Why do bodies at rest weight more than zero?

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  • #26
Doc Al
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I have a point to make here, I dont deny that theoretically it makes perfect sense to say that acceleration exists in the absence of velocity,
in fact at zero velocity most equations would have negative acceleration.
Acceleration describes how velocity changes. You can certainly have zero instantaneous velocity and non-zero acceleration.

My point about the ball going up is simple, the earth continues to retard the ball even as it moves up and here the g acts as retardation.
OK, you can think of it that way. Gravity exerts a downward force on the ball, which produces a downward acceleration. It "retards" the ball on the way up, but that same force speeds the ball up on the way down.

When the retardation overcomes the acceleration of the ball,
:confused: The "retardation" is the acceleration of the ball. Perhaps you mean that the retardation/acceleration overcomes the velocity. (Sloppy terminology, but OK.)
the ball stops and thereby you would have the ball accelerating in the opposite direction.
After reaching its highest point, the ball starts to move in the opposite direction. The acceleration, 9.8 m/s^2 downward, hasn't changed.

Hence my point remains that at zero velocity, the acceleration of the ball would be equal to the retardation and the ball would stop, so acceleration here is discontinuous.
Again, this is meaningless: What you call the "retardation" is the acceleration, which is downward. (Realize that "acceleration" in physics doesn't just mean "speeding up"--it refers to changing velocity, whether speeding up or slowing down.)
 
  • #27
Doc Al
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Ok let me place my case very clearly here,
1. We have a force F which is exerted on the ball as the ball leaves the person's hand, hence the acceleration a on it and F=m*a, where m becomes the mass of the ball
When the ball loses contact with the hand, the hand no longer exerts its upward force on the ball.
2. There is a gravitational force f which is acting counter to this ball moving upward
Gravity is always acting downward. Once the force of the hand stops acting, the only force on the ball is the downward force of gravity. F= mg = ma, thus a = g (downward).
 
  • #28
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Right got the point thanks guys :)
 
  • #29
Borek
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But i think practically it makes no sense, thats like saying in the absence of velocity we can have acceleration.

Assuming we CAN'T have an acceleration in the absence of velocity is like stating "whatever doesn't move, won't move for ever".
 

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