Confusion over 2nd law and conservation of momentum

[steven george]

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.

 

[mfb]

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.

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..

 

[steven george]

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?

 

[nasu]

What do you mean by "carefully"? The answer may depend on this.