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Size of body neglected in Circular motion

Why in  circular  motion the  size  of  the moving body is  neglected?


Not only in circular motion, but in entire kinematics we discuss the case of point objects only. In kinematics we study motion only and not the objects undergoing the motion.

A point object is an object having negligible dimensions compared to the distance travelled by it. For example, a car moving from Delhi to Mumbai can be considered a point object.

Further, when we discuss the concepts in Kinematics, even a large object is dealt considering the entire body to be a single point. This point will be the centre of mass of the body.

If a person is revolving a stone tied to a string, the centre of mass of the stone is taken for determining the length starting from the point about which it revolves.

In dynamics, we have to consider the shape and size of the body also.

Uniform Circular Motion in Horizontal Plane

In uniform horizontal circular motion, how are the vertical forces balanced?

Asked Arjun M Nair


If you are considering an object tied to a string and whirled with uniform speed in a horizontal plane, the object will be moving in a horizontal plane but the string has to make a certain angle with the horizontal so that the tension can be resolved in such a way that the horizontal component provides the centripetal force and the vertical component balances the weight of the body.

The motion will become almost entirely in a horizontal plane when the centripetal force is much larger than the weight of the particle.



The case of rid body structures the situation is complex, but as long as there are no vertical accelerations, the vertical components of  forces will be balanced.

Comments are welcome from students and teachers WHO THINK!

Why Earth does not collide with the sun?

If our earth rotates around sun in discrete orbits, then why it doesn’t collide with Sun because of gravitational force acting between both bodies ?


Earth is moving around the sun in such a way that the distance to be traveled TOWARDS sun is compensated by its circular motion. Therefore, instead of “falling towards” the sun, earth “falls around” and never reaches the sun.


For a circular motion centripetal force is required. Is there any other force which could replace centripetal force to obtain a circular motion? (Asked Sajin)


centripetal forcePlease note that Centripetal Force is the net force that must be acting towards the centre of the circular path. It may be provided by one or more of the forces of different kinds. It is not a kind of force like the frictional force or gravitational force. In different cases the centripetal force is provided by different forces.

For example, when we whirl a stone tied to the end of a rope in a horizontal plane, the tension in the string provides the centripetal force. Here the real force acting is the tension in the string and its magnitude equals the centripetal force mv^2/r.

Read more at the following links

  1. http://hyperphysics.phy-astr.gsu.edu/hbase/cf.html
  2. http://en.wikipedia.org/wiki/Centripetal_force
  3. http://www.physicsclassroom.com/Class/circles/u6l1c.cfm

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