**
***The space shuttle is a spaceship which was designed for transporting a payload to near-earth orbits and to be used many times. When it stands on the launching pad, it consists of the orbiter itself, the external tank, and two booster rockets. The orbiter is 7.32 X 10 ^{4} kg, while the whole assembly is 2.0 X 10^{6} kg. The thrust at liftoff is 2.86 X 10^{7} N, which is achieved by burning 3400 kg of fuel each second. The force provided by the engines is given by the product of the velocity of the exhaust gases relative to the ship and the rate (mass per time) at which fuel is burned.*

*As the space shuttle ascends, the rate of fuel burning is approximately constant, and so the mass of the shuttle decreases.*

*The following chart shows hypothetical data for the liftoff of a shuttle. We are assuming the shuttle moves in one dimension upward. Use 10 m/s ^{2} for the acceleration due to gravity. The force due to gravity is F_{grav} = Mg, where M is the mass of the object, and g is the acceleration due to gravity.*

__t(s) x (m) v (m/s)__

*0 0.0 x 10 ^{4} 0*

*90 2.0 x 10 ^{4} 490*

*180 9.6 x 10 ^{4} 1240*

*270 25.5 x 10 ^{4} 2400*

Consider a situation when the shuttle is in space and fires its engines, creating a force accelerating the shuttle. Which of the following BEST describes the force accelerating the shuttle?