A flashlight emits 2.5 W of light. As the light leaves the...

Name: Problem 72, Chapter 27: Physics: Principles with Applications
Uploaded: 2019-05-16T01:59:10-08:00
Duration: 2 min 28 s
Channel: Jayashree Behera
Description: A flashlight emits 2.5 W of light. As the light leaves the flashlight in one direction, a reaction force is exerted on the flashlight in the opposite direction. Estimate the size of this reaction force.

A flashlight emits 2.5 W of light. As the light leaves the flashlight in one direction, a reaction force is exerted on the flashlight in the opposite direction. Estimate the size of this reaction force.

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Key Concepts

Conservation of Momentum

When a system emits radiation, such as light from a flashlight, the law of conservation of momentum dictates that the momentum carried away by the light must be balanced by an equal and opposite momentum change in the system. This results in a reaction force on the emitter that is related to the momentum carried by the photons.

Force-Power Relationship in Electromagnetic Emission

The power of the emitted light, defined as the energy released per unit time, can be connected to the momentum transfer to compute the reaction force. Since each photon's momentum is related to its energy (p = E/c), the total reaction force exerted by a light source emitting power P can be estimated by dividing the power by the speed of light (F = P/c), highlighting the relationship between electromagnetic energy emission and force.

Photon Momentum

Photons, the quantum particles of light, carry momentum even though they have no rest mass. This momentum is determined by the energy of the photon divided by the speed of light (p = E/c), which is critical for understanding how light can exert forces when emitted or absorbed.

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