This might seem a stupid question but please help me out because I'm very confused.

  1. We say that when an object is dropped from a moving body (moving with constant acceleration $a$) It has the velocity $v$ which was possessed by the body, but its acceleration is $g$ only as soon as it is dropped. Why don't we consider that it has the acceleration of the moving body if we consider it to have its velocity?

  2. Please describe the net acceleration and the acceleration as observed by a person on the ground and a person on the moving object. Neglect air resistance.

P.S. I came across a very similar question but it didn't clear the doubts I had.


Why don't we consider that it has acceleration of the moving body ?

Because force acting on the main body which may be the reason of its acceleration is not acting on the object dropped

Although all the force that is still acting on object will cause acceleration like in the case of gravity

why we consider velocity same?

Because the object, when with the body ,was moving with that velocity and there is no sudden change in momentum or no Impulsive force acting on the object that would inhibit us from taking that velocity as an initial one

person on ground ( frame)

A parabolic motion with downward acceleration of g

person on moving object

Depends on its direction of acceleration, say, it is a(vector)

Then the acceleration of dropped object is g(vector) - a(vector)


In short: velocity is a non-effect, it is like rest, while acceleration is caused by interaction of the system with its environment.

When the initial system breaks in two, what is "nothing" does not change, so the velocity is the same in both new systems. Acceleration now depends for each new system on the net force applied to it.

In the case of an accelerating rocket dropping a piece of its structure for example, there is no engine attached to the dropped part so its acceleration is only due to gravity.

  • $\begingroup$ I find your explanation quite easy to get and pretty obvious! Thank you. $\endgroup$ – user144613 Apr 5 '17 at 7:21

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