# Newton's Cradles vs Ball Bouncing off Wall

• Tik
In summary, the conversation discusses the working of Newton's Cradles and the confusion surrounding the explanation of why the first ball stops and doesn't bounce back. The explanation is attributed to the law of conservation of momentum and energy, but it is questioned why this idea cannot be applied to the case of the cradle as well. The response is that conserving momentum and energy at the same time gives different outcomes and there are multiple solutions to consider. Real world experiments show that the ball may bounce back slightly, but there are also other factors to consider such as the direction of forces and the shifting of the pack of balls.
Tik
Hello everyone!

I was just reading about the working of Newton's Cradles and ended up having some confusion.

When a ball is taken to a certain height and dropped, it comes down and hits the next ball. Now the explanation as to why the first ball stops and doesn't bounce back is attributed to the law of conservation of momentum. Since momentum has direction, if the ball were to bounce back, the momentum would no longer be conserved as the direction would be reversed. Hence the momentum is transferred to the next ball.

Considering another situation, a ball bounces back when it hits a wall. The explanation given in this case is that the momentum of the ball changes from +mv to -mv and a momentum of 2mv is transferred to the wall, hence conserving momentum. Why can't this idea be applied to the case of Newton's Cradles as well?

Tik said:
Now the explanation as to why the first ball stops and doesn't bounce back is attributed to the law of conservation of momentum...
...and conservation of energy.
Tik said:
Why can't this idea be applied to the case of Newton's Cradles as well?
Because it would violate conservation of energy.

Thanks for the reply.

But just like a ball bouncing off the wall, why can't the first ball in the cradle retain some energy and bounce back, passing on the remaining energy to the next ball, hence conserving energy?

Tik said:
But just like a ball bouncing off the wall, why can't the first ball in the cradle retain some energy and bounce back, passing on the remaining energy to the next ball, hence conserving energy?

Conserving momentum and kinetic energy at the same time gives you a unique solution, which depends on the mass ratio of the objects. Hence there different outcomes.

Alright, I get it now. Thanks a lot!

Tik said:
But just like a ball bouncing off the wall, why can't the first ball in the cradle retain some energy and bounce back, passing on the remaining energy to the next ball, hence conserving energy?
There are multiple solutions that conserve both momentum and energy. You need to consider the direction of forces and how the forces propagate through the balls. Real world experiments show that the ball does bounce back a very small amount (not sure if this happens with an idealized cradle). There's also the issue that the pack of balls is shifting back and forth, when results in some component of opposing force from the strings the balls are attached to. Here's a website with a good explanation:

rcgldr said:
There are multiple solutions that conserve both momentum and energy.
For the outcome of the full cradle with > 2 balls, yes. But for the collision of two balls (which seemed to the OPs focus) there is only one.

## 1. How does a Newton's Cradle work?

A Newton's Cradle is a device that demonstrates the conservation of momentum and energy. It is made up of a series of suspended metal balls that are connected by strings. When one ball on the end is pulled back and released, it hits the next ball in line, causing it to swing forward and transfer its energy to the next ball. This continues until the last ball swings forward and then swings back, mimicking the original ball's motion.

## 2. Why do the balls in a Newton's Cradle eventually stop swinging?

The balls in a Newton's Cradle eventually stop swinging due to the effects of friction and air resistance. As the balls continuously collide and transfer energy, some of that energy is lost to friction between the balls and the strings, as well as air resistance. This causes the swinging motion to gradually slow down and eventually stop.

## 3. How does a ball bouncing off a wall differ from a Newton's Cradle?

A ball bouncing off a wall is a simple example of elastic collision, where the ball hits the wall and bounces back with the same speed and direction. In a Newton's Cradle, the balls do not bounce off each other but rather transfer energy and momentum through a series of collisions, with the last ball swinging back and forth. Additionally, a ball bouncing off a wall does not conserve energy, as some energy is lost in the form of sound and heat.

## 4. Can different materials be used in a Newton's Cradle?

Yes, different materials can be used in a Newton's Cradle, as long as they are able to transfer energy and momentum through collisions. However, the materials used can affect the efficiency of the device, with some materials causing more energy loss due to friction than others.

## 5. What are some real-world applications of Newton's Cradle?

Newton's Cradle is primarily used as a demonstration of basic physics principles, but it also has practical applications in engineering and design. It can be used to study and understand the conservation of energy and momentum in systems, as well as to model and test the behavior of materials under impact and collision. It has also been used in the development of impact-resistant materials and structures.

Replies
8
Views
669
Replies
10
Views
2K
Replies
53
Views
3K
Replies
14
Views
2K
Replies
1
Views
1K
Replies
2
Views
1K
Replies
1
Views
1K
Replies
2
Views
2K
Replies
9
Views
2K
Replies
4
Views
3K