Question

If I were able to set a flashlight out in the middle of space and turned it on, would it move?

Asked by: David Buchwald

Answer

You set your flashlight is space to remove friction and all other external forces. In empty space, absolute motion can't be defined without a reference point but we can talk about the CHANGE of motion, or acceleration, of the flashlight.

The flashlight is emitting photons in one direction, and photons, even though they have a REST mass of zero, have momentum equal to hf/c where f is the photon frequency and h and c are constants. Conservation of momentum requires that the momentum of emitted photons in one direction be balanced by the momentum of the flashlight in the opposite direction. Since the flashlight's momentum is mv, where m = its mass, and v = its velocity, the flashlight WILL continue to change its velocity as photons are emitted. That means it will accelerate, just as a rocket accelerates opposite to the momentum of hot gases expelled.

Given the relatively small momentum of photons vs. an average flashlight's mass, however, the resulting acceleration of your flashlight rocket would be very small.

Answered by: Paul Walorski, B.A., Part-time Physics/Astronomy Instructor

Search

Loading



Sponsors

USC University of Southern California Dornsife College Physics and Astronomy Department McMaster University Physics and Astronomy Department

Science Quote

'Beauty is a harmonious relation between something in our nature and the quality of the object which delights us.'

Blaise Pascal
(1623-1662)

Deal of the Day

Image of Hover Craft Kit. Click on this image for more info. Hover Craft Kit
Regular Price: $29.99
Today's Price: $12.00
Learn the science behind hover craft technology with this cool kit! Buy it now! Deal ends at midnight!

Top Selling

Here are our physics & astronomy bestsellers:
Mini Plasma Ball
Slide Making Kit
Top Secret - Spinning Top
Wood Grain Newtons Cradle
Deluxe 3D Mirascope
Solar Radiometer
Periscope
Ultimate Chemistry Set CHEM C3000
Solar Science 4M Kit
3D Magnetic Field Tube