Newtons third law and collision of bodies

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Discussion Overview

The discussion revolves around the application of Newton's third law in various collision scenarios involving bodies of different masses. Participants explore how momentum and energy transfer occur during these collisions, questioning the outcomes based on the masses involved.

Discussion Character

  • Exploratory
  • Debate/contested
  • Technical explanation

Main Points Raised

  • One participant describes a scenario where a mass m collides with a mass 2m, noting a change in direction, while a collision with another mass m results in the first mass stopping.
  • Another participant questions the application of Newton's third law in these scenarios, suggesting inconsistencies in the expected outcomes.
  • Several participants explain that during elastic collisions, kinetic energy is transferred, and the resulting velocities depend on the relative masses of the colliding bodies.
  • There is a discussion about how the conservation of momentum and energy applies differently depending on the mass ratios involved in the collisions.
  • One participant provides examples involving pushing a friend on chairs and kicking a football to illustrate the forces at play during collisions.
  • Another participant emphasizes that the forces exerted during collisions are always equal and opposite, regardless of the resulting motion of the masses.
  • Questions arise about the factors influencing the transfer of kinetic energy between colliding bodies, with a focus on mass ratios and the nature of the collision.

Areas of Agreement / Disagreement

Participants express differing views on the implications of Newton's third law in specific collision scenarios. While some agree on the principles of momentum and energy conservation, the application and interpretation of these principles in the context of different mass collisions remain contested.

Contextual Notes

Limitations in understanding arise from assumptions about the nature of collisions (elastic vs. inelastic) and the specific conditions under which the scenarios are analyzed. The discussion does not resolve these complexities.

Who May Find This Useful

This discussion may be useful for students or individuals interested in understanding the principles of mechanics, particularly in relation to collisions and the application of Newton's laws.

caffe
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Hi friendz
I m new to this forum
I have a problem in a question which is as follows-
If a body of mass m collides with a body of mass 2m it changes its direction of motion buy when it collides with a body of same mass m it stops and when it collides with a body of mass m/2 it does not change its direction of motion but there is only change in its speed.But when we do this by Newtons third law the third case does not prove true.If you got the answer then reply me fast.
Thanks a lot.
 
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Welcome to the Physics Forums!

One can give better help if you give some more detail how the Newton's 3rd law seems troublesome.
 
If a body of mass m collides with a wall it changes its direction because when it collides with wall it transfer its energy to the wall but due to wall's elastic nature it transfer its energy back to a body due to which it get reaction and when it collides with a body of same mass then on collision it tranfer its tranfer its all energy and the rest body start moving and first body stop due tranfer of all its energy.Then please tell me third case with same with the same energy concept.
 
caffe said:
But when we do this by Newtons third law the third case does not prove true.
Newton's 3rd law is true regardless of the masses involved. Why do you think it doesn't work in the third case?
 
sir please tell me how it works in third case
 
caffe said:
sir please tell me how it works in third case
How what works? Newton's third law works exactly the same in all cases.

In terms of energy, when the mass collides elastically with a stationary mass of lesser value, some (but not all) of its kinetic energy will transfer to the other mass. It continues to move in its original direction, just at a lower speed.
 
sir please read this two examples- when you and your friend sit on two.chairs (both of same mass)and you push your friend's forward then you got equal and opposite force in backward direction.but when a body of mass m strikes with a body of same mass m it stops why.another w
example when you kick a football with a force F it gives reaction on your foot with equal and opposite force but when a body of mass M collides with a body of mass M/2 it get a small reaction due to which there is small change in its speed.why? sir please tell me what i am missing.and reply soon.
 
caffe said:
sir please read this two examples- when you and your friend sit on two.chairs (both of same mass)and you push your friend's forward then you got equal and opposite force in backward direction.
Right.
but when a body of mass m strikes with a body of same mass m it stops why.
They exert equal and opposite forces on each other, just like always. Note that the change in momentum of each mass is equal and opposite. While one comes to rest, the other starts moving. In this particular case, things work out just right (due to conservation of momentum and energy) so that the first mass comes to rest.
another w
example when you kick a football with a force F it gives reaction on your foot with equal and opposite force but when a body of mass M collides with a body of mass M/2 it get a small reaction due to which there is small change in its speed.why?
In both case, Newton's 3rd law applies. When mass M collides with mass M/2, they exert equal and opposite forces on each other. In this particular case (due to conservation of momentum and energy) the first mass just slows down while the second mass speeds up.

The effect of the collision on the resulting velocities will depend on the relative masses of the bodies colliding. But Newton's 3rd law always applies.
 
Whether m stops or not depends on your frame of reference. The third law is really about the conservation of momentum. If you think about it in those terms it will make more sense.
 
  • #10
But as we know that when we kick a football with a force f (such that mass of foot is M then in opposite it gives us reaction but when a box of same mass M collide with a body of mass M/2 and apply a force F then it does not get equal reaction because if it gets equal reaction then it must stops running.
 
  • #11
caffe said:
But as we know that when we kick a football with a force f (such that mass of foot is M then in opposite it gives us reaction but when a box of same mass M collide with a body of mass M/2 and apply a force F then it does not get equal reaction because if it gets equal reaction then it must stops running.
Why do you think this? The forces that two colliding objects exert on each other are always equal and opposite.
 
  • #12
O.K sir I got the point.And thanks for reply on my questions.Thank you very... much
 
  • #13
On what factor the transference of kinetic energy from one body to another body depends on colliding bodies.
 
  • #14
caffe said:
On what factor the transference of kinetic energy from one body to another body depends on colliding bodies.
It depends on the relative sizes of the two masses. By applying conservation of momentum and energy, you can figure out the final velocity of each mass in terms of their initial velocities. (At least for simple, straight-line collisions.)

(Note: It also depends on the nature of the collision, whether energy is conserved or not. The examples in this thread all assume a perfectly elastic collision.)
 
  • #15
Thanks a lot...
 
  • #16
You might find this helpful. It describes the special case of head-on elastic collisions where the target object is initially at rest: Elastic collisions, target at rest
 

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