The change of motion of a body is proportional to the applied force, and it takes place in the direction of a straight line in which that force is applied. this describes what law of motion?

Newton's Second Law of Motion  

Explanation:

According to Newton's second law of motion, the change in velocity of a body is directly proportional to the force applied on it. Velocity is a vector quantity. It measures the magnitude of the speed as well as its direction.

F = m a

where, F is the applied force, m is the mass and a is the acceleration.

It can also be expressed as:

where, p = mv ( momentum)

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the answer is a. at the equator