The acceleration of a moving pendulum

[Kyoma]

A pendulum bob is released from a height in a non-ideal situation (that's there is friction). What I don't get it is the fact that the acceleration of the pendulum is actually constant. Why?

 

[Drakkith]

I'm not quite up to speed on pendulums, what do you mean by the acceleration is constant?

 

[Kyoma]

The acceleration of the bob is the same throughout its journey. So, if a = 0, then it will be zero throughout its journey.

 

[Drakkith]

My understanding is that a pendulum dropped from point X has an intial acceleration due to gravity that depends on the angle of the pendulum from its pivot point. The closer to horizontal it is, the greater the inital acceleration. Once it passes that, the accelerations lessens and at the vertical point the acceleration is zero. After passing veritcal, the deceleration increases up to horizontal and then decreases after that. If the pendulum was dropped initially from below horizontal, the deceleration simply decreases until the pendulum reverses direction and then increases again.

That is my understanding of it. Is there something I've missed here?

The acceleration of the bob is the same throughout its journey. So, if a = 0, then it will be zero throughout its journey.

I don't see how the acceleration could be zero. If it was the pendulum wouldn't move at all.
Are you talking about the increase or decrease in acceleration?

 

[Vanadium 50]

If the acceleration is the same, it won't go back and forth, so it's not much of a pendulum.

 

[chrisbaird]

The total acceleration is not constant. The acceleration due to gravity is constant, but the acceleration due to the force exerted by the pendulum's pivot point keeps changing direction. The total acceleration is the sum of gravity and that from the pivot point and it keeps changing. If pivot was cut so that it could no longer provide a force, the pendulum would just keep falling.