Forces on a Rocket


I was wondering if you could help me. I need to know all the forces that influence on a rocket and a person inside. We have information about the mass and the acceleration of the rocket, and we have the mass of the person. I would be very happy for a fast answer.


First let's consider the forces when the rocket is not firing. A rocket in space is subject to the force of gravity from every object in the universe. Gravity is a very weak force that decreases with the square of the distance from the rocket to the object so that it is small enough to be neglected except for large nearby objects. The person in the rocket is subject to the same gravitational force as is the rocket except for the gravitational attraction between the person and the rocket itself. In that case the direction of the forces are opposite. Because of the weakness of gravity, that force between rocket and person is so small that its effects are undetectable.

If the rocket and person are in the vicinity of a planet sized object the force of that planet's gravity will bend the path of the rocket away from the straight line path that it would otherwise take. If the rocket is sufficiently close to the planet relative to its speed, the path of the rocket may even close on itself in an ellipse so that the rocket is captured as a satellite of the planet. Of course the speed of the rocket and the speed of the person inside are very nearly the same so the effect on the person of the planet's gravity will be the same as the effect on the rocket and they will both follow the same path, giving the appearance that the person is floating inside the rocket.

Now suppose we fire the rocket engine. The burning fuel accelerates some mass which is ejected from the rocket. According to Newton's third law the momentum of the rocket-fuel combination must remain constant so the rocket experiences an equal and opposite force from that of the ejected mass and is accelerated. The person inside remains in the same path as before until some pare of the rocket comes into contact with the person and applies a force sufficient to match the persons speed to that of the rocket. As long as the rocket engine continues to fire, the rocket continues to change its speed and contact between the rocket and person is required to keep their speed matched. The magnitude of the force on the person by the rocket is found by multiplying the mass of the person times the rate of change of the person's speed. Once the rocket engine stops, the person and rocket again coast along the same path so the person appears to float inside the rocket again.