[Renelyn]

Does anyone knows an answer for this question?

1. Suggest the conditions required for a pendulum to swing continuously.

2. If a pendulum is made to swing in water, compare the time taken for this pendulum to come to a complete stop with the time taken by a pendulum swinging in air. Explain the difference.

[ebaines]

Yes, I'm sure there are many people on this board who can answer this question. However, since this is a homework problem - rather than us doing it for you, you should tell us what you think the answers are, and then we'll give you feedback.

[Unknown008]

Yes, its pretty easy to answer with a bit of thinking!

[Renelyn]

Erm... what I think is...

1. I think a pendulum can swing continuously in vacuum because of the absence of friction. Is there any answer apart from vacuum?

2. The time taken for a pendulum to come to a complete stop in the water is faster than in air. This is because water has higher density. Thus, more energy is needed to overcome the water friction than air friction. So, energy for pendulum to swing is used up faster in water.

[Unknown008]

Yeah, you're on the right track... You can also consider air and water resistance, which is nearly the same as your answers.

[Renelyn]

Yeah, you're on the right track... You can also consider air and water resistance, which is nearly the same as your answers.

Haha.. thanks! By the way, is there any answer beside my answer? I need more information.

[Unknown008]

If your pendulum is in a water tank, then there will also be waves generated. These will even more slow the pendulum down as they rebound from the walls of the tank in the opposite direction.

[ebaines]

Regarding getting a pendulum swing continuously - you are correct that putting it in a vacuum would certainly help, as obviously this would remove any air friction that would cause the pendulum to slow down. But you would also need some way to eliminate all friction at the pivot point - you can reduce friction there by making the arc of the pendulum as small as possible. Hence pendulums with really long arms and relatively slow arcs of movement tend to run longer than short pendulums. Another method might be to add a mechanism that gives the pendulum a little "kick" on each swing to help overcome the friction - that's how the pendulum in a grandfather's clock works.

[Unknown008]

Thanks for the additional info ebaines. Always here when needed!

[Renelyn]

Regarding getting a pendulum swing continuously - you are correct that putting it in a vacuum would certainly help, as obviously this would remove any air friction that would cause the pendulum to slow down. But you would also need some way to eliminate all friction at the pivot point - you can reduce friction there by making the arc of the pendulum as small as possible. Hence pendulums with really long arms and relatively slow arcs of movement tend to run longer than short pendulums. Another method might be to add a mechanism that gives the pendulum a little "kick" on each swing to help overcome the friction - that's how the pendulum in a grandfather's clock works.

Thanks a lot ebaines! This really give me a lot of info!

[galactus]

On topic, I have a question. What about the pendulum that swings in the Smithsonian? I believe it is called a Foucault pendulum? I forget, duh! One that moves as the Earth rotates. What keeps it swinging imperpetuity? Very little friction, I assume. There is also one at the local University where I live. They are cool.

[ebaines]

On topic, I have a question. What about the pendulum that swings in the Smithsonian?. I believe it is called a Foucault pendulum?. I forget, duh!. One that moves as the Earth rotates. What keeps it swinging imperpetuity?. Very little friction, I assume. There is also one at the local University where I live. They are cool.

Focault pendulums typically have very long arms, and hence swing through a short arc (a few degrees at most). So the mass at the end swings slowly, which minimizes air resistance. But they do not go forever - no perpetual motions allowed - typically they have to be restarted every few days.

[lauriea]

What can reduce friction on a pendulum