# Homework Help: What affects the swing of a pendulum w/ and w/o air resistance

1. May 20, 2007

### The_Lobster

1. The problem statement, all variables and given/known data
I'm having troubles figuring out what affects the swing of a pendulum.

Hypotheses:
1) The period is dependent on the amplitude
2) The period is dependent on the mass of the bob
3) Air resistance and friction will affect the period
4) The period is dependent on the length of the thread
2. Relevant equations
None...
3. The attempt at a solution
Well, as I've understood so far, hypothesis 1 is wrong, as a higher amplitude will give the bob greater kinetic energy, but the distance it has to travel is longer; hence they will cancel each other out and the period is the same?

Hypothesis 2: This one I'm having most trouble with. Without air resistance, any bob with any mass and density will travel with the same velocity. But with air resistance, is the period affected by the mass or by the density of the bob? Say the mass is constant but the density varies (big wood-bob 0.2 kg, small iron-bob 0.2 kg); will they both travel with the same velocity (w/ air resistance)? What about if the density is the same, but the mass varies (small iron bob 0.2 kg, large iron bob 1 kg)? What conclusions can be raised here? (some explanations as well would be great!)

Hypothesis 3: Well of course I guess? No need for further explanation?

Hypothesis 4: True. If we use the same amplitude on two bobs, one with a long thread the other with a short thread, the bob on the short thread will be in a higher position than the one with the long thread at the same amplitude, and therefore it will gain a higher kinetic energy than the other, though travel the same distance; hence quicker...

I guess it's hypothesis 2 I'm having most troubles with... It's probably very simple

Help please with confirming and correcting (especially hypothesis 2)? :)

Cheers,
Joachim

2. May 20, 2007

### Astronuc

Staff Emeritus