SHM - the max force of pendulum

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fakedoctor123

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Hello,

I have some conflicting sources on the maximum force of a pendulum. TBR physics, on chapter of periodic motion, page 229, says the max force occurs when the bob is straight down, but Chad's Videos says when the bob is straight down the force is zero. Can someone explain to me when the max force is on a pendulum and whether which of these sources is correct?

I am leaning towards Chad being correct and because on the next page of TBR, it shows that the acceleration is max when the bob is at its maximum displacement from equilibrium; thus, if acceleration is max then that also means the force is max.

Also, how does the PEpendulum = - mgL (1-cos theta), can't we just use PE= mgh?

TBR in the wrong here or am I mixing some concept up? Thank you for any help.

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I think it's important to distinguish WHERE the force is acting: the tension in the rope/bar holding the bob or the forces on the bob itself.
If you refer to the TENSION in the rope holding the bob, then it will be maximum when the bob is straight down because at this point the tension in the rope is opposing both the gravitational and centripetal forces. Hence: T = mg + mv^2/R
If you refer to the RESTORING FORCE acting on the bob, then it's maximum at greatest displacement as you said.
 
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