Conservation of momentum and energy problem

Click For Summary

Discussion Overview

The discussion revolves around a problem related to elastic collisions, specifically focusing on the conservation of momentum and energy in a scenario involving two objects with differing initial and final velocities. Participants explore the conditions under which these conservation laws apply, particularly in the context of mass differences between colliding objects.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant requests an elastic collision problem involving two objects, one initially at rest and the other with a final velocity of zero.
  • Another participant proposes a specific scenario involving two identical balls, where one is at rest and the other moves towards it, suggesting a straightforward elastic collision setup.
  • A participant describes their attempt to assign values to masses and velocities, noting that only energy conservation was observed in their calculations, raising questions about the validity of their assigned values.
  • Some participants assert that for the first mass to come to rest after an elastic collision, the two colliding objects must have the same mass.
  • There is a query about the feasibility of such a scenario occurring in nature if the objects have different masses, which is affirmed by others as being impossible in elastic collisions.
  • A later reply introduces a frame of reference perspective, suggesting that viewing the collision from a specific moving frame can clarify the outcomes of the collision, where both objects appear to exchange momentum and kinetic energy without contradiction.

Areas of Agreement / Disagreement

Participants generally agree that for an elastic collision to result in one object coming to rest, the masses must be equal. However, there is disagreement about the implications of mass differences in real-world scenarios, with some asserting that such collisions cannot occur in nature if the masses differ.

Contextual Notes

Participants express uncertainty regarding the assignment of values and the resulting conservation laws, indicating potential limitations in their assumptions or calculations. The discussion does not resolve these uncertainties.

Who May Find This Useful

This discussion may be of interest to those studying mechanics, particularly in the context of collisions and conservation laws in physics.

Ibraheem
Messages
51
Reaction score
2
Hello,
Could someone please provide me with a elastic collision problem where there are two objects one object with an initial velocity=0 m/s and the other object with a final velocity= 0 m/s.

Thank you
 
Physics news on Phys.org
How about this: A ball of mass m is resting on a frictionless table. A second ball, identical to the first, moves with speed v and collides dead on and elastically with the first ball. (All motion is along a single direction.)
 
This is what I did, but I have assigned the values of the masses ,randomly, and the velocities based on the conservation of energy and something weird showed up.
The Problem quantities that I assigned are the following :
m1=2 kg
initial velocity 1 = 2m/s
final velocity 1 = 0 m/s
m2= 4kg
Initial velocity 2 = 0 m/s
final velocity 2 = sqrt(2) m/s
When I wanted to check if the energy and momentum is conserved, only the energy was conserved. How is that possible?
should energy and momentum be conserved in elastic collisions or am I assigning wrong values?
 
They have to have the same mass for the first mass to come to rest.
 
  • Like
Likes   Reactions: Ibraheem
So is it impossible for this to happen in nature if the objects have different mass?
 
Ibraheem said:
So is it impossible for this to happen in nature if the objects have different mass?
In an elastic collision, yes.
 
  • Like
Likes   Reactions: Ibraheem
Thank you guys for the help
 
You might think it's a bit strange that in this special collision the moving body, A, stops and the struck body, B, takes on its momentum and KE. But have you looked at it from the frame of reference moving at half the speed of A and in the same direction as A? In this frame, A and B are approaching each other with equal and opposite velocities, and bounce back after the collision with equal and opposite velocities. Nothing strange now!
 

Similar threads

  • · Replies 6 ·
Replies
6
Views
2K
  • · Replies 30 ·
2
Replies
30
Views
3K
  • · Replies 53 ·
2
Replies
53
Views
5K
  • · Replies 21 ·
Replies
21
Views
2K
  • · Replies 3 ·
Replies
3
Views
2K
  • · Replies 4 ·
Replies
4
Views
1K
  • · Replies 6 ·
Replies
6
Views
2K
  • · Replies 10 ·
Replies
10
Views
2K
  • · Replies 1 ·
Replies
1
Views
2K
  • · Replies 2 ·
Replies
2
Views
2K