Why does Newton's Cradle behave the way it does?

  • Thread starter sungholee
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In summary, the Newton's cradle relies on the principle of momentum to send balls off to the other side. If the masses of the balls are not the same, then the balls will not send off the same velocity and the cradle will not work.
  • #1
sungholee
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If you pull and let go three "balls" from one side of a Newton's cradle, three will be pushed from the other side. But why is that? Why can't one ball be pushed three times as fast (or far?) as a result of the three balls? Or 2, for that matter?
 
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  • #2
That's a very good question - in fact: how does the cradle know how many balls to send off the far side?
Have you tried tapping the cradle with a hammer too? What happens?

What have you done so far to try to figure it out ;)

Other things to try
- hold the fifth ball still when the first drops.
- raise one ball from each end and drop them at the same time
- raise one ball from one end and two balls from the other end...

- raise all five balls: why doesn't just one ball go five times as high?
 
  • #3
There have been previous threads about this. The reason has to do with how the force is distributed and transmitted through the balls, which compress slightly (like a very stiff spring) during the collision. Link to web page:

http://www.lhup.edu/~dsimanek/scenario/cradle.htm
 
  • #4
sungholee said:
Why can't one ball be pushed three times as fast
Simon Bridge said:
how does the cradle know how many balls to send off the far side?
rcgldr said:
The reason has to do with how the force is distributed and transmitted through the balls, which compress slightly
The answer is very simple: just because a ball doesn't know how many balls are behind it. Each collision takes place separately between a ball at rest and a ball with KE and with same mass.
 
  • #5
@bobie: simple aye? Have you visited rclgdr's link?

What you've described it the "series of 2-ball collisions" approach - the only way "[e]ach collision takes place separately...", physically, is if there is a small gap between each ball - which is not required for the result.
Please read the link if you have not already done so.
 
  • #6
sungholee said:
Why can't one ball be pushed three times as fast (or far?) as a result of the three balls? Or 2, for that matter?

Things like that can happen, but only if the balls have different masses. In most "toy" cradles the balls are all identical, but there is no reason why you can't make your own where you can change the pattern of different sized balls. Steel ball bearings work well. The bigger and heavier, the better.
 
  • #7
Steel ball bearings work well. The bigger and heavier, the better.
Up to a point ... the bigger and heavier they are the more rigid they need to be. At some point you won't find steel (or other material) rigid enough.

Seen the demolition-ball Newton's cradle thing?

But, the bigger the balls, the more likely you will see other solutions pop up.
That link in post #3 is essential reading.
 
  • #8
Simon Bridge said:
@bobie: simple aye? Have you visited rclgdr's link?.

I have, I think last ball cannot be pushed 2 or 3 times faster because it starts moving before the energy of the first or second ball is discharged. the third balls discharges its energy and stops , then takes the energy of the second ball. discharges it to the penultimate... and so on .
Isn't time delay the main reason?
 
Last edited:
  • #9
bobie said:
The answer is very simple: just because a ball doesn't know how many balls are behind it. Each collision takes place separately between a ball at rest and a ball with KE and with same mass.

I notice that the concept of Momentum hasn't been brought into the discussion and this sort of problem is best treated inn terms of Impulse. For any collision, an equal and opposite impulse is applied to each of a colliding pair. For equal mass balls, this will stop the first and send the second off at the same velocity.
If you want to treat the intermediate balls as a single mass then it gets more complicated and you need to consider the first and last collisions together but you can still consider the impulses in each collision. Equal times again and equal contact forces (as long as incoming and outgoing balls have the same mass). Two simple simultaneous equations can be obtained for each collision and the velocity of the middle mass drops out - giving the same overall result.
 

What is Newton's Cradle?

Newton's Cradle is a device that demonstrates the laws of conservation of momentum and energy. It consists of a series of pendulums with identical mass that are suspended from a frame. When one of the pendulums is pulled back and released, it swings and collides with the other pendulums, causing the one on the opposite end to swing out. This creates a chain reaction, with the last pendulum swinging back and forth for a period of time.

What are the laws of conservation of momentum and energy?

The law of conservation of momentum states that in a closed system, the total momentum remains constant. This means that the total mass times velocity of the system before and after a collision must be the same. The law of conservation of energy states that in a closed system, energy cannot be created or destroyed, it can only be transferred from one form to another.

Who invented Newton's Cradle?

Newton's Cradle was not actually invented by Sir Isaac Newton, but rather by English actor and artist Simon Prebble in 1967. However, the device was named after Newton because it demonstrates his laws of motion.

What are some real-life applications of Newton's Cradle?

Newton's Cradle is often used as a model for understanding collisions and how they conserve momentum and energy. It can also be used to demonstrate the concept of resonance and the transfer of energy between objects. In addition, it has been used in the development of toys, educational tools, and even as a desktop decoration.

Is Newton's Cradle a perfect representation of the laws of motion?

No, Newton's Cradle is a simplified model and does not account for all factors that may affect a collision, such as friction and air resistance. Additionally, in real-life situations, energy is often lost due to imperfections in the materials or other external factors. However, it is still a useful tool for understanding the basic principles of the laws of motion.

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