What causes a 'simple pendulum' to stop?

[student321]

Homework Statement

I need all the variables that causes the pendulum to stop.

Homework Equations

The Attempt at a Solution

I know friction causes the pendulum to stop but what type of friction is it and what from?

thanks

 

[Hootenanny]

Homework Statement

I need all the variables that causes the pendulum to stop.

Homework Equations

The Attempt at a Solution

I know friction causes the pendulum to stop but what type of friction is it and what from?

thanks

Welcome to Physics Forums.

A simple pendulum will never stop once set into motion.

 

[student321]

doesn't it stop due to air resistance etc.

 

[Hootenanny]

doesn't it stop due to air resistance etc.

By definition a simple pendulum ignores air resistance, friction, etc.

 

[student321]

By definition a simple pendulum ignores air resistance, friction, etc.

is that even possible ignoring air resistance?

 

[Hootenanny]

is that even possible ignoring air resistance?

I don't quite understand what you mean.

 

[student321]

But real pendulums are subject to friction and air drag, so the amplitude of their swings declines.

 

[negitron]

http://hyperphysics.phy-astr.gsu.edu/HBASE/pend.html

 

[Hootenanny]

But real pendulums are subject to friction and air drag, so the amplitude of their swings declines.

Is a simple pendulum the same a real pendulum?

 

[student321]

Is a simple pendulum the same a real pendulum?

OK, what causes a real pendulum to stop?

 

[Hootenanny]

OK, what causes a real pendulum to stop?

Well, you've already given one cause:

doesn't it stop due to air resistance etc.

Another cause could be friction from the pivot.

 

[student321]

how does the pivot cause friction?

Thanks Hootenanny.

 

[Dadface]

Energy is also transferred from the pendulum via the support system.If the support is firmly fixed in position the pendulum will swing for a longer time.

 

[Hootenanny]

how does the pivot cause friction?

The usual way is friction between axle and the bearing. Take the simple case of a piece of string tied round a pin. If the knot is loose then as the pendulum oscillates the string will 'rub' against the pin.

 

[noblegas]

SHM is just a mathematical picture right? It would not exist in the real world unless it were place in a vacuous region of the universe

 

[Hootenanny]

SHM is just a mathematical picture right? It would not exist in the real world unless it were place in a vacuous region of the universe

You are indeed correct. We would also presumably need to invent frictionless bearings or ideal springs to practically realize SHM. We would also need to find massless strings and point particles.

 

[student321]

is 'SHM' what a pendulum creates?

 

[Dadface]

is 'SHM' what a pendulum creates?

A simple pendulum is just one example of a system that vibrates with SHM.If we were to be fussy we would say that the motion is only exactly simple harmonic if the string were massless the bob had zero size and if the amplitude of swing was negligible,in other words,a real pendulum does not move with SHM.I suspect,however,that this fussiness is confusing you and that at your current level you do not need to be fussy.Provided that the size of the bob is small,the string is light and the amplitude of swing is small the motion is approximately SHM.

 

[ideasrule]

A pendulum can actually approximate SHM extremely well. The string doesn't need to be massless, nor does the bob need to be a point mass; extended pendulums still swing with SHM provided the amplitude of its oscillations is small.

 

[Dadface]

A pendulum can actually approximate SHM extremely well. The string doesn't need to be massless, nor does the bob need to be a point mass; extended pendulums still swing with SHM provided the amplitude of its oscillations is small.

It's true that the pendulum can approximate SHM very well but it is still only an approximation.By definition a system moves with SHM if it accelerates towards a fixed point it's acceleration being direcly proportional to the displacement from that point and these criteria are not met exactly with a real pendulum.As I stated above this is being too fussy and may confuse the OP.

 

[student321]

A simple pendulum is just one example of a system that vibrates with SHM.If we were to be fussy we would say that the motion is only exactly simple harmonic if the string were massless the bob had zero size and if the amplitude of swing was negligible,in other words,a real pendulum does not move with SHM.I suspect,however,that this fussiness is confusing you and that at your current level you do not need to be fussy.Provided that the size of the bob is small,the string is light and the amplitude of swing is small the motion is approximately SHM.

Is it possible to prove that a real pendulum does not move with SHM? and does SHM create a sin graph?

 

[Dadface]

Is it possible to prove that a real pendulum does not move with SHM? and does SHM create a sin graph?

Hello student 321.The proof can be theoretical or it can be experimental and achieved by taking very precise measurements,however, I still think this is being too fussy.With a well constructed pendulum swinging through a very small angle the motion differs from perfect SHM by an amount that is so tiny that ,I guess,for most purposes it can be considered as negligible.
If we ignore damping(energy losses)The displacement, velocity and acceleration can be displayed graphically as sin or cos curves but the three quantities are out of phase with each other.For example when the displacement is a maximum the velocity is zero and the acceleration is a maximum but in the opposite direction to the displacement.

 

[student321]

Hello student 321.The proof can be theoretical or it can be experimental and achieved by taking very precise measurements,however, I still think this is being too fussy.With a well constructed pendulum swinging through a very small angle the motion differs from perfect SHM by an amount that is so tiny that ,I guess,for most purposes it can be considered as negligible.
If we ignore damping(energy losses)The displacement, velocity and acceleration can be displayed graphically as sin or cos curves but the three quantities are out of phase with each other.For example when the displacement is a maximum the velocity is zero and the acceleration is a maximum but in the opposite direction to the displacement.

I'm doing an experiment with a real pendulum and I am able to calculate the period quite accurately using a motion sensor which draws a graph for me on a computer and provides information. how do i prove that a real pendulum doesn't move in a SHM. for e.g

Length of pendulum : 0.5m
amplitude: 30 degrees
and using the formula i get T = 1.418s

while from my experiment i get T = 1.515s

does this show that a real pendulum does not move in SHM? or am i wrong. I know my amplitude is large but also am doing 5 degrees.

 

[Hootenanny]

I'm doing an experiment with a real pendulum and I am able to calculate the period quite accurately using a motion sensor which draws a graph for me on a computer and provides information. how do i prove that a real pendulum doesn't move in a SHM.

The simplest way would be to note that the amplitude of the real pendulum does not remain constant. Set the pendulum in motion and observe whether the pendulum eventually stops (which it will).

 

[Dadface]

I'm doing an experiment with a real pendulum and I am able to calculate the period quite accurately using a motion sensor which draws a graph for me on a computer and provides information. how do i prove that a real pendulum doesn't move in a SHM. for e.g

Length of pendulum : 0.5m
amplitude: 30 degrees
and using the formula i get T = 1.418s

while from my experiment i get T = 1.515s

does this show that a real pendulum does not move in SHM? or am i wrong. I know my amplitude is large but also am doing 5 degrees.

There is fairly close agreement between your two answers and the difference could be attributable to experimental errors these mainly being in the length measurement, errors within the sensing system and errors caused by the large amplitude used.When measuring the length accurately you need a clearly defined support point,For example the string could be clamped between two coins and you need to measure from the point of support to the centre of gravity of the bob.By the way, when presenting an answer it is best to estimate the errors ,express these as a percentage and round your answer down to the correct number of significant figures.

 

[Dadface]

Following on from hootenannys idea why not repeat your experiment but start with a large amplitude. From your graph you can measure the time period at different times as the amplitude reduces.

 

[Redbelly98]

I'm doing an experiment with a real pendulum and I am able to calculate the period quite accurately using a motion sensor which draws a graph for me on a computer and provides information. how do i prove that a real pendulum doesn't move in a SHM. for e.g

Length of pendulum : 0.5m
amplitude: 30 degrees
and using the formula i get T = 1.418s

while from my experiment i get T = 1.515s

does this show that a real pendulum does not move in SHM? or am i wrong. I know my amplitude is large but also am doing 5 degrees.

To follow SHM, it's angle must follow a sine-dependence on time. Just having an amplitude and a period does not show whether it is or isn't a sine function. After all, non-sine functions can have an amplitude and a period as well.

You are looking for departures from a sine function. If you could measure the angle at many times within a single period and graph it, then you might be able to show that it is not a sine, but that would be easier with a larger amplitude (perhaps 90 degrees).

Or, as others said, the fact that the amplitude is not constant also represents a departure from a sine function, and hence is not SHM.

EDIT: Oops, I see this thread is a little old. It came up in a search, and I forgot to check the date.