# Curious questions about a pendulum

1. Jan 27, 2005

### nemzy

If the size of the ball that is attached to the pendulum increases, will it affect the swing at all?

What if you changed the mass of the spring, will that influence any change in T?

Also, why is that an iron bob is more suited than an aluminum?

2. Jan 27, 2005

### dextercioby

For the third question,i have no idea...Fot the second and the first what are your answers and WHY??

Daniel.

3. Jan 27, 2005

### vincentchan

i'll answer the last one than... the iron is heavier (higher density) than aluminum

4. Jan 27, 2005

### nemzy

for the first part, well since it isn't a point mass this isnt an ideal pendulum. So if i treat this like a physical pendulum, then I, moment of inertia is directly proportional with T, so if size of ball increases , so should T? is my conceptual thinking right or am i missing an important point

Also, when the mass of a string increases, i remember that it will change the swing of the pendulum, and i remember i saw a formula for it somewhere but i just cant remeber where. But changing the mass of a spring , i have no idea how it will effect the swing of a pendulum
thanks

5. Jan 27, 2005

### dextercioby

Air friction is modelled by Stokes force and aerodynamical force (coming from the dynamical pressure)...They do not imply the density of the object that is moving in that medium...Only size,velocity and rigidness (and of course the "ita" coefficient)...Of course,the density of the medium,too... :tongue2:

Daniel.

http://scienceworld.wolfram.com/physics/StokesFlowSphere.html

EDIT:Despite the indirect effect,the formulas still do not contain the density,but "derived" elements,like radius and cross-section...

Last edited: Jan 27, 2005
6. Jan 27, 2005

### vincentchan

the mass of the ball increase sure won't affect the air friction...... however, F=ma, if the force doesn't change, and increase the mass.... the accelaration will decrease... that's mean the effect of air friction on the ball will decrease , which eventually affect the swing period...

7. Jan 27, 2005

### dextercioby

I would have been an idiot to claim that air friction would not affect the swing...

Daniel...

Last edited: Jan 27, 2005
8. Jan 27, 2005

### nemzy

ok, so what about my reasonings for quesoitn 1 and 2?

9. Jan 27, 2005

### dextercioby

For bobs of Al & Fe with the same mass,air friction is smaller for Fe,since its volume is smaller and hence the surface of the sphere (cross-section) is smaller.

So the effect of density is INDIRECT,through:density------>volume----->surface/radius------->friction---------->period of oscillation.

Daniel.

Last edited: Jan 27, 2005