Acceleration = Force / Mass
Acceleration = Measured in meters per second
Force = Measured in Newtons (N)
Mass = Measured in kilograms
An Apollo CSM leaving lunar orbit after jettisoning the Ascent Stage of the LEM is about to execute a Trans-Earth Injection (TEI) burn with the following parameters:
Change: 3,279 fps (999.44 m/sec)
Spacecraft Mass: 36,965.7 lbs (16,767.35 kg)
Spacecraft Thrust: 20,500 lbf (91,200 newtons)
91,200 Newtons / 16,767.35 kg = 5.44 m/sec acceleration
999.44 / 5.44 = 183.75 second burn.
This compares favorably with the actual 151.4 second burn that Apollo 11 executed to return to Earth. The difference between the actual and the calculated burn is due to the fact that the spacecraft gets lighter as propellant is burned off, while the formula above assumes the spacecraft stays at a constant mass.
At the end of Apollo 11's TEI, the mass of the CSM was 26,792.7 lbs; for a rough propellant consumption rate of about 67.19 lbs per second.