Momentum question: Object dropped onto a moving railcar

[kasheee7zark7]

Could someone please explain, in terms of the momentum, a rail car slowing down if an

item is dropped vertically downwards on to it. Thanks.

 

[Ibix]

Can you state the momentum of the cart (say its mass is $M$) and weight (say its mass is $m$) before and after the collision?

 

[kuruman]

What's the difference (if any) between this problem and a perfectly inelastic collision?

 

[A.T.]

Could someone please explain, in terms of the momentum, a rail car slowing down if an item is dropped vertically downwards on to it.

Vertically in the rest frame of the ground or of the rail car?

 

[kasheee7zark7]

The object and train are perpendicular to the ground.​

 

[Vanadium 50]

The object and train are perpendicular to the ground.

Please show some work.

I doubt the train is perpendictular to the ground. Is it vertical and riding on its caboose?

 

[kasheee7zark7]

See the video below (question 3, 6 minutes 54 seconds from the start).

The explanation given describes the interaction between the vertical side of the cart and the rock dropped, but what if there is no interaction between the two and it is dropped, say, in the middle of the cart? While I am writing these words, I am thinking that at the moment it hits the cart, the rock will move to the right, creating a leftward force on the cart (using Newton’s 3rd law), slowing the cart down. Presumably the interaction here would the friction of the floor of the cart and the rock.

 

[A.T.]

Presumably the interaction here would the friction of the floor of the cart and the rock.

If the rock ends up moving at the same horizontal speed as the cart, then there must have been some interaction that accelerated the rock horizontally. For the final speed of the cart the details of that interaction do not matter.

 

[kasheee7zark7]

Thanks for that. I was geting caught up on focussing on the interaction with the side of the cart.

 

[A.T.]

Thanks for that. I was geting caught up on focussing on the interaction with the side of the cart.

The power of conservation laws is the ability to make certain predictions without having to know much about the details.