## Physics professor reacts to "Troll Physics" comic strips This actually makes an interesting point, sort of. Why don't you recoil when you shine a beam of light? The answer is -You actually do! But not at the same SPEED as the light - with the same MOMENTUM as the light. The momentum (p) of a single photon of light is equal its energy divided by the speed of light (E = c p). So how much momentum does a beam of light carry? Well, suppose we shine a 100 Watt spotlight - we are producing 100 Joules of light energy per second (a Watt is a Joule of energy per second). So every second we are giving that light a momentum of (100J) / (300,000,000m/s) = .00000033 kg m/s.

To give a sense of how hard of a push that is, let's assume that the stupid troll thing in the wagon weighs 50 kg. To accelerate the wagon up to a speed of 1 m/s (around 3 feet per second... which is still pretty slow) would take (33 million * 50) seconds, which is 52 years. And that's assuming no friction!

(In reality, it would never move at all because of friction, so we'd have to do it in space. In fact, even though this seems like a feeble form of propulsion, NASA has plans for prototypes of a "solar sail" to push space probes out of the solar system using no propulsion other than that provided by reflected sunlight!)

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