- #1

sachin

- 61

- 7

- Homework Statement
- I am posting another query on my previous post in the below link

https://www.physicsforums.com/threads/tt.ps://www.physicsforums.com/threads/n-semi-circular-track.1044825-laws-block-sliding-up-a-frictionless-semi-circular-track.1044825/

The question says find the common horizontal velocity of the blocks when the smaller block of mass m reaches the point A of the bigger block of mass M, the smaller block was given an initial velocity v to the left as in the figure, all surfaces are frictionless.

- Relevant Equations
- m x v + M x 0 = (m + M)v'

The question is solved in a single step by taking the blocks as a system and using conservation of linear momentum in the horizontal direction as there is no net force acting in the horizontal direction.

Conserving the momentum we get,

m x v + M x 0 = (m+M)v',

so,,v' = mv/(m +M).where v' is the common horizontal velocities of the blocks at the point A.

My concern is quite trivial,

while choosing the system why did we consider only the two blocks,why did not we consider the Earth into the system ?

what is the necessary condition of choosing a system while using conservation of linear momentum ?

Conserving the momentum we get,

m x v + M x 0 = (m+M)v',

so,,v' = mv/(m +M).where v' is the common horizontal velocities of the blocks at the point A.

My concern is quite trivial,

while choosing the system why did we consider only the two blocks,why did not we consider the Earth into the system ?

what is the necessary condition of choosing a system while using conservation of linear momentum ?