# Proving F is a Central Force: Alonso & Finn's Physics

• Anastasis Pk
In summary, the conversation is about proving that a body moving under the force F = k⋅u × v, with u as an arbitrary unit vector and v as the velocity, has a circular motion with angular velocity ω = k ⋅ u or a spiral parallel to u. The discussion also touches upon the concept of a central force and the equation for acceleration in order to demonstrate the motion as a helix.

#### Anastasis Pk

Good afternoon!
Exercise: Prove that if a body moves under the action of a force F = k⋅u × v, where u is an arbitrary unit vector and v the velocity, the motion is circular with angular velocity ω = k ⋅ u or, in a more general case, a spiral parallel to u.

Source: (Alonso & Finn: Fundamental university physics)

I attempted to prove that F is a central force, so that the angular momentum is constant. Is that wrong?

Would showing F is a central force buy you anything? The force of the Sun on a planet is a central force. but the orbits aren't circular.

Anastasis Pk said:
I attempted to prove that F is a central force
That won't do it. F = k.v would satisfy that. Get an equation for the acceleration.

Thank you for your responses! In order to prove that it's motion is circular wouldn't it be sufficient to prove that ω is constant?
Could you "boost" me a bit?

Anastasis Pk said:
Thank you for your responses! In order to prove that it's motion is circular wouldn't it be sufficient to prove that ω is constant?
Not really. The question is asking you to demonstrate that the equation of motion corresponds to a particle moving at constant speed in a helix. (It says spiral, but it means helix.)
It might help if you could write down the general equation for such a motion, so that you could see what the answer has to look like. Hint: pick a suitable set of orthogonal unit vectors.
Anyway, the first (easy) step, as I posted originally, is to obtain an equation for the acceleration of the particle.