• 2 Posts
  • 303 Comments
Joined 1 year ago
cake
Cake day: June 24th, 2025

help-circle












  • Newton’s third law works well in explaining this. Think of where the momentum is going in each case: in the jet engine, normally the exhaust is directed rearwards, which creates an equal and opposite force forwards. When the reversers are deployed, the exhaust is directed partially forward, which creates an opposite force rearward, thus slowing the plane.

    In a sailboat with a fan pointed at the sail, the fan initially pushes air forward, which would push the boat backwards. But the air immediately hits the sail and either stops or is directed out the sides, which creates a forward force. The forward and reverse forces roughly cancel each other, so there’s little or no net motion of the boat. This assumes the fan and sail are in the configuration shown in your picture in another comment. It is possible to arrange the fan and sail in such a way that you actually get forward motion, but at that point it’s just an air boat with more steps.


  • It would be really difficult to design a spacecraft to go to Pluto and then return. You’d have to carry enough fuel to come to a stop at Pluto, then reverse back to Earth, then come to a stop at Earth. The extra fuel adds up quick because the more you carry, the more you need. A ship that could carry enough fuel for a round trip would be incredibly large and would have to be constructed in orbit rather than on the surface of the Earth.

    You could reduce fuel requirements somewhat by going slower, but it already takes so long to get to Pluto that you’d soon be limited by human life expectancy.