A question about motion

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ian
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A question about motion

Postby ian » Thu Dec 18, 2008 7:35 pm UTC

Hello, I'm a bit worried I've gone in totally the wrong direction on this question, so hopefully someone can see if I'm right or point me in the right direction.

A child on a swing with a combined mass of 16.0 kg is pulled back 0.98 m, and I need to calculate the inital energy transfered to the child. So far I have (excuse the jpg, I'm not used to the formula coding)

Image

(I've already calculated w = pi/2). Does this look right? I'm a bit worried over what 'pulled back 0.98m' means.

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danpilon54
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Re: A question about motion

Postby danpilon54 » Thu Dec 18, 2008 10:17 pm UTC

You are waaaaay overthinking it. Im assuming by pulled back it means vertical height? How much energy does the child/swing have in the beginning? What kind of energy is gained when the child is pulled back? How much energy is this? This can be done in 1 line.
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Carnildo
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Re: A question about motion

Postby Carnildo » Fri Dec 19, 2008 1:02 am UTC

It's a little harder if "pulled back" means horizontal distance, and a good bit harder if it means arc length.

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Re: A question about motion

Postby danpilon54 » Fri Dec 19, 2008 1:36 am UTC

if that is the case though, you would have to have been given the length of the rope on the swing.
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Re: A question about motion

Postby oxoiron » Fri Dec 19, 2008 2:54 am UTC

That's what I thought immediately. "How can I solve this if I don't know the length of the pendulum?"
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ian
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Re: A question about motion

Postby ian » Fri Dec 19, 2008 8:43 am UTC

well the next question is to find the length, which is what confuses me. i believe from the other questions it is indeed pulled back 0.98m horiozontally, would the above be correct if it was for horizontal distance?





danpilon54 wrote:You are waaaaay overthinking it. Im assuming by pulled back it means vertical height? How much energy does the child/swing have in the beginning? What kind of energy is gained when the child is pulled back? How much energy is this? This can be done in 1 line.


I don't think it is as a later question deals with coservation of momentum of two pedulums, and finding the velocity of one by calculating potential energy from height raised, converting to kinetic energy, and calculating the velocity from this. The thing is that question is worth more than the one above, despite it's relative simplicty

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danpilon54
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Re: A question about motion

Postby danpilon54 » Fri Dec 19, 2008 2:34 pm UTC

well your problem then is that a pendulum is not a simple harmonic oscillator unless you are considering very small angles (<3-5 degrees) about the eqilibrium. Even if that is the case however, the spring constant k would depend on the length of the rope. Pendulum motion is completely determined by gravity, which is strictly vertical. If you don't know the vertical displacement, you can't do the problem.

how did u get w = pi/2? I would think 2pi/w = period = 2pi(L/g)^0.5 so w = (g/L)^0.5 This depends on L.
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oxoiron
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Re: A question about motion

Postby oxoiron » Fri Dec 19, 2008 4:51 pm UTC

Bringing us back to, "How can I solve this if I don't know the length of the pendulum?"

By the way, that isn't a hint. It seems like you are missing information.
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ian
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Re: A question about motion

Postby ian » Fri Dec 19, 2008 7:02 pm UTC

danpilon54 wrote:well your problem then is that a pendulum is not a simple harmonic oscillator unless you are considering very small angles (<3-5 degrees) about the eqilibrium. Even if that is the case however, the spring constant k would depend on the length of the rope. Pendulum motion is completely determined by gravity, which is strictly vertical. If you don't know the vertical displacement, you can't do the problem.

how did u get w = pi/2? I would think 2pi/w = period = 2pi(L/g)^0.5 so w = (g/L)^0.5 This depends on L.


The period is 4s, so w = 2pi/T = 2pi/4 = pi/2

I guess the missing information was the period.

Though the second part of what you wrote means l = (T/2π)^2 x g from which i get l to equal roughly 4m

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danpilon54
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Re: A question about motion

Postby danpilon54 » Fri Dec 19, 2008 7:35 pm UTC

ah ok then you should be able to work it out from here. All you need is vertical displacement.
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