Question

if you burn a matchstick and keep it straightly the fire will be straight.But if you keep the matchstick downwards fire will be straight .why?

Answer 1

becoz the fire burns and gives out flames which moves up and only up irrespective of the position of the matchsticks

Answer 2

it's precisely because of gravity that they go upwards. The gravity is pulling on both it and the air. The air, being cooler and thus denser, puts pressure on the lighter flame when it is displaced. That pressure is stronger on the bottom than the top, and pushes the flame upwards. That's buoyancy; it's the same thing that makes boats float.

When the gravity is absent, the cooler, denser air doesn't flow downwards. It continues to put pressure on the flame from all directions, which means it comes out as a sphere rather than going upwards