Why Isn't Momentum Conserved on All Axes in a Bouncing Ball Scenario?

  • Context: Undergrad 
  • Thread starter Thread starter ZxcvbnM2000
  • Start date Start date
  • Tags Tags
    Law Momentum
Click For Summary

Discussion Overview

The discussion revolves around the conservation of momentum in the context of a bouncing ball scenario and collisions between two balls. Participants explore the implications of external forces, the law of restitution, and the conditions under which momentum is conserved or not, focusing on both theoretical and practical aspects of these concepts.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant asserts that momentum is conserved only when no external forces act on the system, questioning why momentum conservation cannot be applied on the y-axis during a bounce.
  • Another participant agrees, explaining that the reaction force from the ground is an external force that affects the momentum of the ball, leading to a reversal of direction upon bouncing.
  • A participant expresses confusion about their restitution formula, suggesting that their system only includes the ball and not the ground, leading to a misunderstanding of the coefficient of restitution.
  • In a hypothetical scenario involving two colliding balls, a participant questions why momentum is not conserved along both the line of impact and the perpendicular axis, despite both balls being part of the system.
  • Another participant challenges the assumption that momentum isn't conserved in all directions, prompting further inquiry into the conditions of momentum conservation.

Areas of Agreement / Disagreement

Participants express differing views on the application of momentum conservation in various scenarios, with some asserting that external forces disrupt conservation while others question the assumptions behind these claims. The discussion remains unresolved regarding the specific conditions under which momentum is conserved or not.

Contextual Notes

Participants highlight limitations in their understanding of the system boundaries, the role of external forces, and the application of the law of restitution. There are unresolved mathematical steps and assumptions regarding the definitions of the systems involved.

Who May Find This Useful

This discussion may be useful for students and enthusiasts of physics, particularly those interested in mechanics, collision theory, and the principles of momentum and restitution.

ZxcvbnM2000
Messages
62
Reaction score
1
Hello everyone , here we go :

I understand that momentum is always conserved as long as no external forces act on a given system . I also understand that the law of restitution applies to the axis perpendicular to the line of impact between two objects .However i have a problem .

For example . A ball hits the ground at an angle of 45 degrees and bounces up at an angle of 30 degrees.

So from the law of restitution : e= - ( Vball*sin30 - Vground)/(Uball*sin45 - U ground) , the ground does not move therefore Uground = Vground=0 so

e = - Vball*sin30/(Uball*sin45).

My question is , why can't we apply momentum conservation on the y-axis ? Is it because the reaction force when hitting the ground is considered an external force ?

I am very confused please explain :S

Thank you !
 
Physics news on Phys.org
ZxcvbnM2000 said:
My question is , why can't we apply momentum conservation on the y-axis ? Is it because the reaction force when hitting the ground is considered an external force ?
Exactly. If you look at the ball itself, its direction reverses when it bounces. So obviously, momentum of the ball is not conserved--it's being smacked by the ground!

If you expand your 'system' to be 'ball + ground/earth', then momentum will be conserved again. (The force between ground and ball would then be an internal force.)
 
So my restitution formula is wrong and the only thing i have in my "system " is the ball , no ground etc . So it should be e= - V*sin30/(u*sin45) but e can't be negative ...argh ! :S
 
Let's assume that two balls(m1=m2=m ) collide with one another .

The first ball is traveling at a speed u while the second ball is stationary.The first ball strikes the second one at an angle θ to the line of impact.If the coefficient of restitution is e , find the angle at which the second ball travels after the impact.

What i don't understand in this case is why isn't momentum conserved both along the line of impact and on the axis perpendicular to it as well ? I mean , both balls are part of our system so there are no external forces ?!

Could you please solve this exercise and explain each step thoroughly so i can finally understand ? Thank you very much !
 
ZxcvbnM2000 said:
What i don't understand in this case is why isn't momentum conserved both along the line of impact and on the axis perpendicular to it as well ? I mean , both balls are part of our system so there are no external forces ?!
Why would you think that momentum isn't conserved in all directions?
 

Similar threads

High School Why Do Objects Bounce? Exploring Momentum and Energy Conservation
  • · Replies 6 ·
Replies
6
Views
3K
High School Why is KE not conserved when momentum is?
  • · Replies 53 ·
2
Replies
53
Views
6K
High School How to characterize a rubber ball moving horizontally and bouncing off of a vertical wall?
  • · Replies 8 ·
Replies
8
Views
2K
Undergrad Forward momentum of mass due to inertia
  • · Replies 4 ·
Replies
4
Views
1K
Undergrad Momentum is not conserved with elastic bouncing ball?
  • · Replies 2 ·
Replies
2
Views
2K
Undergrad Is Mechanical Energy Conservation Free of Ambiguity - follow up
  • · Replies 77 ·
3
Replies
77
Views
7K
Undergrad Why is friction minimal in a non-ideal ball bounce situation?
  • · Replies 10 ·
Replies
10
Views
2K
Undergrad Conceptual understanding of moment transfer for bouncing ball
  • · Replies 1 ·
Replies
1
Views
2K
Angular momentum conservation on a merry-go-round
  • · Replies 5 ·
Replies
5
Views
3K
Undergrad Newton's Cradles vs Ball Bouncing off Wall
  • · Replies 6 ·
Replies
6
Views
4K