The weight of a shadow


Do shadows have any weight? If not, does a shadow make the object on which it is cast weigh any lesser?

At first glance these questions seem stupid but after doing a little research, realized that these are valid questions with real world implications. These are things that astrophysicists take into consideration all the time. 

To clarify, shadows have no weight. However, when a shadow a cast on an object, the object weighs a little (well, very very little) lesser, because it is no longer being pushed by light.

We all know that light has energy. Just like a ball that is hurled at a wall pushes the wall a little bit, light photos that hit any surface push the surface a tiny bit and then bounce off. In fact, all the photons from the Sun hitting the Earth's surface push the Earth away from the Sun all the time. As you might have guessed, the force with which the photos push is extremely low. To be precise, the force with which Earth is being pushed by sunlight is 10-9 lb/inch2. Don't even bother to figure out how small it is - its a very little force. 

However, things get a little interesting if we look at larger surface areas instead of square inches. For example, United States of America, on a bright sunny day, is being pushed with a force of 14,925,122 pounds. This means that United States weighs so much more on a sunny day than at night just because it is being pushed by light!

Okay, but what are the real world implications? The effects of 'light pushing' are more pronounced once we leave the Earth's atmosphere and move into space where solar winds are much stronger. For example, NASA should take into account while planning trajectories, the fact that a space shuttle travelling from Earth to Mars will be pushed by as much as 950 Kilometers during its journey just because of light hitting the shuttle. 

Then we have Solar sails which are space crafts designed to use the solar pressure for propulsion.  

Going back to the question we started with, shadows do not have weight but they do reduce the weight of the objects. The next time you think you have put on too much weight, just go stand in a shade and boom, you weigh lesser!

Did the simple science for kids in this post make you a little smarter?
If you have any ideas / questions you want to learn about, please leave a note in the comments section. 
Would be glad to know if there are any inaccuracies in the article.

1 comment:

  1. thala mahesh ku ooooo poduinga pa sama intresting site to spend time valuable

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