Revision: Motion Science English Medium Class 9 CBSE

"The motion of an object that does not move in a straight line is called ‘non-linear motion’."

A train or a moving vehicle on a road may travel in a straight line or in the same direction. This motion of an object is called linear motion. An object in linear motion shows displacement along a straight line."

When a body covers equal distances in equal intervals of time (however small may be the time interval) in a specified direction, the body is laid to be moving with uniform velocity.
Example: A car moving on a straight road with constant speed has uniform velocity.

The ratio of the total distance travelled in a specified direction to the total time taken by the body to travel the distance is called average velocity.
Example: If you walk to a campsite 1 km away and then back to your starting point within 1 hour, then your average velocity will be zero because your initial and final position is the same.

• Displacement is the shortest distance in a straight line between the starting point and the final point, along with a direction. It is a vector quantity, meaning it includes both magnitude (distance) and direction.
• Example: In Ranjit’s case, the displacement is the straight line AD from his house to the school.

The length of path travelled by a body in certain interval of time is called distance.

Displacement of an object between two points is the shortest distance between these two points.
“It is the unique path that can take the body from its initial to the final position.”

The displacement of a moving body is defined as the change in its position along a particular direction

• Distance is the total length of the path travelled by a moving object, regardless of the direction. It is a scalar quantity, which means it only has magnitude (size), not direction.
• Example: If Ranjit walks from point A to B to C to D, the distance is the total of AB + BC + CD.

When a particle moves with a constant speed in a circular path, its motion is said to be the uniform circular motion.

The force directed towards the centre along the radius, required to keep a body moving along a circular path at constant speed, is called centripetal force.

When a particle moves with a constant speed in a circular path, its motion is said to be uniform circular motion.

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The motion of a body moving with constant speed along a circular path is called uniform circular motion.

The angle traced out by the radius vector at the centre of the circular path in a given time, expressed as Δθ = θ₂ − θ₁​, is called angular displacement.

The rate of change of angular displacement of a body undergoing circular motion is called angular velocity.

The rate of change of angular velocity of a body is called angular acceleration.

The component of acceleration directed towards the centre of the circular path is called centripetal acceleration (or radial acceleration).

The time taken by a particle performing uniform circular motion to complete one revolution is called time period.

Angular velocity of a particle is the rate of change of angular displacement.

• In UCM, speed is constant, but velocity continuously changes direction, always remaining tangential to the path.
• Angular displacement is the angle swept by the radius vector; angular velocity is its rate of change.
• Even at constant speed, centripetal acceleration is never zero — it always acts towards the centre of the circular path.
• Centripetal force is always directed towards the centre and is essential to maintain circular motion — it does no work on the body.
• If speed is constant in circular motion, tangential acceleration = 0, but radial acceleration ≠ 0.