Spanish experiment

[gordonjudd]

the result of the experiment can show a longer distance for the spring or a longer distance for the string.

Aitor,
I do not understand why you keep revering to relative distances, especially since the distance force is being applied to the brick in you experiment is the same whether or not it is being pulled by the string or via the spring. Can you see from this force over distance plot that the force applied with the spring is different than it is with the spring although the distance those forces are applied is the same as the dropping distance of the lead mass in Lasse's experiment with a 2.4 N/m spring?

Remember time is running from right to left in this plot, so the break in the slope of spring force marks the point where the lead mass hit the ground and thus the acceleration on that end of the spring went to zero. The deflection of the spring was around .12 m when the lead mass hit the ground in that simulation, a distance that would be easy to see in a video.

Since the area under the green curve for the spring is larger than the red area for the string, the work energy applied to the brick will be larger with the spring and thus it will produce a larger launch velocity.

I think you need to focus on the relative launch velocities, not the relative distance the brick has traveled when the lead mass hits the ground. Why do you think the relative distance at the point the lead mass hits the ground is important? Is it because that is something you can see in comparative videos?

What I would suggest is that you measure the velocity of the brick as it passes a point that is equal to the dropping distance of the lead mass from its zero velocity starting point. Tom's timing lights would be a neat way to do it, but you could also use Tracker to analyze a high speed video as Alejandro is very adept at using it for velocity analysis.

As Lasse did you could also measure where the brick hits the floor to get an ideal of which case had the higher velocity. In that case I would make the edge of the table close to that .56 m launch point so that friction losses would not have any time to reduce the launch velocity.

If the impact distance of the brick hitting the floor is what you are referring to, then that would required a bit more analysis in the model to calculate the impact distance expected for a fixed height and different horizontal launch velocities. From an experimental standpoint, the impact distance would make for a simple comparison of the relative launch velocities, so if that is what you are referring when you are talking about longer distances then you are making perfect sense.

Gordy

[gordonjudd]

If the impact distance of the brick hitting the floor is what you are referring to, then that would required a bit more analysis in the model to calculate the impact distance expected for a fixed height and different horizontal launch velocities.

Aitor,
If I have done the simple drag-free calculation correctly (always a big if for me) the time for an object with an initial y velocity of zero to fall from a .7 m high table would be:
t=sqrt(2.*.7/9.81)=.377 s.

For a 1 m/s difference in the horizontal launch velocity the corresponding difference in the impact distance would be
d=v*t=1*.377=.377 m.

Thus measuring that distance should be easy to do for the velocity differences we are talking about. Hats off to Lasse for coming up with that simple impact distance criteria.

Gordy

[Unregistered]

the result of the experiment can show a longer distance for the spring or a longer distance for the string.

Aitor,
I do not understand why you keep revering to relative distances, especially since the distance force is being applied to the brick in you experiment is the same whether or not it is being pulled by the string or via the spring.

Gordy

Again.
The brick (or marble, which is what I probably will use) is a simile of a fly line. Right?
The weight driven by gravity is the caster. Right?
The spring is a bendy rod. Right?
The inelastic string is a rigid rod. Right?

Then the caster falls and makes a cast. Does the cast end when the caster stops the rod (in this case when it hits the ground)? Not at all the cast ends when the lline unrolls and falls to the ground, and every element in the system is at rest.

I think you need to focus on the relative launch velocities, not the relative distance the brick has traveled when the lead mass hits the ground. Why do you think the relative distance at the point the lead mass hits the ground is important? Is it because that is something you can see in comparative videos?

I have never talked about considering the distance covered by each marble at the instant the brick touches down, and can not understand why you keep over it. I need to consider the total distance covered by each marble when the cast ends. So I will release the weight, will smoke a cigarrette, and then I will measure the distance covered by the marble from its starting position to its final position.

So I will ask again:
What is you predicition for the result? My feeling (probably wrong, but my feeling anyway) is that, when the cast is finished, the marble/spring combo will never cover more distance than the marble/string combo. Take notice that I say "when the cast is finished", not when the weight hits the ground but five minutes later so to speak. Both marbles will cover, more or less, the same distance but I don't expect to see any advantage for the spring in any case.
Do yo expect an advantage for the spring in all cases?

Again, a HS video can not show energy, work or any other thing, it can only show visual clues. And the visual clue that anybody expects from a model that states that a spring has a mechanical advantage is to see that the marble attached to it has gone a longer distance than the other one. Right?

P.S.
In my experiment the marbles will not fly. I have aavilable a sort of elevated floor and the marbles will only over a flat surface form the start to the end of their motion. Then I will measure the final distance covered by each one when the cast is finished and the marble is at rest. If the distance covered by the spring/marble is longer than the distance covered by the string/marble it is a closed case for me.

[James9118]

Aitor,

Are you going to supply the figures that are needed to model your experiment, masses, spring constant, drop height etc?

Cheers, James

[Merlin]

Aitor,

If the marbles cover the same distance, it will tell that the spring was not adequate. Just like a fly rod, you know, if the stiffness is not within a reasonable range, you have difficulties to cast the line.

I have the same question than James, since it will allow to compare with model(s).

Many thanks in adavnce.

Merlni

[Unregistered]

Gordy,
I asked:

So my question again: Will the marble spring/combo always cover more distance than the marble/string combo or not?

Merlin replied:

We explained several times that the result depends on the parameters (masses and spring stiffness). What you will see will depend on them. If you tell us what they are we can compare with the model. For the time being the answer is any of the two bricks.

And you replied:

A very stiff spring would produce the same results as the string driven case. A very weak string would still add to the launch velocity, it just would not be as big as the improvement you would get with an optimum spring.

Don't you see a difference? I do. Merlin says that, depending on the parameters, the result of the experiment can show a longer distance for the spring or a longer distance for the string. You say that the distance is either the same for both or longer for the spring, but the string will never cover more distance than the spring.

Merlin and James,

So back again to the question I posed yesterday. Gordy said A very stiff spring would produce the same results as the string driven case. A very weak string would still add to the launch velocity, it just would not be as big as the improvement you would get with an optimum spring.
And that implies that the spring will always have an advantage, and that is not what you are saying.

I will give data when I have the materials. Anyway I don't know how to measure the spring constant of a conventional rubber band used in any office.

[Merlin]

Aitor,

It is all a matter of numbers, what do we call weak or strong if we speak about spring stiffness? I do not understand Gordy's statement as "the spring has always an advantage", it could just be negligible.

Merlin

[Lasse Karlsson]

Hi Aitor

In the experiment I tried, the brick on the string had the lead until the weight hit the floor, then the brick on the spring overtook, and as it can be seen on the clip, it launched over the tble before the brick on the string indicating a higher velocity.
Starting position for the two bricks where the same.

Cool table you have Looking forward to seeing the clip

Cheers
Lasse

[gordonjudd]

I have never talked about considering the distance covered by each marble at the instant the brick touches down, and can not understand why you keep over it. I need to consider the total distance covered by each marble when the cast ends. So I will release the weight, will smoke a cigarette, and then I will measure the distance covered by the marble from its starting position to its final position.

Aitor,
I hope you have a long table for that implementation of the experiment.

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."

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. I tried to point that out to you in the other thread when you noted:

I see in the distance vs. time graph that when both bricks stop at t= 1.6

and I responded:

I think that 1.6 second value is the point where Merlin stopped his calculations not where the brick came to a stop in those two cases. Since there are no losses, they would continue to travel with the maximum velocity produced with the string (5.1 m/s) or the spring (6.2m/s) connections for all time after their launch.

But I guess that did not get through to you since you are still thinking about doing the experiment that way.

However, measuring the impact distance the marble travels as it falls from the table does make sense and I think is a simple, direct way to see the difference in the launch velocities. You may need to come up with some sort of hook and loop arrangement so that the marble disengages from the pulling string as it falls, but I think something could be worked out so the marble is not entangled by the string or spring as it goes through the free-fall path to the floor.

And that implies that the spring will always have an advantage,

If you consider the fact that a stiff spring producing the same velocity as a string is an advantage I would agree with that statement, but as Merlin noted same=a negligible advantage.

Anyway I don't know how to measure the spring constant of a conventional rubber band used in any office.

Just do the same thing as measuring the spring constant of a fly rod. Hang a bag from the rubber band, and note the deflection or strech you get for different numbers of coins that have a known mass. I will be glad to work out the spring constant value for you as I did for Lasse's very stiff elastic earlier.

Gordy

Gordy

[Unregistered]

Cool table you have Looking forward to seeing the clip

Lasse

As I wrote in a previous post it is not a table, it is a sort of elevated floor; just think of something like a loading dock for trucks.