Whenever a body exerts a force on another body, the latter exerts a force of equal magnitude and opposite direction on the former.
Velocity is a vector quantity, which consists of both a direction and a magnitude.
The velocity of an object is said to have changed if either its direction or its magnitude is changed. Like velocity, acceleration is also a vector quantity. Newton stated that the force is in the direction of the acceleration. The law of inertia suggests that the Moon would be flying off in a straight line unless there is a force that pulls it back and keeping it in a circular orbit around the Earth. This force Newton called gravity .