When a card placed over the mouth of a tumbler is flicked sharply with a finger, the coin resting on the card falls straight down into the tumbler. Which law of motion does this demonstrate?

Introduction / Context:

This classic classroom demonstration with a card, coin, and tumbler illustrates a fundamental principle of mechanics. When the card is flicked away quickly, the coin appears to fall almost straight down into the tumbler instead of moving sideways with the card. This behaviour is a vivid example of inertia, as described by Newton's first law of motion.

Given Data / Assumptions:

• A coin is placed on a card, which itself rests horizontally on the mouth of a tumbler.
• The card is flicked sharply in a horizontal direction.
• Air resistance and friction between coin and card are relatively small during the very short flick.
• The question asks which law of motion this setup demonstrates.

Concept / Approach:

Newton's first law of motion, also known as the law of inertia, states that a body at rest will remain at rest, and a body in uniform motion will continue in that state of motion, unless acted upon by a net external force. Initially, the coin is at rest. When the card is flicked quickly, only a small horizontal frictional force acts on the coin for a very short time, so the coin tends to remain in its state of rest. As the card moves out of the way, the coin no longer has support and falls vertically into the tumbler under gravity. This shows the tendency of matter to resist changes in its state of motion, that is, inertia.

Step-by-Step Solution:

Verification / Alternative check:

Similar demonstrations of inertia include pulling a tablecloth quickly from under dishes or yanking a sheet of paper from under a glass. In each case, objects resist change in their state of motion and tend to remain at rest if the applied force acts for only a short time. Textbooks explicitly describe the coin and card experiment as an example of Newton's first law, reinforcing that the correct law here is the law of inertia.

Why Other Options Are Wrong:

• Law of conservation of energy: Total energy is indeed conserved, but this experiment is not primarily used to illustrate energy transformations.
• Newton's third law of motion: This law deals with action and reaction forces, which are not the main focus of the demonstration.
• Law of conservation of momentum: While momentum concepts are present in many interactions, this simple setup is not usually framed as a momentum conservation demonstration.

Common Pitfalls:

Students sometimes choose Newton's third law for any situation involving motion and forces, but it is important to match the key idea. Here the key is that the coin tends to remain at rest despite the motion of the card. Another error is to think that because the coin falls into the tumbler, energy or momentum laws are the main lesson. In reality, the experiment is used in most textbooks specifically to show inertia and Newton's first law.

Final Answer:

This demonstration illustrates the law of inertia, that is, Newton's first law of motion.