The change in direction of all free-moving objects ?

Moving in a direction is called having a "velocity". A velocity is just having a speed, but only in one direction. Because of a thing called "Newton's First Law", in order to change the speed or direction of an object, it must be accelerated, which is a fancy way of saying it's velocity must change with respect to time, meaning that for every second it travels while be accelerated, its velocity will change by this much, either in speed or direction or both. Due to "Newton's Second Law", and acceleration is gained by applying a force. Using the equation "F = ma" where F is force applied, m is mass of the object, and a is the acceleration of the object. This means that it is harder to move things with more mass.
Specifically relating to change in direction, force is also a thing which has a direction. A force can be any size, and also have any direction. When a force is applied in a certain direction, the acceleration of the object will be in the same direction of the force.

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