# Centrifugal Force, Centripetal Force, and Space

1. Oct 21, 2013

### evaengineer

I've got a weekly physics assignment, and I've been racking my brain on it. Any input on the accuracy of my response, what I could do to improve it, and where I may have gone wrong?

1. The problem statement, all variables and given/known data
Some years ago, a newspaper story contained the following statement" "The space shuttle orbits the earth at an altitude of 200 miles and is traveling at a speed of 18,000 mph. The shuttle remains in orbit because the gravitational force pulling it towards the Earth is balanced by the centrifugal force (the force of inertia) that is pushing it away from the Earth."

A friend who knows you have taken Physics 107 asks you to explain this statement to them. In particular, this friend is mystified by the reference to a "centrifugal force".

2. Relevant equations
None that I can think of.

I will however include my basics of understanding:
Centrifugal force is fictitious and the opposite or centripetal force.
Centrifugal force pushes outward in a circle.
I understand the "Rotor Carnival Ride" example in physics (I note this because I am curious if it has to do with the question at hand).

3. The attempt at a solution
To start, I believe the newspaper's statement is to a degree incorrect. This is because the suggested action/reaction pair of centripetal/centrifugal force does not work, as Third Law Forces never act on the same object (Essentially, one object cannot be acted upon by the action AND reaction in the same instant).

To describe why the shuttle stays in orbit is simple enough. All that is necessary is for the shuttle to have a centripetal force (F=mv2/r) that counter-balances the gravitational force (F = mg). When that happens, the shuttle will continue along its path outside the Earth's atmosphere in a circular motion; Not breaking past gravity fully, but not being pulled by it enough to counteract its rotational force.

The centrifugal force does not counter-balance the centripetal force. Gravitational force does. This is because after the shuttle is in orbit, it would have a tendency to stay in motion, unless something else acted upon it. The shuttle doesn't need to constantly use different thrusters to move around the planet. The gravitational force pulls on the shuttle while the shuttle itself continues its forward motion, thus creating the rotation around the earth.

So, to de-mystify the friend about the "centrifugal force reference," I would have to tell him that a centrifugal force is a fictional, or "pseudo" force. It was named as such in order help describe why one feels that they are being pushed outwards when in a circular motion.
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

2. Oct 21, 2013

### willem2

Gravity doesn't counter counterbalance the centripetal force, gravity IS the centripetal force.
An object moving in a circle will be accelerated all the time, so the force on it is not zero.
To move a mass m in a circle of radius r with speed v, you need a centripetal force of mv^2/r to produce the needed acceleration, and gravity is that force.

3. Oct 21, 2013

### Bryson

The centrifugal force is not an inertial reference frame, as you said fictitious.