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#1 2005-10-30 12:25:18


Newton's Cradle.. Silly Question

This is probably the most ridiculous question but I have this idea in my head that when the balls of a newton's cradle are set in motion that they are 'never' suppose to stop and my mind just won't rest until I know if this is true or not. I bought one as a gift and I'm convinced it's broken because it stops within seconds, and I'm wondering if it's not balanced right. The bent strings and not-so-perfect alignment of it makes me think this, too.

Thanks in advance.

#2 2005-10-30 13:02:30

Registered: 2005-08-25
Posts: 59

Re: Newton's Cradle.. Silly Question

Ideally, it won't stop. However, several factors get into our way, the two main ones being:
1. Air resistance
2. The bar holding them is not perfectly rigid. Energy can transfer through the cords and the bar from one ball to another, and also to the ground.


#3 2005-10-30 17:13:10

Registered: 2005-01-21
Posts: 7,684

Re: Newton's Cradle.. Silly Question

I agree, the inevitable friction will slow it down, but it should run for a minute or so ... long enough to be satisfying.

"The physicists defer only to mathematicians, and the mathematicians defer only to God ..."  - Leon M. Lederman


#4 2005-10-31 06:18:17


Re: Newton's Cradle.. Silly Question

Thank you for replying. I'm glad I'm not insane for thinking this. I wonder if its size could be a factor? Each ball is very tiny and it only stands at maybe 3 inches tall. It's just a little gimmicky version so I'm thinking of buying a proper one with a wooden stand.

What can I say? The kid wants a Newton's cradle and I don't want to disappoint smile

#5 2005-10-31 06:30:14

Registered: 2005-06-22
Posts: 4,900

Re: Newton's Cradle.. Silly Question

If it's gimmicky, then it's likely that not so much care would have been made to make sure it works properly, so the forces would be unbalanced and it would stop relatively quickly. As you've said, the strings are bent and the alignment isn't perfect, so every time one of the balls hits another some of the energy is being wasted. The size of it matters as well though. If you have two Newton's Cradles that are made in exactly the same way but one is bigger, then the bigger one will go for longer.

With a big one, you lift up the first ball higher than you would for the smaller one and so you give it more gravitational energy, which means that it will take longer for the energy to be dissipated by the friction, because there's more of it. Also, the bigger one would have heavier balls so the air resistance would have less effect on it. Think falling hammer and feather.

I've got one that's currently tangled up (if you're buying it for a child you'll need to watch out for that), but before it went into the unfixable mess that it's in now, it could last for about a minute before it slowed down significantly.

Why did the vector cross the road?
It wanted to be normal.


#6 2005-10-31 12:21:17


Re: Newton's Cradle.. Silly Question

Thank-you for the explanation mathsyperson. It makes sense (says the person with dyscalculia:)) I will just give him this gimmicky version when I see him next and surprise him with a better one at Xmas time than.

He's not actually a child but even I'm not convinced of that sometimes! roll

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