Question

A (somewhat risky) way of telling if a train that cannot be seen or heard is approaching is by placing your ear on the rail. Explain why this works.

Short answer

Answer: Placing your ear on a railway track helps detect an approaching train because sound waves travel faster and more efficiently through the solid steel track than through air, allowing you to sense the vibrations created by the train's movement even if it's too far away to be heard through the air. However, this method is not recommended for safety reasons as it puts individuals at risk of being too close to the tracks when a train approaches.

Step-by-step solution

Understanding sound waves

Sound is a vibrational energy transmitted through mediums like air, water, and solids. The vibration causes the particles in the medium to oscillate back and forth, creating sound waves. These sound waves are longitudinal waves that travel at different speeds depending on the medium.

Properties of solids for sound transmission

In solids, particles are more densely packed than in air or water, allowing them to transmit sound waves faster and more efficiently. The speed of sound in steel, which is the primary material used in railway tracks, is around 5,120 meters per second (m/s), while in air, it is approximately 343 m/s. Thus, sound travels significantly faster through steel than through air.

Detecting vibrations through the ear

Our ears can detect the vibrations created by sound waves in the environment. When you place your ear on a solid surface, like a railway track, you can effectively detect the vibrations that travel along the track. This means that if a train is approaching, even from a distance, the vibrations from the train's movement will propagate through the steel track and your ear will sense the vibrations more easily when in direct contact with the track.

Why it works for detecting approaching trains

Placing your ear on the railway track allows you to effectively detect an approaching train because the solid track can transmit the sound waves faster and more efficiently than through air. As a result, you can sense the vibrations created by the train's movement even if it's too far away to be heard through the air. This helps in determining if a train is coming without actually seeing or hearing it through the air, although it is a risky method and not recommended for safety reasons.