Here's why:
* Thrust is the force that propels the space shuttle forward. It's generated by the engines burning fuel and expelling hot gas out the back. This expulsion of gas creates an equal and opposite force on the shuttle, pushing it in the opposite direction.
* Gravity is also acting on the shuttle, pulling it towards Earth. However, gravity becomes less significant as the shuttle gains altitude.
* Air resistance is a force that opposes the motion of the shuttle through the air. It's negligible in space, but during launch and re-entry, it can play a significant role.
So, while gravity and air resistance might be present, the force that directly drives the acceleration of the space shuttle is thrust.
