# Confusions about rotational dynamics and centripetal force

I am a high school student. I am having confusions about the centripetal force and rotational motion.

I have known that a body will be in rest or in uniform velocity if any force is not applied. But a force will be needed to create a rotational motion in a body. That force is called the centripetal force and it acts vertically with the direction of linear velocity of the body in rotational motion.

My question is, how an object (e.g. stone) attached with a rope rotates by the force we apply by our hand to the system (rope and the stone) while holding the other end of the rope? Though I have made an answer I am not satisfied with it.

When we attach a rope with a stone and hold the other end with our hand and we rotate our hand and create a rotational force to rotate the system , the vector (axial vector) of the rotational force by our hand acts vertically with the plane of rotation of our hand and acts through the rope. This force which acts through the rope and through the axial vector of the rotating force created by our hand acts as the centripetal force to the stone. Another force which is same and opposite to this centripetal force is the centrifugal force which is a pseudo force and acts through the rope and straightly outside the system.

The direction of centripetal force is towards the centre of rotation through the rope and that of centrifugal force is towards the object through the rope. This centrifugal force tries to get the stone out of the rotating circle through the vector of the rope stretched outside. But due to the tension of the rope and centripetal force, the object cannot get out. Due to the tension and centripetal and centrifugal force, the rope and the stone together acts as ONE tough system (like a single stick) and as we rotate a stick and as we rotate our hand the stick rotates, likewise the system also rotates.

But this explanation is not satisfying because I can't get how it is rotating? I mean, the centrifugal and centripetal forces are just maintaining the system to be rigid and stable but what is the force that is helping the stone and the rope to rotate? How does that force work? Is that force (which helps the system to rotate) centripetal? Or the rotating force applied by our hand?

There's are many things wrong in your concepts .

Let's attend them one by one ,

A body without a force acting on it always can never rotate as its velocity is changing at each hence momentum is changing at each instant . But to maintain constant velocity , the force must be such that it never does any work , so as to maintain the constancy of Kinetic Energy.

Second there are two types of forces : Contact and Field ,

Your hand exerts a force on rope and rope exerts a force on the stone attached it and that force needn't be equal to the force you're applying on the rope .

Third , this force which is being applied to rotate is called centripetal force , but it is a real force like Tension/Normal reaction gravity . Centripetal is a term just to denote that one of these forces is helping the body to rotate .

Fourth and the biggest flaw , centrifugal force is nothing . It is not a force , it is a mathematical trick you apply to apply newton's laws in your frame , hence from your frame(if you're rotating the stone) , there will never be a centrifugal force but if your are the stone then you will see yourself at rest , and that means forces must be balanced and to balance those force , you will apply centrifugal force and you will apply that centrifugal force on everything in the universe . It is a pseudo force , a mathematical trick , but somehow, that becomes your reality but for now , you can just assume it is a mathematical trick.

Some of this might not make sense , but it will later . Keep on reasoning like you are. And read a bit more about centripetal force and pseudo forces.

Perhaps from Ramamurti Shankar's lectures on youtube or the book resnick halliday krane , physics volume 1

• Many things have become clear. But to rotate a stone attached with a rope we need to rotate our hands? Isn't it? Is this rotating force anyhow responsible for the rotation ? If yes then how? – shiladitya basu Apr 26 '13 at 8:34
• Basically you have to rotate because you have to always apply force perpendicular to the motion of the rotating object , if you don't rotate your hand , then your force won't remain perpendicular to the direction of motion of the stone , it will get to a make an angle and then path won't remain circular . did you follow this one ? and if you think you've understood , then you can rate my answer if you like – user23503 Apr 26 '13 at 8:43
• I can't vote up as I don't have much reputation. Anyways, It helped a lot. Thanks! – shiladitya basu Apr 26 '13 at 8:52