Here, let’s say I have a tea cup.

enter image description hereIn that cup , let us say there is a ball inside the cup which is temporarily fixed to the surface of cup. Now , this ball is fixed very loose but strong enough that if I tilt the cup downwards , it doesn’t fall.

Now , what I noticed is that if I tilt my cup face in downward direction and then I move my cup down with a haste while holding the cups handle. The ball falls down.

I did not apply any pressure or force on the ball from the top of cup or from below the cup. How is it that the ball falls down ?.

enter image description here

What I think is that if I Look at this movement from an inertial frame.

1)I can notice if the cup moves down with 5m/s , then so does the ball moves down with same speed. Also , there is a $9.8m/s^2$ acceleration acting all the time. Due to these two factors , it contributes to more acceleration I.e more Force since F=m*a.

  1. There should be a force from the air molecules in upward direction . Due to Newton’s third law , the ball pushes the air down. Therefore , also forces itself to fall down.

This is all what I think should happen.

Please let me know if you have any other suggestion or somewhere I went wrong.


1 Answer 1


The ball gathers velocity and when you decelerate it the inertia results in the development of forces that separate it from the cup.

Basically the inertia force which is required to develop the deceleration exceeds the maximum force of the bond between the ball and the cup.

  • $\begingroup$ Ok. Thanks a lot. $\endgroup$
    – S.M.T
    Jun 11, 2021 at 17:38
  • $\begingroup$ Right. Because unless and until I don’t stop my cup after moving. The ball doesn’t fall till that time. $\endgroup$
    – S.M.T
    Jun 11, 2021 at 17:51
  • 2
    $\begingroup$ yes. Also, the ball falls only when you decelerate the cup hard enough $\endgroup$
    – NMech
    Jun 11, 2021 at 17:52

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