 ## What is Circular Motion?

Circular motion happens when an object moves around a point in space, never getting farther from it or closer to it. The distance from the center point is the radius of the circle. In order to travel in a circular motion, an object must constantly change direction. Though its path forms a circle, the object is always moving tangentially to the circle. One tangent is simply interrupted by the next with each tiny directional shift.

Circular motion can be classified into uniform and non-uniform circular motion:

Remember, unless otherwise mentioned, it is assumed (by convention) that objects undergoing circular motion do so uniformly (i.e. with constant speed).

## Uniform Circular Motion • An object undergoinguniform circular motion moves with a constant speed (only the direction changes).

For an object to move along a curved path, the direction of its velocity must change. A change in velocity means that there must be an acceleration that is not in the same direction as the velocity. For an object to move along a circular path, this means that there must be an acceleration that is always perpendicular to the velocity. This acceleration vector is therefore always directed toward the center of a circle formed by the object’s path. This acceleration is either called the “radial acceleration”, because it is at a certain radius from a central point, or the “centripetal acceleration”, which means that it is “center seeking”.

Uniform circular motion means that the magnitude of the velocity is a constant, and the direction of the velocity changes at a constant rate. This means that the object’s path forms a circle, and that the object will complete repeated trips around the path in the same amount of time each time.

Examples of circular motion include:

• Earth revolving around the sun for which gravitational force acts as centripetal force
• A stone which is tied to a rope and is being swung in circles: here rope provides the centripetal force in the form of tension
• A car turning through a curve in a race track for which friction provides the centripetal force required
• Also, inside atoms, electrons revolve around the nucleus due to Coulombic force. In other words, we can say centripetal force originates from an external source.

## Non-Uniform Circular Motion • An object undergoing non-uniform circular motion moves with varying speed.

Non-uniform circular motion means that there is another acceleration that changes the magnitude of the velocity of the object. One example of non-uniform circular motion would be a roller coaster car going around a vertical loop. At all points in the loop, the car experiences an inward radial acceleration, though the car also experiences acceleration due to gravity.

The radial acceleration is always perpendicular to the velocity vector of the object, though in non-uniform circular motion there is also an acceleration component that is parallel to the velocity vector. This is the “tangential acceleration,” since this vector is always a tangent to the curve of the circular path.

Example: If a car covers 10 meters in first two seconds and 15 meters in next two seconds.

If a body is involved in rectilinear motion, and if the motion is non-uniform, then the acceleration of the body must be non-zero.

There are a lot of fascinating real-life instances of circular motion with explanations. For example, the reason why the object does not fall down when subjected to only downward forces is a simple one. Think about what keeps an object up after it is thrown. Once an object is thrown into the air, there is only the downward force of earth’s gravity that acts on the object. That does not mean that the once an object is thrown in the air, it will fall instantly. What keeps that object up in the air is its velocity. A fair understanding of all the concepts of Circular Motion helps to learn the notion of rotating bodies about different axes. The first of Newton’s laws of motion states that an object’s inertia keeps it in motion, and since the object in the air has a velocity, it will tend to keep moving in that direction.

A prior knowledge of Vectors, Pseudo force, and Frame of reference is required for the extensive study of Circular Motion. It is certainly an important chapter for all competitive examinations like JEE, NEET, Board exams, etc.

Practice in the most time-efficient way with Toppr’s Practice section! The artificial intelligence fed into our system will ensure an increasingly difficult level of questions for you, along with avoiding questions on repeating concepts. You can check out our physics practice section for Circular Motion here!

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(Writer Credits: Rupesh Verma, IIT Guwahati)

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