Irodov's Challenging Gift to Every Physics Lover : Find Time Before Collision | IRODOV 1.138

Описание: In this Physics video in Hindi for the Chapter of "Laws of Motion" for Class 11, we solved a very interesting and conceptual question from the book “Problems in General Physics” by I.E.Irodov.

The question states: A horizontal plane supports a stationary vertical cylinder of radius R and a disc A attached to the cylinder by a horizontal thread AB of length l₀ (top view). An initial velocity v₀ is imparted to the disc as shown in the figure. How long will it move along the plane until it strikes against the cylinder? The friction is assumed to be absent.

This problem belongs to the chapter Laws of Motion from Class 11 Physics. It combines concepts of tension, constrained motion, and dynamics of circular paths. In this Hindi explanation video, I have carefully broken down the situation to make it easier to visualize and understand. The motion of the disc is restricted by the thread, and as it moves, the thread gradually winds around the stationary cylinder. Because the surface is frictionless, there is no energy loss due to friction, and the only horizontal force acting on the disc is the tension in the thread, which changes direction as the disc moves.

The solution is based on Newton’s Laws of Motion and the understanding of motion under a variable direction of force. The tension continuously pulls the disc towards the cylinder, creating a changing radial acceleration. Since there is no friction, the magnitude of the velocity remains the same, but its direction changes due to the tension force. This causes the disc to move along a spiral-like trajectory until it finally collides with the cylinder.

The problem beautifully demonstrates how Laws of Motion govern a system where the direction of motion changes without any change in speed. Using the relationship between angular displacement and linear motion, and understanding how the thread winds around the cylinder, we can find the total time of motion before impact. The setup combines geometry and dynamics in a single elegant problem.

This question is an excellent example of how Irodov combines geometry and Newtonian dynamics into one problem. For IIT-JEE Advanced aspirants, this type of question is a must-solve because it enhances one’s understanding of constraint motion, circular motion, and the real application of Newton’s second law in curvilinear motion. The chapter Laws of Motion often features problems involving tension and motion on frictionless surfaces, making this a perfect problem to strengthen those concepts.

In the video, I explain in Hindi how to visualize the winding of the thread, how the velocity direction changes gradually, and how to determine the instant when the disc finally touches the cylinder. The reasoning is presented step-by-step, with emphasis on physical understanding rather than memorized formulas.

By the end of this video, you will have a deep conceptual grasp of motion under tension, the geometric constraints of circular motion, and the physics behind the disc’s final collision with the cylinder. This question also highlights the importance of applying Laws of Motion in non-trivial scenarios — where the net force acts continuously perpendicular to the motion, altering the path without changing speed.

This explanation, delivered completely in Hindi, helps IIT-JEE Advanced aspirants and Class 11 students connect theory with visualization. It provides an intuitive and complete understanding of how Newton’s Laws control every part of constrained motion in a frictionless environment. Through this video, you will gain the ability to apply the Laws of Motion confidently in any similar advanced-level problem seen in IIT-JEE Advanced or IIT-JEE Main.

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