Hypotheses I and II are correct. Definitions for speed, velocity, acceleration, force and mass in words. Average speed of an object during a given time interval is the change in its position divided by the length of that time interval. Velocity is a measurement including both the speed of and the change in direction of an object. To completely specify an average velocity , one must describe both its average speed and its average direction over a given time interval.
Acceleration is a measurement of both the change in speed and the change in direction of an object.
Average acceleration is the change in these two divided by the time interval. Experiments show that the acceleration of an object is directly proportional to the net force acting on it. If the acceleration of an object is observed to decrease, then the net force acting on it must also have decreased in magnitude. The direction of the acceleration is the same as the direction of the net force. If no net force is exerted on an object, then its acceleration is zero.