Confusion over 2nd law and conservation of momentum

In summary, the conversation discusses the concept of conservation of momentum and how it applies to a moving cart with added weight. It is explained that the cart will slow down due to the added weight, but can also accelerate if the weight is added at the same speed as the cart. The conversation also mentions a similar scenario involving a figure skater and how they can accelerate without any torque. Finally, the conversation touches on the concept of removing the added weight and how the method of removal may affect the acceleration.
  • #1
steven george
11
1
Hi everybody, I apologize if something like this has been asked before but I have been unable to find an answer through searching.

If a weight is carefully added to a moving cart then the cart should slow down due to conservation of momentum. How is it that the cart has accelerated without a horizontal force?

This issue originally came up while discussing rotational motion. If a figure skater is spinning and brings his hands in he will speed up. How can he accelerate without any torque?

There must be something very basic that I am missing here, so thank you to anybody who can help with an answer.
 
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  • #2
steven george said:
If a weight is carefully added to a moving cart then the cart should slow down due to conservation of momentum.
It depends on how the weight is added. I'll guess: You add things at zero speed relative to the ground.
steven george said:
How is it that the cart has accelerated without a horizontal force?
There is a force between cart and new weight, speeding up the weight to match the speed of the car. The new weight accelerates forwards, the remaining cart accelerates backwards..
 
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  • #3
mfb said:
It depends on how the weight is added. I'll guess: You add things at zero speed relative to the ground.There is a force between cart and new weight, speeding up the weight to match the speed of the car. The new weight accelerates forwards, the remaining cart accelerates backwards..

I knew there must be a simple answer. Thanks for that. Now, what if that weight is taken off by carefully lifting it straight up?
 
  • #4
What do you mean by "carefully"? The answer may depend on this.
 
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Related to Confusion over 2nd law and conservation of momentum

1. What is the second law of thermodynamics?

The second law of thermodynamics is a fundamental principle in physics that states that the total entropy of a closed system will always increase over time. This means that the disorder or randomness of a system will always increase, and energy will naturally flow from a more concentrated form to a more dispersed form.

2. How does the second law of thermodynamics relate to the conservation of momentum?

The second law of thermodynamics does not directly relate to the conservation of momentum. The conservation of momentum is a separate principle that states that the total momentum of a closed system remains constant, unless acted upon by an external force. However, the second law of thermodynamics can help explain why momentum is conserved in everyday situations.

3. Can the second law of thermodynamics be violated?

No, the second law of thermodynamics is a fundamental law of nature and cannot be violated. However, there are some rare situations where it may appear to be violated, such as in systems with extremely low entropy or in situations involving quantum mechanics. But overall, the second law of thermodynamics holds true in all physical systems.

4. How does the concept of entropy relate to the confusion over the second law and conservation of momentum?

Entropy is a measure of the disorder or randomness in a system. The confusion over the second law and conservation of momentum may arise because the concept of entropy is often used to explain how momentum is conserved in certain situations. For example, in a collision between two objects, the total entropy of the system will increase, which helps to explain why momentum is conserved.

5. What are some real-life examples that demonstrate the second law of thermodynamics and conservation of momentum?

One example is a game of billiards, where the balls will eventually stop moving and the total entropy of the system will increase. Another example is the heat transfer from a hot object to a cold object, where the heat will always flow from the hot object to the cold object, increasing the overall entropy of the system. In both cases, momentum is conserved as energy is transferred and dispersed, following the second law of thermodynamics.

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