Rotating planet moving through space

[Bofors]

Its velocity, Vp, is towards some other part of the universe, It rotates in the same plane as its direction of travel. The linear velocity of an object on the surface of the planet relative to the centre is Vr. That means the object's total linear velocity is Vp+Vr when it's at one point in its path, and Vp-Vr when it's directly opposite. That means it's accelerating and decelerating. But where is the force that causes that? (I know there's a flaw in my reasoning somewhere, but I can't spot it).

 

[DrStupid]

But where is the force that causes that?

That's the sum of gravity and (electromagnetic) normal force.

 

[RPinPA]

That's the sum of gravity and (electromagnetic) normal force.

Specifically the forces that make a solid a solid. You can ask this same question about any rotating solid object on earth. Or Earth itself. We are being pushed toward the sun at sunrise and away from the sun at sunset. What caused the acceleration?

 

[Dale]

That means it's accelerating and decelerating. But where is the force that causes that?

The combination of gravity and the normal force. In other words, the centripetal force.

Edit: @DrStupid and @RPinPA for the win!

 

[Bofors]

Thanks for the replies. But so far as I'm aware, neither gravity nor centripetal force can cause any acceleration or deceleration in a direction tangential to the circle of rotation.

 

[Delta2]

Thanks for the replies. But so far as I'm aware, neither gravity nor centripetal force can cause any acceleration or deceleration in a direction tangential to the circle of rotation.

We don't need to have tangential acceleration for this difference in velocities between one point and its diametrical opposite. The centripetal acceleration is enough. To see this let's examine a particle that does uniform circular motion. At one point its velocity is V and at diametrical opposite the velocity is -V . It is only the centripetal force that acts and gradually changes the direction of velocity from +V to -V (from angle $0$ to angle $\pi$ gradually passing through all the directional angles ). The magnitude of velocity remains constant, but the direction changes from $0$ to $\pi$ so it becomes -V at angle $\pi$.

 

[Dale]

Thanks for the replies. But so far as I'm aware, neither gravity nor centripetal force can cause any acceleration or deceleration in a direction tangential to the circle of rotation.

You should work out the math. It will be illuminating for you. Easiest way is to use some math software.

 

[RPinPA]

Thanks for the replies. But so far as I'm aware, neither gravity nor centripetal force can cause any acceleration or deceleration in a direction tangential to the circle of rotation.

Well, OK, I'll concede that in a manner of speaking.

Again, this question applies to any rotating object, including you sitting in your chair on earth. Why do you rotate with earth? What keeps you in that constantly-accelerated frame? In the case of you and the air around you, it's friction. But the cause of that friction is the normal force due to gravity.

But how about the ground under your feet? What makes it follow the rotation of the earth, again undergoing a constant acceleration? And that's why I said "everything that makes a solid a solid".

And once again I'll say all of this applies to any rotating object. If you twirl a baton from the center, why do the ends of the baton rotate? What causes them to undergo a state of constant acceleration?

 

[Bofors]

Thanks again. I've thought about it some more, and I see that I made a statement that is wrong. I said "neither gravity nor centripetal force can cause any acceleration or deceleration in a direction tangential to the circle of rotation". That would be true if that tangent is seen as continually changing, but my original scenario was in relation to a fixed tangent - one pointing in the direction the whole planet is moving through space. In that fixed direction, of course, the velocity does change under centripetal force, going from +v to 0 to -v to 0 to +v...

 

[zanick]

Cant you make this even more simple and look at a spinning tea cup (like the amusement park ride) say you sitting on one side of the tea cup and the tea cup was rotating at an angular velocity of one rev/sec. say that equated to a tangential velocity of 10mph... NOW, picture the tea cup on a train track where the tea cup also has a constant velocity of 10mph. same as the planet rotating and moving through space, would the occupants feel the starting and stopping viewed from the inertial frame of reference?

example … but picture the rotating object being changed 90degrees to horizontal. https://cdn.discordapp.com/attachme..._Should_Have_Speed_Changes_at_the_Surface.mp4