# Why this incident happens? [closed]

If there's a rope which two sides are attached to the wall and a rigid body is hanged down from the rope (that body has light mass )in a such situation if a force is applied on the rope at any point on it then why that rigid body starts to rotate?

• This doesn't seem very clear to me. You might need to include a sketch or link to an example. Jul 14, 2018 at 12:13
• I've included one picture. Jul 14, 2018 at 12:24
• The rigid body is hanging down from the rope by a string (or something similar)? I would think the details of the mechanical connection to the rigid body might be important. What other evidence do you have that this happens, other than your sketch? Jul 14, 2018 at 12:31
• I assume only applying the force won't be enough to cause the rotation. If you pull the rope down, you have to also let go of it and the rope then snaps back in the other direction. Correct? Jul 14, 2018 at 12:32
• @KaustavGuhaRoy One possible scenario is that the force abruptly moves the suspension point diagonally - if the displacement is large enough, I think it might cause what you describe. Could you share a video of this experiment? Jul 15, 2018 at 7:35

The question is still a little unclear; however, two things I can think of are:

1. The downwards displacement of the horizontal rope results in a small change of the horizontal position of the connection between the rope and the string that the body is suspended from. This then imparts some sideways force to the suspended body, so that it starts to swing like a pendulum.
2. If the horizontal rope is being excited at some frequency, the suspended body may start to move if that frequency is close to its resonant frequency, even though the excitation force is vertical.

I think what you are describing is the plucked string spinner which used to be a popular home-made toy.

Instead of a mass on a string (like a simple pendulum), a button or washer or card works even better. The card should either have a large hole concentric with the centre of mass (like a washer) or a smaller hole which is slightly off-set from the centre of mass (like a button). The key factor is that the centre of mass should not coincide with the string (about which the object rotates), but neither should it be far away.

When the hanging mass/washer/card is moved up and down in a vertical plane there should in theory be no sideways motion. However, this kind of motion is unstable : very small sideways motion is rapidly amplified by the up and down motion and quickly become a rotation. The only stable position is with the mass above the string - which has to be replaced by a rigid rod. See Kapitza's Pendulum.

Something similar happens when you 'pump' yourself on a swing by alternately standing up and crouching down as the swing passes through the lowest point. With practice you can quickly execute a 360 degree swing - provided that the ropes/chains are replaced by rigid rods.

Another similar motion is spinning a hula hoop around your waist or wrist. After a slight push to start the rotation you can maintain or increase the speed using a very small forward-and-backward wiggle.

A similar toy is described in Vibration to Torque: Magic Motor. This converts the small amplitude vibration of a stick into a large circular motion of a propeller.

I think you are referring to the highly formalized case of a strand in a thread. It is true that broken strands rotate around the string if you pull it, but it has nothing to do with physics!

Strings are made by spinning strands together. It gets unwind-ed if you pull it. This causes the strands standing out to rotate!

• I am sure it has something to do with physics -- as Feynman says, everything has to do with physics. Jul 14, 2018 at 15:24