I open doors like this

Classical Mechanics Level 1

Is it possible to open doors by pushing on their hinges?

No Yes

2 solutions

Agnishom Chattopadhyay
Jun 15, 2015

Zero Torque is not likely to produce any change in angular momentum

A hard enough push breaks the hinge... The door opens.

Mark Leahy - 5 years, 10 months ago

The question didn't specify what angle from which you must push on the hinge. If you were pushing on the inner face of the hinge you could push the door open.

Taylor Spencer - 5 years, 10 months ago

You do need to explain why the torque is zero.

Venkata Karthik Bandaru - 5 years, 12 months ago

I do. But I am really sleepy today.

Can you add it for me?

Agnishom Chattopadhyay - 5 years, 12 months ago

I don't think I am patient enough. Well, you can add it later :).

Venkata Karthik Bandaru - 5 years, 12 months ago

$\tau = r \times F$ so when $r = 0$ the cross product torque is also the zero vector.

Jake Lai - 5 years, 12 months ago

Noel Lo
Jun 16, 2015

If you push on a door's hinges, the distance from the fulcrum is zero since the hinge itself is the fulcrum. We know moment = force $\times$ perpendicular distance from fulcrum. Since perpendicular distance from fulcrum is zero, you can apply as much force as you want but the door simply won't budge because the moment will be zero once the perpendicular distance from the fulcrum is zero.

The pivot of the hinge is the fulcrum, not the entire hinge. You could push on the flange of the hinge.

Taylor Spencer - 5 years, 10 months ago

I open doors like this

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