Horizontal force on a pendelum - what drives it upwards?

[Nikitin]

Two questions:

1) If you have a horizontal force in the x direction on a pendelum supported by a wire, then it will go upwards and in the x-direction. What is the vertical component which drags it upwards?

Is it the normal force which adjusts itself along the wire to match the horizontal force?

2) During the act of motion, will the speed be constant if the force isn't too big? Ie, will the normal force adjust itself in such a manner that it matches the horizontal force and gravity in magnitude?

 

[HallsofIvy]

Two questions:

1) If you have a horizontal force in the x direction on a pendelum supported by a wire, then it will go upwards and in the x-direction. What is the vertical component which drags it upwards?

Is it the normal force which adjusts itself along the wire to match the horizontal force?

Yes, if I understand "normal force" to mean "force normal to the motion", this is just the tension force in the wire.

2) During the act of motion, will the speed be constant if the force isn't too big? Ie, will the normal force adjust itself in such a manner that it matches the horizontal force and gravity in magnitude?

It should be obvious that this is NOT true. Since the bob of the pendulum is moving in a vertical circle, its height, and so potential energy, changes. By "conservation of energy", its kinetic energy, and so its speed, must change. (And, of course, it has to come to a stop in order to go back the opposite direction.)

 

[pbuk]

1) If you have a horizontal force in the x direction on a pendelum supported by a wire, then it will go upwards and in the x-direction. What is the vertical component which drags it upwards?

Gravity acts downwards and the applied force only has an x component: what other force is acting on the pendulum bob? What direction does it act in - so what is its upwards component?

2) During the act of motion, will the speed be constant if the force isn't too big? Ie, will the normal force adjust itself in such a manner that it matches the horizontal force and gravity in magnitude?

Initially the pendulum is at rest. What is the speed after 1 second? What about after 0.001 seconds?

To understand what happens you really need to look at the direction of the forces and the velocity of the pendulum bob. Can you draw vectors to represent the forces at any point in time?

 

[Nikitin]

It should be obvious that this is NOT true. Since the bob of the pendulum is moving in a vertical circle, its height, and so potential energy, changes. By "conservation of energy", its kinetic energy, and so its speed, must change. (And, of course, it has to come to a stop in order to go back the opposite direction.)

I meant while the horizontal force is still active.

mranchovy: yeah, I'll do that later.