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If I remember correctly, less than 10% of all IB physics candidates got it
right.
The net force at the bottom is *not* zero because the pendulum bob is
changing direction (a circular path). Hence there must be a net force
acting, in this case the net force = tension - weight. So the tension
force has greater magnitude than the weight of the pendulum bob.
Tangential acceleration at the bottom is indeed zero but normal
acceleration is not.
In other words, the NET FORCE, and thus the acceleration,
is zero at the bottom and non-zero at an angle, large or small.
Clearly it is moving the fastest at the
bottom and an instant later it must be moving slower.
This is a change in velocity - this is an acceleration. To
have an acceleration, a force must be present.