Answer
Although we know all the forces acting on an object, we do not necessarily know the direction of motion. For example, imagine that an astronaut on the moon has thrown a ball upward. After the ball leaves the astronaut's hand, there is only one force acting on the ball (gravity). This force is the same while the ball is moving upward or moving downward. Therefore, even though we know that the only force acting on the ball is gravity, we don't necessarily know which direction the ball is moving.
Work Step by Step
Although we know all the forces acting on an object, we do not necessarily know the direction of motion. For example, imagine that an astronaut on the moon has thrown a ball upward. After the ball leaves the astronaut's hand, there is only one force acting on the ball (gravity). This force is the same while the ball is moving upward or moving downward. Therefore, even though we know that the only force acting on the ball is gravity, we don't necessarily know which direction the ball is moving.
