# B The force of the stone on the rope

1. Dec 29, 2017

### Luigi Fortunati

Is the force of the stone on the rope "centrifuge"?

Is the force of the stone on the rope "real" or "apparent"?

2. Dec 29, 2017

### Luigi Fortunati

The rope sling

3. Dec 30, 2017

### PeroK

What force, what stone, what rope?!

4. Dec 30, 2017

### Luigi Fortunati

The strength of the stone on the rope of the sling.

5. Dec 30, 2017

### jbriggs444

The terms that I learned are "centrifugal" and "fictitious".

Centrifugal: Literally meaning "directed away from the center".

Centripetal: Literally meaning "directed toward the center".

Real force: An ordinary physical force. It is present regardless of what reference frame is used. Real forces have third law partner forces.

Fictitious force: A force that is invented to allow Newton's second law to apply in an accelerated or rotating frame of reference. Fictitious forces do not have third law partner forces.

In the case of a stone being whirled in a sling there are three forces that may be considered.

1. Centripetal force. This is the real force of the sling on the stone. It is called "centripetal" simply because it is directed toward the center. https://en.wikipedia.org/wiki/Centripetal_force

2. Centrifugal reaction force. This is the real force of the stone on the sling. It is the third law partner force to the centripetal force. https://en.wikipedia.org/wiki/Reactive_centrifugal_force

3. Centrifugal force. This is the fictitious force. If one adopts a frame of reference that rotates with the sling then the stone is stationary. Yet it is still subject to the real centripetal force. In order to explain how it can remain motionless one invents a "centrifugal force". https://en.wikipedia.org/wiki/Centrifugal_force

Last edited: Dec 30, 2017
6. Dec 30, 2017

### Luigi Fortunati

Ok, if I understand correctly, there exists the "real" centrifugal force and there is also the "apparent" centrifugal force.

However, in the case of the slingshot, there are no "apparent" forces, right?

7. Dec 30, 2017

### jbriggs444

By "slingshot", you mean the Y-shaped device with rubber bands that boys use to shoot projectiles? And not the leather strap-and-pouch device that David used to slay Goliath?

Edit: If so, then we are considering the linear motion of a projectile as it is released and is accelerated under the force of the rubber bands. It rests on a pad during this acceleration phase. If one adopts a frame of reference where the stone is at rest while being accelerated then a rearward fictitious force can be considered. It explains why the stone is judged to be at rest [in this reference frame] while still experiencing the real force from the pad. https://en.wikipedia.org/wiki/Fictitious_force

Last edited: Dec 30, 2017
8. Dec 30, 2017

### Luigi Fortunati

Whirling sligshot, not Y-shaped device.

Last edited by a moderator: Jan 2, 2018
9. Dec 30, 2017

### jbriggs444

There is indeed an apparent force. It is the centrifugal force on the stone that appears to hold it in place in the sling as it is whirled. It is a fictitious force -- it only appears if we consider the stone to be "in place".

By considering the stone to be "in place", at rest in the sling we have implicitly adopted the rotating frame of reference where the sling is at rest.

Last edited: Dec 30, 2017
10. Dec 30, 2017

### Luigi Fortunati

11. Dec 30, 2017

### Luigi Fortunati

If it appears to "hold it" it is a "centripetal" force, not "centrifugal"!

12. Dec 30, 2017

### jbriggs444

The force that holds the stone in place against the inward/centripetal force is outward/centrifugal.