# Direction of the net force acting on a pendulum

• songoku
Thank you for the correction. In summary, the conversation discusses the forces acting on a pendulum and the direction of the net force. It is determined that tension must be greater than W sin θ to provide centripetal force and the resultant of the force in the y-axis is in the direction of A and the resultant of the force in the x-axis is in the direction of C. The direction of the net force can be anywhere between A and C, depending on the position of the pendulum. The answer key (C) may be incorrect because it assumes the pendulum is either at the peak of its swing or at the top of its swing, when in reality it is at an intermediate point. Additionally, the statement about
songoku
Homework Statement
The picture shows a pendulum in its upward swing, that is, the velocity vector for the pendulum is pointing in the direction of E. What is the direction of the net force acting on the pendulum?
Relevant Equations
Not sure

I imagine y - axis is parallel to direction of A and x - axis is parallel to direction of E. There are two forces acting on the pendulum: tension in direction of A and weight in direction of D.

I break the weight into 2 components: W sin θ in opposite direction to tension and W cos θ in direction of C (where θ is angle between D and C). Tension should be bigger than W sin θ to provide centripetal force so the resultant of force in y - axis is in the direction of A and resultant of force in x - direction is in direction of C hence the direction of net force will be B.

But the answer is C. Where is my mistake?

Thanks

songoku said:
Tension should be bigger than W sin θ to provide centripetal force so the resultant of force in y - axis is in the direction of A
Yes.
songoku said:
resultant of force in x - direction is in direction of C
Yes.
songoku said:
hence the direction of net force will be B.
You seem to be showing B as horizontal. The direction could be anywhere between A and C.
songoku said:
But the answer is C.
Only if it is at the peak of the swing.

songoku and PeroK
haruspex said:
You seem to be showing B as horizontal. The direction could be anywhere between A and C.

But for this case, B is the only logical answer, right?

Only if it is at the peak of the swing.

So the answer key (C) is wrong because from the diagram it is not at its peak?

Thanks

songoku said:
But for this case, B is the only logical answer, right?
So the answer key (C) is wrong because from the diagram it is not at its peak?

Thanks

Yes, I'd say that if it's a multiple choice and it has to be one of the answers, then the pendulum must (logically) be just at the point where the acceleration is horizontal.

Either that or it is supposed to be at the top of its swing, when the answer would be C.

I suspect the question setter forgot that acceleration is not necessarily in the instantaneous direction of motion.

songoku
PeroK said:
Yes, I'd say that if it's a multiple choice and it has to be one of the answers, then the pendulum must (logically) be just at the point where the acceleration is horizontal.

Either that or it is supposed to be at the top of its swing, when the answer would be C.

I suspect the question setter forgot that acceleration is not necessarily in the instantaneous direction of motion.

Yes it is multiple choice question.

By " top of its swing" you mean when the string is horizontal?

Thanks

songoku said:
By " top of its swing" you mean when the string is horizontal?

No. When it reaches its highest point.

songoku
PeroK said:
No. When it reaches its highest point.

When the string is horizontal, the tension will be directed to the left and weight downwards so the resultant will also be C, right?

Thanks

songoku said:
When the string is horizontal, the tension will be directed to the left and weight downwards so the resultant will also be C, right?

Thanks

If the string reaches the horizontal just as the pendulum stops, there will be no tension.

Anyway, the diagram clearly shows the pendulum at an intermediate point (which is what you would expect). With the string neither vertical nor horizontal.

Also, the statement about the velocity vector pointing in the direction of E infers that the pendulum is still moving upwards.

songoku and jbriggs444
PeroK said:
Also, the statement about the velocity vector pointing in the direction of E infers that the pendulum is still moving upwards.
Nicely noticed.

However "implies" is what the writer does while "infers" is what the reader does. The former would be appropriate here.

PeroK
PeroK said:
If the string reaches the horizontal just as the pendulum stops, there will be no tension.

I see, miss this point. No tension if it stops and there is tension if it keeps moving

Thank you very much for the help Perok and haruspex

jbriggs444 said:
Nicely noticed.

However "implies" is what the writer does while "infers" is what the reader does. The former would be appropriate here.

Yes, I thought it didn't sound quite right.

## What is the direction of the net force acting on a pendulum?

The net force acting on a pendulum is always directed towards the center of the pendulum's swing, which is also known as the equilibrium point. This force is often referred to as the restoring force, as it works to bring the pendulum back to its equilibrium position.

## How does the direction of the net force affect the motion of a pendulum?

The direction of the net force determines the direction of the pendulum's motion. When the force is directed towards the center, the pendulum will swing back and forth in a regular pattern. But if the force is directed away from the center, the pendulum's motion will be disrupted and it may not swing as expected.

## Does the direction of the net force change during a pendulum's swing?

Yes, the direction of the net force changes as the pendulum swings. As the pendulum moves away from its equilibrium position, the force will change from being directed towards the center to being directed away from the center. This change in direction is what causes the pendulum to swing back and forth in a continuous motion.

## How does the length of a pendulum affect the direction of the net force?

The length of a pendulum does not directly affect the direction of the net force. However, the length does impact the speed and period of the pendulum's swing, which in turn can affect the direction of the net force and the pendulum's motion. A longer pendulum will have a longer period and a slower swing, while a shorter pendulum will have a shorter period and a faster swing.

## Can the direction of the net force be changed?

Yes, the direction of the net force can be changed by altering the factors that affect it, such as the length or weight of the pendulum. For example, if the weight at the end of the pendulum is changed, the direction of the net force will also change, potentially impacting the pendulum's motion.

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