Net force on a Swinging Pendulum

[Conservation]

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.

 

[Dale]

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]

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.

 

[PJC01]

I would like to clarify that the net force on a swinging pendulum is not necessarily inward. The net force on an object is the sum of all the forces acting on it. In the case of a swinging pendulum, there are two forces acting on the weight - the tension force from the string and the force of gravity. These forces are in opposite directions, with the tension force pulling the weight inward and the force of gravity pulling it downward. The net force on the weight is the vector sum of these two forces.

In the case of a horizontal circle, the weight is constantly changing direction, which means it is constantly accelerating. This acceleration is caused by the net force on the weight, which is a combination of the tension force and the force of gravity. The tension force provides the centripetal force needed for the weight to move in a circular path, while the force of gravity provides the downward force that keeps the weight from flying off in a straight line.

So, to answer the question, the net acceleration inward is not due to a single force, but rather the result of the combination of multiple forces acting on the weight. This is in accordance with Newton's second law, which states that the net force on an object is equal to its mass multiplied by its acceleration.