Girl on a swing, stands up and becomes stationary?

[kudoushinichi88]

Homework Statement

Homework Equations

Conservation of Energy

The Attempt at a Solution

The girl has potential energy of 400(1.2) J at A. At B, her total energy is 400(0.6) J plus her kinetic energy.

But at this point, she's also doing work against gravity, which is 400(0.6) J.

So is it correct for me to say that this takes away her kinetic energy, therefore giving her a zero velocity at B and h=0.6m?

Or is this only true if she gradually stands up?

In the case of her instantaneously standing up, does this means the work done against gravity by the girl adds energy into the system, therefore increasing the Ek of the girl?

 

[Delzac]

Well, if you were to do this in real life, you wouldn't expect the swing to suddenly stop isn't it?

And if you consider energy wise, its a bit hard to see what's going on.

You should think in terms of angular momentum.

 

[kudoushinichi88]

but wouldn't gravitational force exert a torque on the system therefore making the angular momentum not conserved?

 

[Delzac]

But your energy will always be conserved.

There is rotational kinetic energy at the bottom and this energy is lost due to gravity. So angular momentum is not conserved, but just because it is not conserved doesn't stop you from making energy calculations isn't?

Is there is conservation of angular momentum, then omega doesn't change, and calculations become impossible.

-Edited for typo-

 

[Delzac]

And i am considering only an instant. Between the girl and the swing, when the girl stands up, angular momentum is conserved.

 

[kudoushinichi88]

Ohhh! That is clever!

But I don't quite understand your English in your post before that.