolling

Adjusting the height of the hanging weight will solve the problem.

Adjusting the height of the hanging weight will solve the problem.

Aitor,
A small amount of input energy may magnify the relative impact of friction effects on the string and impact the rubber band stiffness required to get an optimum effect, so it might "muddy" your results a bit. But I think you will still see an advantage for the spring.
Gordy

In the experiment I tried, the brick on the string had the lead until the weight hit the floor,

Lasse,
In your implementation of the experiment did you have both masses tied to the lead mass at the same time and thus get your comparison from a single drop? That is what it appears to be in your video.

If that was the case then your implementation of Aitor's experiment would produce a different result than if you dropped them separately. In your case the amount of energy dissipated when the lead mass hit the ground would be the same. If you did it separately the velocity of the lead mass when it hit the ground would be less with the spring connection than it was with the string. Thus the advantage you would get for the spring would be less using a single drop than it would be if you had done separate runs.

Gordy

Hi Godon

Seperate weights and seperate bricks. Getting the weights to drop at the same time is difficult, that's why I used a book to suspend the weights until dropping them.

Cheers
Lasse