The gravity.
In fact, the gravity will "pull" the bullet towards the ground, and the magnitude of the force is equal to the weight of the bullet:
[tex]F=mg[/tex]
where
m is the mass of the bullet
[tex]g=9.81 m/s^2[/tex] is the acceleration of gravity
Due to the presence of this force, the bullet will have a parabolic motion, which consists of two independent motions on the horizontal axis and on the vertical axis:
- on the horizontal axis, the bullet moves by uniform motion with constant speed
- on the vertical axis, the bullet moves by uniformly accelerated motion, with constant acceleration g towards the ground.
