Question

In Lecture 5 (Uniform Circular Motion), how do we know that the white particles move to the edge of the glass because of the large "artificial gravity"? What I'm really asking is: Why don't the white particles act like the marble that travels in the direction it was going, in the glass tube? How do we know the white particles experience a centripetal force?

Answer 1

It is centrifugal force. I don't like centripetal force. For me is centripetal force always really force in system (like tension in string).But centrifugal force is somthing what has influence in object in circular motion.

Answer 2

We have our centrifuge which is rotating the glass container around a center point. Imagine a single marble inside the glass container. When the centrifuge in spinning, the marble will roll to the end of the tube and stay ''stuck'' there. The marble is moving with uniform circular motion and does not fly off on a tangential path because the normal force with the bottom of the container pushes the marble in towards the center of rotation.

The marble 'feels pushed' towards the bottom of the container, which is a sort of perceived or artificial gravitation. It also 'feels' the bottom of the container pushing on it in the opposite direction of the perceived gravity--much like we feel gravity push us towards the ground and the ground push back up on us with a normal force.

Now imagine thousands--even millions--of tiny white marbles in a sea of millions more dark marbles. And imagine all of them are evenly mixed together in the container. Then you stick the container in the centrifuge and give it a uniform circular motion. ALL the particles will now experience the 'artificial' gravity in our one marble example--but the white marbles have a different mass than the dark marbles. Can you imagine how they will all act now? This models what happens with the chemical mixture in Lect 5.

Answer 3

Actually the white particals expirance a centifugal force that is nearly 2000 times greater than the original gravity. so the particals get seperated as the get heavy...

Answer 4

when we first learn physics, we learn newton's three laws. we learn that a body moving at constant velocity (constant speed along the same direction) will not change its motion unless acted upon by a force.

if a body is moving in a circle, then its velocity is CONSTANTLY CHANGING, and there must therefore be a force. that force is a CENTRIPETAL FORCE because through geometry we figure out that the force must be directed towards the center of the circle, and centripetal means force directed towards the center.

CENTRIFUGAL FORCE is a useful concept, but it can also be confusing. it means force pushing outward--in our centrifuge, it would be a force pushing the white particles down to the bottom of the flask. BUT THERE IS NO FORCE PUSHING IN THAT WAY. it's an 'illusion' on circular motion!

centrifugal force is NOT A 'REAL' FORCE, JUST A TRICK THAT we sometimes use to think about how it "feels" to be in an object that is moving with circular motion.

again, CENTRIFUGAL FORCE IS NOT A 'REAL' FORCE--so what is it? well, it's got to do with something call the "reference frame." see, physics doesn't just describe how things happen--it describes WHAT WE OBSERVE TO HAPPEN--THE OBSERVERS ARE INCLUDED IN THE PHENOMENA THAT THEY OBSERVE! specifically, if we are already accelerating when we observe, then we will observe 'phantom forces' that don't exist according to someone who is standing still, and we will fail to observe other forces that do exist from the viewpoint of someone standing still.

the 'centrifugal force' that the white particles 'feel' is a 'phantom force' due to the fact that the white particles are living inside a reference frame that is being accelerated in the motion of a circle!

from our viewpoint in the laboratory, watching the particles, there is NO MEASURABLE FORCE PUSHING THE PARTICLES OUTWARD. if we draw one into our force diagrams or try and put it in our newtonian equations, it will produce wrong answers!