| Topic: |
Science > Physics |
| User: |
"" |
| Date: |
27 Aug 2005 09:11:57 PM |
| Object: |
a simple dynamic problem consultion |
Dear all,
Here is the problem: a chain lies on the frictionless desk, the mass
per unit length is u, we pull one end of the chain with the constant
force F, so that there is increasing portion of the chain being pulled
out with constant velocity v.
then the force we applied should be u*v^2 and the power done by the
external force F sould be u*v^3. But the kinetic energy increasing rate
is 1/2 u v^3 !!?? why the energy is not conserved?
Thanks in advance.
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| User: "srp" |
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| Title: Re: a simple dynamic problem consultion |
28 Aug 2005 09:05:05 AM |
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<idyllic.math@gmail.com> a ecrit dans le message de news:
1125195117.052455.227330@g43g2000cwa.googlegroups.com...
Dear all,
Here is the problem: a chain lies on the frictionless desk, the mass
per unit length is u, we pull one end of the chain with the constant
force F, so that there is increasing portion of the chain being pulled
out with constant velocity v.
If you pulled with a constant force, your velocity would not remain
constant.
If your chain is frictionless and moving at constant velocity, then
no force is being applied except for an initial impulse, during which
you will have had acceleration. First Newton law.
then the force we applied should be u*v^2
No. Force is applied only if there is acceleration. v^2 is not
acceleration.
and the power done by the
external force F sould be u*v^3. But the kinetic energy increasing rate
is 1/2 u v^3 !!?? why the energy is not conserved?
Energy (from the initial impulse) is conserved, since no external force
is acting afterwards on the chain (since its velocity remains constant).
Look up Newton's first law.
Andre Michaud
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