# Moving Pendulum

When a pendulum is moving

will the length of the string that the bob is attached to appear to be contracted?I have this doubt as the string is sometimes in direction of motion and at one point absolutely not in the direction of motion

## Answers and Replies

Hootenanny
Staff Emeritus
Science Advisor
Gold Member
Appear to be contracted from whom's point of view?

From an observer's point of view whoo is in an inertial frame.If he wants to calculate the time period of the pendulum,how will he measure the length as it changes from position to position

Please answer!!:D

Please sum1 help me out here!

When a pendulum is moving

will the length of the string that the bob is attached to appear to be contracted?I have this doubt as the string is sometimes in direction of motion and at one point absolutely not in the direction of motion
Each infinittesimal portion of the string is a linie segment which is moving perpendicular to its motion. According to SR the length of the string will remain unchanged.

Pete

Doc Al
Mentor
I assume that the OP meant that the swinging pendulum was in a moving rocket (say) being observed from the earth.

I assume that the OP meant that the swinging pendulum was in a moving rocket (say) being observed from the earth.
That's the usual way. Another way is to to have a charged particle on a string at a fixed distance from a charged plate. But you're right, he was probaby speaking of in a grav-field or in a rocket shift.

Pete

I assume that the OP meant that the swinging pendulum was in a moving rocket (say) being observed from the earth.

No,i just meant to have an experiment of a pendulum,moving at a constant velocity relative to a frame.Now the string of the pendulum,once is perpendicular to the direction of motion,and when it is at its maxima,it has some component in the direction of velocity,so my question was,wouldnt this affect the length of the string?

Also the time period as viewed from another rest frame would be the
$$T = \gamma T_0$$?

where T is the time period as viewed by the rest frame and $$T_0$$ ,$$T$$the time period as viewed by the moving frame.

Last edited:
PLease help.I would really ,like to know whether the above reasoning is correct or not!