Question

In movies you often see scenes where A spaceship activates a “force field”, which acts like a physical shield, preventing any projectiles from hitting the ship by causing them to bounce off it. Why isnt this logical according to physics? What concepts can you use to prove this is "bad physics" and how can you change this common scene to portry physics correctly?

Answer 1

We will use Kinetic energy conservation law to show that shields would violate conservation of momentum. Basically, it boils down to: the shield doesn't change speed, so its momentum remains the same, however the incoming object does change speed, so its momentum changes. Since those are the only two objects in the system, the total momentum changes, which is not physical , thats why it can be illogical.

In order for shields to be possible, we'll have to make a key adjustment: the momentum from the object is transferred to the shield generator. From here it can transfer the momentum to a large mass, like the Earth. (This is exactly what a wall does, transfer momentum into the ground via a momentum current, i.e. a force.)

We need a way for the shield generator to apply a force to the object that it wants to stop. We can use electromagnetic force for it as it is most suitable here.

I hope this helps.