Simple Pendulum Experiment questions

[nousername]

HI ALL!

Im new to physics forums, so ill see how i go :)

I have some questions below regarding a simple pendulum:

1) Why does the string need to be massless? What will happen if it isn't massless?
2) What does a "frictionless pivot mean"? If i have two plates which have the string in between and then i clamp the two plates together, does that mean i have a frictionless pivot or a pivot with friction? I don't get what is meant by a frictionless pivot. If you had a choice between a fishing line and string, which one would you choose and why? I would think that the fishing line is too stiff and would cause a retarding force on the pendulum... so i would choose the string... not sure.
3) What is the PHYSICS term given to the 'stiffness' of a string.


Thanks in advance

 

[tiny-tim]

welcome to pf!

hi kirollos! welcome to pf!

1) Why does the string need to be massless? What will happen if it isn't massless?

if it isn't massless, we need to include the moment of inertia of the string

2) What does a "frictionless pivot mean"? If i have two plates which have the string in between and then i clamp the two plates together, does that mean i have a frictionless pivot or a pivot with friction? I don't get what is meant by a frictionless pivot. If you had a choice between a fishing line and string, which one would you choose and why? I would think that the fishing line is too stiff and would cause a retarding force on the pendulum... so i would choose the string... not sure.

no pivot will be completely frictionless

"frictionless" is a word used in exam questions to make the questions easier

i'd choose the lightest one … the friction makes virtually no difference

3) What is the PHYSICS term given to the 'stiffness' of a string.

stiffness is a physical term … see http://en.wikipedia.org/wiki/Stiffness"

 

[Drakkith]

HI ALL!

Im new to physics forums, so ill see how i go :)

I have some questions below regarding a simple pendulum:

1) Why does the string need to be massless? What will happen if it isn't massless?
2) What does a "frictionless pivot mean"? If i have two plates which have the string in between and then i clamp the two plates together, does that mean i have a frictionless pivot or a pivot with friction? I don't get what is meant by a frictionless pivot. If you had a choice between a fishing line and string, which one would you choose and why? I would think that the fishing line is too stiff and would cause a retarding force on the pendulum... so i would choose the string... not sure.
3) What is the PHYSICS term given to the 'stiffness' of a string.


Thanks in advance

1. It only needs to be massless because the question is staying simple and they want you to avoid having to calculate for the mass of the string. If you were to design one in real life you would absolutely have to take into account the mass of the string.

2.The string is kind of a weird one to use when referring to friction of the pivot. Think of a big clock with a metal pendulum swinging back and forth turning the gears. The pivot itself has friction in real life. With the string I think there is still friction within the string itself. Again, frictionless is being used to simplify the materiel.

3. See above!