Laws of rocket motion. Rocket thrust
A rocket moves even in empty space: it throws part of its mass backward at high speed and moves forward through conservation of momentum (Newton’s third law). If the exhaust velocity relative to the rocket is u and the mass flow per second is μ = dm/dt, the thrust is F = u · μ (in vacuum). The change of the rocket’s speed is given by Tsiolkovsky’s formula: Δv = u · ln(M₀ / M₁), where M₀ is the initial mass and M₁ the mass after the propellant is used. So the larger the exhaust velocity and the mass ratio, the larger Δv. The specific impulse is I = u / g₀ (in seconds): kerosene–oxygen ≈ 300–350 s, hydrogen–oxygen ≈ 450 s. To reach low Earth orbit ≈ 9–10 km/s is needed (including losses); a single-stage rocket cannot do this, hence multi-stage rockets.
A safe experiment: thread an inflated balloon on a string and watch it move opposite to the escaping air (with an adult). NEVER try to make rocket propellant at home – there is a risk of explosion.