How does propulsion work?

What is resistance?

Resistance is anything that slow or stops movement or keeps movement from happening.

For a space ship, resistance will counter the thrust of a rocket and keep the rocket from going as fast as it could. Once the ship is out of the Earth's atmosphere and gravitational pull, resistance will drop off considerably.

Examples of resistance to motion on Earth include: air resistance slowing a parachutist's fall; resistance of friction slowing or stopping a book from sliding across a rug or a desk; and gravity resisting something traveling upwards or keeping something on the ground. There is also electrical resistance, which slows or stops electrical current, often transforming that energy into heat or light.

Is there gravity in space?
How do objects in space travel?

Is there any resistance to movement in space for DS1?

Why do ion engines need a vacuum to work?
What's a resistor?