Animated Solution for Physics - Laws of Motion: Assertion A cloth covers a table. Some dishes are kept on it. The cloth can be pulled out without dislodging the dishes from the table. Reason For every action there is an equal and opposite reaction.

The Tablecloth Trick

Imagine you are at a fancy dinner party. The table is set with exquisite dishes, crystal glasses, and a beautiful silk tablecloth. Suddenly, a magician steps up, grabs the edge of the tablecloth, and with a swift, sharp yank, pulls it completely off the table! Miraculously, not a single glass tips over, and the dishes remain perfectly in place.

Is this magic? Not at all. It is a pure, unadulterated demonstration of physics in action. This classic parlor trick is the exact scenario described in our Assertion. To understand why the dishes don't end up shattered on the floor, we need to dive into the fundamental laws that govern motion in our universe.

The Law of Inertia

The Assertion states that a cloth can be pulled out from under dishes without dislodging them. To evaluate this, we must look at Newton's First Law of Motion, often called the Law of Inertia.

Newton's First Law tells us that an object at rest will remain at rest, and an object in motion will remain in motion with a constant velocity, unless acted upon by a net external force. The key word here is inertia, which is the inherent resistance of any physical object to a change in its state of motion. The amount of inertia an object has is directly proportional to its mass.

When the dishes are sitting on the table, they are at rest. They "want" to stay at rest. When you pull the tablecloth, you are applying a force to the cloth, not directly to the dishes.

You might be wondering, "Wait, isn't there friction between the cloth and the dishes?" Yes, absolutely! If you pull the cloth slowly, the force of static friction will drag the dishes along with the cloth.

However, the secret to the trick lies in the speed of the pull. By yanking the cloth extremely fast, the time of contact is minimized. The frictional force acts for such a tiny fraction of a second that the impulse (which is $\text{Force}\times \text{Time}$) delivered to the dishes is incredibly small. This tiny impulse is insufficient to overcome the inertia of the heavy dishes. Consequently, the dishes barely move, maintaining their state of rest. Therefore, the Assertion is physically True.

The Action-Reaction Pair

Now, let's shift our focus to the Reason statement: "For every action there is an equal and opposite reaction."

This is a direct quotation of Newton's Third Law of Motion. It is a profound and universally true statement about the nature of forces. Forces never exist in isolation; they always come in pairs. If object A exerts a force on object B, then object B simultaneously exerts a force of equal magnitude and opposite direction on object A.

In the context of our table, if the cloth exerts a tiny forward frictional force on the dish, the dish exerts an equal and opposite backward frictional force on the cloth. Because this statement correctly describes a fundamental law of physics, the Reason is also True.

Why the Reason Fails to Explain the Assertion

Here is where the trap lies in Assertion-Reason questions. We have established that both statements are factually true. But does the Reason explain the Assertion?

Let's test it by combining them with the word "because": The cloth can be pulled out without dislodging the dishes BECAUSE for every action there is an equal and opposite reaction.

Does that make physical sense? No. The dishes stay at rest because of their inertia (Newton's First Law), not because forces exist in pairs (Newton's Third Law). The Third Law is completely irrelevant to explaining the phenomenon of the dishes resisting a change in their state of rest.

This problem perfectly illustrates the core strategy for tackling Assertion-Reason questions in JEE and NEET: 1. Evaluate the Assertion independently: Is it a true statement based on physics principles? (Yes, due to inertia). 2. Evaluate the Reason independently: Is it a true statement based on physics principles? (Yes, it's Newton's Third Law). 3. Check the linkage: Does the Reason provide the correct scientific cause for the Assertion? (No, First Law vs. Third Law).

Since both statements are true, but the Reason is not the correct explanation for the Assertion, we confidently select option (b). Always remember to separate the factual truth of a statement from its explanatory power!