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glueball8
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I don't understand how I can be at rest and then start walking or when a ball hits the wall it bounces back while the wall get no momentum at all?
Bright Wang said:I don't understand how I can be at rest and then start walking or when a ball hits the wall it bounces back while the wall get no momentum at all?
As you move forward with momentum +P, the ground/Earth is pushed back with momentum -P. Since the ground/Earth is so massive, you won't notice its backward movement. (As mathman and aerospaceut10 already explained.)Bright Wang said:So what about that I can walk from rest? How is the momentum conserved here?
Phlogistonian said:When you walk, you push against the earth. Theoretically, this gives it a very slight velocity in the opposite direction, but it's too small to be detected.
Cyrus said:"Theoretically"?... "velocity in the opposite direction?" ..."To small to be detected"...? ...
Hint:
[tex]m_ev_{e,1}+m_pv_{p,1}=m_ev_{e,2}+m_pv_{p,2}[/tex]
Do some algebraic manipulation and get this"
[tex]v_{1,e}-\frac{m_pv_{p,2}}{M_e}=v_{e,2}[/tex]
Now, how did you deduce the Earth moves "in the opposite direction?" Something should have yelled to you, wait a minute, that doesn't make sense by the sound of what I just wrote.
Phlogistonian said:Cyrus,
Ah, I see your point now. I didn't see it at first. Your point is still stupid.
I have to specify that the Earth is moving before I take a step? Why? What absolute reference frame are you using?
It's not nice to be rude.Cyrus said:"Theoretically"?... "velocity in the opposite direction?" ..."To small to be detected"...? ...
Doc Al said:As you move forward with momentum +P, the ground/Earth is pushed back with momentum -P. Since the ground/Earth is so massive, you won't notice its backward movement. (As mathman and aerospaceut10 already explained.)
aerospaceut10 said:Remember that it's mass times velocity to get that momentum figure, so if you have such a large mass, the Earth, it'll have to have an insignificant velocity, essentially, when your tiny mass moves at the given velocity.
mathman said:In the cases you mentioned there is momentum transfer to the Earth (via the wall in the second case). Since you and the ball are a lot less massive than the earth, the velocity effect on the Earth is extremely small.
Conservation of energy, linear momentum, and angular momentum are three distinct concepts. Suppose you take a drive in your car down the freeway, and wham, your car instantaneous reverses its velocity vector, throwing you against the steering column at 120 mph. The car's speed is 60 mph before and after the velocity reversal, so kinetic energy is conserved. Fortunately, that never happens because of conservation of momentum.Ed Aboud said:As far as I know, conservation of energy underlies conservation of momentum
What's your point? You seem to relish giving out more heat than light.Cyrus said:Ok, sorry if I was rude.
DH, I read what they wrote. They were using a reference frame of the Earth being stationary, I did not. In the case where its not stationary, the Earth does NOT move backwards.
I just showed this using the simple conservation of momentum equation. I have no problems with you stating I am wrong, but then show me where my equation is incorrect.
Doc Al said:What's your point? You seem to relish giving out more heat than light.
Of course, the Earth "moving backwards" is from the initial inertial frame in which everything started at rest. Duh! You do understand the concept of velocity and reference frames, don't you?
Bright Wang said:So what about that I can walk from rest? How is the momentum conserved here?
Momentum is a measure of an object's motion, and is equal to the product of its mass and velocity.
Momentum is always conserved because of the law of conservation of momentum, which states that the total momentum of a closed system remains constant.
In collisions, the total momentum of the system before and after the collision is the same. This means that the sum of the momenta of the objects involved in the collision remains constant.
Yes, momentum conservation applies to all types of collisions, including elastic and inelastic collisions.
Some examples of momentum conservation in everyday life include car crashes, ball games, and rocket launches.