# Conditions for conservation of momentum

Gold Member

## Homework Statement

Consider a classic wedge and block system, (block on top of wedge(inclination theta)). there is friction between the block and wedge (not enough to prevent block from sliding). All other surfaces are smooth. For the motion that follows after releasing the block from rest, is momentum in the horizontal direction conserved?

## Homework Equations

There is no exact formula for momentum conservation but I guess p1=p2 in general

## The Attempt at a Solution

Momentum is conserved when net external force is zero. Since friction is an internal force here momentum conservation should be applicable along x axis but it seems counter-intuitive so I'm just confirming

Last edited:

kuruman
Homework Helper
Gold Member
2021 Award

## Homework Statement

Can we apply momentum conservation in the horizontal direction if friction is present between the wedge and block and nowhere else, I suppose we can since friction here is an internal force (although we can't apply energy conservation since friction is dissipative) but it feels somewhat counter-intuitive so just making sure. Thank you very much

all relevant

## The Attempt at a Solution

Mentioned above
Can you provide a more complete statement of the problem in (1), include the relevant equations instead of using a wild card in (2) and explain your reasoning in (3) separately from (1)? The template is meant to be used so that we can understand what's on your mind and what difficulties you have. Thank you.

Gold Member
Can you provide a more complete statement of the problem in (1), include the relevant equations instead of using a wild card in (2) and explain your reasoning in (3) separately from (1)? The template is meant to be used so that we can understand what's on your mind and what difficulties you have. Thank you.
Alright, sorry. I've changed it- thanks for pointing it out :D

RPinPA
Homework Helper
We don't know what "the wedge and the block" are or what you're trying to analyze.

An internal force is one between components of a system, and that depends on where you draw the boundaries of your system. If I catch a ball and I consider the ball to be the system, then I am an external force. The ball's momentum changes. But if I am standing on ice I might want to include myself as part of the system, in which case the action of my catching the ball is considered an internal force, and the total momentum of the system (myself and ball) is unchanged.

Gold Member
We don't know what "the wedge and the block" are or what you're trying to analyze.

An internal force is one between components of a system, and that depends on where you draw the boundaries of your system. If I catch a ball and I consider the ball to be the system, then I am an external force. The ball's momentum changes. But if I am standing on ice I might want to include myself as part of the system, in which case the action of my catching the ball is considered an internal force, and the total momentum of the system (myself and ball) is unchanged.
Lets say I want to find the velocity of the wedge when the velocity of the block is given. Considering the block+wedge as the system, can I write initial momentum=final momentum. Its a simple question really :D
Here's an image-

#### Attachments

• 3.8 KB · Views: 249
Gold Member
in which case the action of my catching the ball is considered an internal force, and the total momentum of the system (myself and ball) is unchanged.
So I guess yes. This is the same case since there is friction between your hand and the ball

Gold Member
Also I just found a solved example in my book which states the same thing so my question is solved. Thank you very much, the "ball catching" analogy was really helpful

CWatters
Homework Helper
Gold Member

## Homework Statement

Consider a classic wedge and block system, (block on top of wedge(inclination theta)). there is friction between the block and wedge (not enough to prevent block from sliding). All other surfaces are smooth. For the motion that follows after releasing the block from rest, is momentum in the horizontal direction conserved?

If your system is the block and wedge then yes. There are no external horizontal forces, no friction between wedge/block and table, so the table is irrelevant in the horizontal plane.

If your system was just the block then no. There is an external force (friction with the wedge) that has a horizontal component.

If your system was just the wedge then no. There is an external force (friction with the block) that has a horizontal component.