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Actually see this video first.

I'll try to explain what happened in the video :

When the pen is dropped in such a way that there is no gap inside the cap (or in between the tip of the cap and the bottom of the pen) then the pen bounces up when dropped.

But when the same pen is dropped in such a way that there is some gap inside the cap (or between the tip of the cap and the bottom of the pen) then the pen doesn't even go up a little. It kinda sticks with the ground which is strange.

So how does the gap inside the cap (or gap between the tip of the cap and the bottom of the pen) making such a huge difference?

Or

Why does the pen get stuck on the surface in the second case ?

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It takes work to jam the cap onto the pen so it seats without a gap. When the pen is dropped with a gap, two things happen: 1) the cap wants to continue falling even though the tip has struck the ground, and 2) meanwhile, the pen wants to bounce up and away from the ground after striking it.

This means the cap is exerting a downward force on the pen, and the pen is exerting an upward force on the cap. these opposing forces "snap" the cap tightly into place on the pen and the gap disappears.

That "snapping work" absorbs the energy in the pen bounce and instead of jumping back into the air, it stops dead.

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  • $\begingroup$ @Ruslan which spring are u talking about ? $\endgroup$
    – Ankit
    Commented Jun 11, 2021 at 2:47
  • $\begingroup$ @niels Nielsen if I take both as my system then the forces would be internal (right ?) So shouldn't it bounce up since the system has normal force acting up ? $\endgroup$
    – Ankit
    Commented Jun 11, 2021 at 5:10
  • $\begingroup$ yes but work has been done in pressing the cap onto the pen and that work is not recovered. Are you familiar with something called a "dead-blow hammer"? $\endgroup$
    – niels nielsen
    Commented Jun 11, 2021 at 5:32
  • $\begingroup$ @niels nielsen no .. what's that ? $\endgroup$
    – Ankit
    Commented Jun 11, 2021 at 7:04
  • $\begingroup$ @Ruslan didn't u see the video I linked ? $\endgroup$
    – Ankit
    Commented Jun 11, 2021 at 8:42

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