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ian
Posts: 706
Joined: Fri Mar 07, 2008 3:55 pm UTC
Location: Sealand

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)

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

danpilon54
Posts: 322
Joined: Fri Jul 20, 2007 12:10 am UTC

### Re: A question about motion

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.
Mighty Jalapeno wrote:Well, I killed a homeless man. We can't all be good people.

Carnildo
Posts: 2023
Joined: Fri Jul 18, 2008 8:43 am UTC

### Re: A question about motion

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

danpilon54
Posts: 322
Joined: Fri Jul 20, 2007 12:10 am UTC

### Re: A question about motion

if that is the case though, you would have to have been given the length of the rope on the swing.
Mighty Jalapeno wrote:Well, I killed a homeless man. We can't all be good people.

oxoiron
Posts: 1365
Joined: Fri Jul 13, 2007 4:56 pm UTC

### Re: A question about motion

That's what I thought immediately. "How can I solve this if I don't know the length of the pendulum?"
"Whenever you find yourself on the side of the majority, it is time to reform (or pause and reflect)."-- Mark Twain
"There is not more dedicated criminal than a group of children."--addams

ian
Posts: 706
Joined: Fri Mar 07, 2008 3:55 pm UTC
Location: Sealand

### Re: A question about motion

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

danpilon54
Posts: 322
Joined: Fri Jul 20, 2007 12:10 am UTC

### Re: A question about motion

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.
Mighty Jalapeno wrote:Well, I killed a homeless man. We can't all be good people.

oxoiron
Posts: 1365
Joined: Fri Jul 13, 2007 4:56 pm UTC

### Re: A question about motion

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.
"Whenever you find yourself on the side of the majority, it is time to reform (or pause and reflect)."-- Mark Twain
"There is not more dedicated criminal than a group of children."--addams

ian
Posts: 706
Joined: Fri Mar 07, 2008 3:55 pm UTC
Location: Sealand

### Re: A question about motion

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

danpilon54
Posts: 322
Joined: Fri Jul 20, 2007 12:10 am UTC

### Re: A question about motion

ah ok then you should be able to work it out from here. All you need is vertical displacement.
Mighty Jalapeno wrote:Well, I killed a homeless man. We can't all be good people.