Mass determines the force of gravity like this:
If an object weight a lot it will affect the mass and gravity pulls on the object lots more when an object has lots of weight/mass.
Example: An airplane. An airplane probably weighs a ton. That is why gravity pushes it down in order for the plane to go down when it is landing or else it would be flying but never coming down. But the gravity pulling down the airplane is still there so the airplane has to be pulling the steer up in order for it to be heading straight.
Glad to help! :)
