337Hello.....anyone???
Newton's 3rd law states that to every action there is an equal and opposite reaction on two different bodies....
But,
A ball has mass m. it goes and hits a wall with f=ma. let this force be f1.
now change the frame of reference....
we are travelling along with the ball. so for us, ball is at rest and wall is in motion. After sometime, the wall hits the ball with f=Ma if mass of wall is M. Let this force be f2.
According to 3rd law, f1 should be equal to f2 but here it can be seen that f1 ≠f2.......
So, where is the problem?
-
UP 0 DOWN 0 2 8
8 Answers
1161There are so many loop holes in this question.
1) Whether ball is moving with constant velocity or not?
2) If yes then we have to apply psudo force when we r seeing from ball frame.
3) During impact the force that come in picture is not f = ma, rather impulse(a very large force for very short time ..yet not constant) comes in picture, so we cant put f=ma there.
4)You give me on logic that why f1 anf f2 is not equal?
- Sayan Sinha Sir,
For nos. 1 and 2:
The ball is moving with constant velocity. But, why should we apply a pseudo force if it is so? [We assume all these things are taking place in vacuum.]
For no. 3, a=(final velocity of the ball or the wall [as the case may be] - 0)/time taken to retard. Time taken to retard is a very small fraction of a second.
For no. 4, f1 is not equal to f2 because in f=ma, 'a' is equal for both the frame of references as stated above in the explanation of point number 3, but in the first case, m is of the ball while in the second case, m is of the wall.
Thank You.Upvote·0· Reply ·2013-01-24 08:46:07
226The forces acting between two objects do not depend on the coordinate system or referemce frame you use to measure them. So the third law always holds.
The second law is different: the apparent accelerations that you measure DO depend on the reference frame, hence the second law does not hold in a accelerating frame unless you add another term to the equation (a.k.a a fictitious force) to compensate for the acceleration of the frame.
- Sayan Sinha We are on earth and our frame of reference is such that the earth appears at rest. But if you see from the sun, the earth is also accelerating (as it is rotating). So, if our frame of reference is the wall and the ball hits the wall, then too, the frame of reference is an accelerated frame of reference as the wall is rotating along with the earth. Now, if you see from the center of the Milky Way, the sun is accelerating and if you see our galaxy from Andromeda, the entire Milky Way is accelerating. So everything is in an accelerated frame of reference. THERE IS NO PRACTICAL EXISTENCE OF A NON-ACCELERATING FRAME OF REFERENCE. In that case, Newton's laws should not be applicable anywhere.
- Abhas Agarwal non accelerated frame of reference is a frame which is either at rest or uniform velocity . .w.r.t another frame
and when u move from a inertial frame of reference to a non inertial frame of reference u feel a force known as IMPULSIVE INERTIAL FORCE OR PSEUDO FORCE it's direction is always opposite to that of acceleration
- Sayan Sinha Can you give me an example of a non-accelerating frame of reference?
226WHEN U R IN A TRAIN AND THE TRAIN IS MOVING WITH A UNIFORM VELOCITY SAY V KM/HR OK THIS IS NON ACC. FRAME OF REFERENCE
337But I will say no. If you see from the sun, the train is revolving around the earth and so, the train is accelerating too.
Can you tell me another thing? In my question, I had considered the ball as my frame of reference in the second case. In the first case, the wall was the frame of reference. Was this frame of reference (the wall) a non-accelerating frame of reference (according to your logic)?
Thanx for your reply...
226the earth is not accelerating it is moving with uniform acceleretion and so the earth is not feeling any pseudo force
Manish Shankar earth is accelerating. The amount of pseudo force is very small. So we neglect it
337"uniform acceleretion" is also a form of accelereation.....right?
And I can also prove that it is not uniform acceleration.....because ... Earth moves in an elliptical orbit....