Why didn't the pen bounce up when dropped for the second time?

Question

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 ?

score 3 · Accepted Answer · answered Jun 10 '21 at 20:07

3

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.

answered Jun 10 '21 at 20:07

niels nielsen

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@Ruslan which spring are u talking about ? – Ankit Jun 11 '21 at 02:47
@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 ? – Ankit Jun 11 '21 at 05:10
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"? – niels nielsen Jun 11 '21 at 05:32
@niels nielsen no .. what's that ? – Ankit Jun 11 '21 at 07:04
@Ruslan didn't u see the video I linked ? – Ankit Jun 11 '21 at 08:42
@Ankit ah, OK, I misinterpreted what I saw. – Ruslan Jun 11 '21 at 08:43

Why didn't the pen bounce up when dropped for the second time?

Or

1 Answers1