# Centripedal acceleration and force question

1. Mar 25, 2010

### Lachlan1

1. The problem statement, all variables and given/known data
i can't fully understand this concept. is this correct?

standing on the earths surface i have two forces acting on me. the force due to the fact that i have mass, and so does the earth. given by equation f = (Gm1m2)/r (this force would be acting if the earth was not spinning) i will call this force gravity here.

then there is a normal force that the earth is acting on me, upwards. as i am stationary, this force will countering the force i described above.

but, what about the centripedal acceleration i have (assuming i am standing on the equator),and the earth is spinning, due to the fact that i have a tangental velocity. this acceleration is given by the equation
ac = (v^2)/r
with this acceleration acting on my mass, is this not creating another force on me too? and is this force acting in the same direction as the gravity force or the normal force.

I know that my centripedal acceleration is towards the centre of the earth, so is this the direction that the force acts as well.

i guess im not really sure which direction the centripedal force acts in a rotational motion situation. with the bucket being spun in a circle above your head, eaxample, i thought that the reason the water does not fall from the bucket when it is above your head is that the force due to gravity acting, as it alwas does, downwards, was equal or less than a force acting in the opposite direction. is this the support force of the bucket bottom? wouldnt that act in the same direciton as gravity force?? or is it the centripedal force?? opposing gravity? but doesn't the centripedal force have to act in the same direction as the acceleration, which is the same direction as gravity? so then why does the water not fall?

edit: ok, so i have just read that the centrpideal force is not actually a force in itself, it is just another name for the force that is creating the cetripedal acceleration, which in the case of the bcket, for example, is the tension force in the persons arm. this i understand.

however it does not help me with my first problem, about the forces acting on a person standing at the equator. what force causes the centripedal acceleration here

2. Relevant equations

3. The attempt at a solution

Last edited: Mar 25, 2010
2. Mar 26, 2010

### rl.bhat

standing on the earths surface i have two forces acting on me. the force due to the fact that i have mass, and so does the earth. given by equation f = (Gm1m2)/r (this force would be acting if the earth was not spinning) i will call this force gravity here.

then there is a normal force that the earth is acting on me, upwards. as i am stationary, this force will countering the force i described above.

According to the newtons third law, action and reaction won't act on the same body.
So A acts on B and B reacts on A.
when the earth is not spinning the force on you is due to gravity. It remains the same even if the earth is spinning.
When the earth is spinning, centripetal force due to rotation acts towards the center. You push the earth with that force earth pushes you back with that force. If this force is less than the gravitational pull you stay on the earth. As the spinning speed increases, this force is greater than the gravitational pull so that you were thrown out of the surface.