# Net force on a Swinging Pendulum

Hello everyone,

Consider a weight tied to a string spinning in a horizontal circle.

According to Newton's third law, the force of the string on the weight (inward) is opposite and equal to the force of the weight on the string (outward).

If this is the case, how can there be a net acceleration inward?

Thanks.

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Dale
Mentor
The forces in a third law pair act on different objects, so you don't sum them to get a net force.

A.T.
how can there be a net acceleration inward?
Because the sum of all forces on the weight is inward.

sophiecentaur
Gold Member
Hello everyone,

Consider a weight tied to a string spinning in a horizontal circle.

According to Newton's third law, the force of the string on the weight (inward) is opposite and equal to the force of the weight on the string (outward).

If this is the case, how can there be a net acceleration inward?

Thanks.
The speed of the mass in a tangential direction does not change but the direction does. That means the only force is inwards towards the centre. That makes the acceleration radial as well.
This only applies if there is no extra forces applied at the time - the string is just going around under its own steam. But a totally flexible string can only transmit a force along its length so getting it up to speed, you need to be moving your hand round in a circle; things get more complicated then.