The force of gravity, which is constantly drawing all bodies to the earth.
What else may check the motion of a body?
The air, the water, and solids of all kinds may check the motion of a body.
In what direction will a body move when its course is not obstructed?
It will move in a straight line.
Why does a ball struck by a bat make a curve to the earth?
Because the force of gravity gradually overcomes the force given by the bat, so that the ball moves in a curve.
Why does a bullet make a curve to the earth?
Because the force of gravity gradually overcomes the force given by the powder, so that the bullet moves in a curve to the earth.
When a person aims directly at a mark some distance from him, where will the bullet strike?