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View Full Version : Newton's 3rd law / instantaneous acceleration


Hiro
Aug 4, 2008, 01:20 PM
If for every action there is an equal and opposite reaction then when an object A strikes object B shouldn't the force involved be infinitely reflected between the two objects?

On the same note, does the opposite reaction bit mean that when an object is acted upon it will move with twice the force it was hit with?


As for instantaneous acceleration. This name sounds like a misnomer, it seems to me illogical that something can have acceleration that is not measured between 2 points. Can anyone explain this to me?

Capuchin
Aug 4, 2008, 02:47 PM
If for every action there is an equal and opposite reaction then when an object A strikes object B shouldn't the force involved be infinitely reflected between the two objects?

I'm not sure what you mean here. Can you clarify?


On the same note, does the opposite reaction bit mean that when an object is acted upon it will move with twice the force it was hit with?

Again this is a bit unclear to me. When one object hits another object. Object A produces a force on Object B. The law states that Object B produces the same amount of force on Object A. I suppose, from the point of view of object A, if the force were to all go into velocity, then object B would have twice the applied force. But to an outside observer it's fairly obviouis that this is not so.


As for instantaneous acceleration. This name sounds like a misnomer, it seems to me illogical that something can have acceleration that is not measured between 2 points. Can anyone explain this to me?

This is just to help with the calculation, of course no force is applied instantaneously, because then the impulse (=Ft) would be infinite. It's the same when you assume perfectly spherical objects, or frictionless environments. It's just to help with the calculation, and is a decent approximation to the real world situation.

Hiro
Aug 5, 2008, 01:08 PM
When an object hits another object the 1st object will transfer its kinetic energy to the 2nd object. In turn, newton's 3rd states that the 2nd object will react by acting on the 1st object with the same amount of force.

However if Newton's 3rd law is true then shouldn't the 1st object react by acting upon the 2nd object with the same amount of force that the 2nd used on it? And so on it goes.

Capuchin
Aug 5, 2008, 02:00 PM
when an object hits another object the 1st object will transfer its kinetic energy to the 2nd object. In turn, newton's 3rd states that the 2nd object will react by acting on the 1st object with the same amount of force.

However if Newton's 3rd law is true then shouldn't the 1st object react by acting upon the 2nd object with the same amount of force that the 2nd used on it? and so on it goes.

In a word, no.

Remember that the collision is instantaneous. The objects come into contact, object a produces a force on object b and object b produces a force on object a, these forces make the objects move apart.