Centripetal force demo dishonesty

[d4rr3n]

I agree with others. You should draw a free body diagram. Maybe then you'd stop thinking there is a tangential force, among other things.
Best would be to draw two: one in the rotating the other in non-rotating frame, as you keep on mixing the two.

Why does the shaft of a motor spin, I hope people are not going to say centripetal force or that there is some imaginary inward force pointing into the center of the motor shaft. The motor turns because the windings are produce a couple and it is turning about the center of that couple. Now put a wagon wheel on that motor shaft and magically people start talking about this inward pull. The wheel wants to expand, spin it fast enough and it will fly apart. Which force overcomes which in the end, which is more primary the inward or the outward?

 

[russ_watters]

If you put a rocket on a tether so it cannot fly in one direction but instead is forced to move in a circular path (we have all seen these fireworks right) which force is more primary the tether resisting the straight line motion of the rocket or the force provided by the propulsion. Which force is primary and which one is reactive?

Those two forces are not directly associated. The force that starts the linear/tangential motion is not at issue in any of these scenarios.

 

[d4rr3n]

Those two forces are not directly associated. The force that starts the linear/tangential motion is not at issue in any of these scenarios.

Ok I'm going to say the tangential force is the primary and these inward/outward force are only reactive secondary forces.

 

[jbriggs444]

I don't have difficulty in seeing the centripetal force in the case of an orbiting satellite that's why I said Vs a wagon wheel. Explain to me instead why the spokes of the wheel are in tension and not compression.

In a wagon wheel the spokes are made of wood and are in compression. The iron-shod rim of the wagon wheel is in compression. This holds the entire assembly together with no need for glue, nails or other fasteners. The construction of a wooden barrel held together by hoops is similar. The tension is normally created by pre-heating the rim (or hoops) before applying them to the spoke/hub assembly (or barrel staves).

In a bicycle wheel the spokes are made of steel and are in tension. The steel or aluminum rim of the bicycle wheel is in compression. The tension is normally created by tightening tiny nuts threaded onto the end of each spoke.

None of this has the slightest thing to do with rotation, centripetal or centrifugal force as they are used in classical physics.

 

[jbriggs444]

Ok I'm going to say the tangential force is the primary and these inward/outward force are only reactive secondary forces.

It is a false dichotomy. There is no such thing as a "primary" or a "reactive" force.

 

[ZapperZ]

Ok I'm going to say the tangential force is the primary and these inward/outward force are only reactive secondary forces.

There are NO tangential forces necessary for circular motion! Stop making things up as you go along or adding complications to this scenario!

Again, draw the free-body diagram, or at the very least, look it up!

Zz.

 

[russ_watters]

Ok I'm going to say the tangential force is the primary and these inward/outward force are only reactive secondary forces.

No. In constant speed motion, the tangential force is zero.

Whether on purpose or not, your adding of additional forces to the scenario is a red herring that is leading you away from understanding the issue.

 

[ZapperZ]

This standard derivation (which I believe can be found in many textbooks) starts NOT by assuming any kind of forces acting on the system, but rather from the motion of the particle in a uniform circular motion. In other words, if we have something moving uniformly in a circle, what is the acceleration (and thus, force) on the system?

http://ruina.tam.cornell.edu/Book/Chapter13_5.4.07.pdf

The result for the acceleration, shown in Eq. 13.6, and letting the second derivative of theta to be zero (for uniform circular motion), shows that the ONLY direction of acceleration is inwards for such a motion. This means that this is the direction of the applied force that caused this type of motion.

So without assuming in the beginning about the nature of the force acting on a uniform circular motion, one can already derive mathematically the direction of the acceleration and the force of the system.

Zz.

 

[gmar]

Khan academy has a good video lecture "Centripetal force and acceleration intuition"

He draws the diagrams and explains them.

https://www.khanacademy.org/science.../centripetal-force-and-acceleration-intuition