Answer:
Newton's second law
Step-by-step explanation:
Mathematically, Newton's second law can be written as

where
F is the net force on the particle
m is its mass
a is the acceleration
This can also be rewritten as

which means that the acceleration is directly proportional to the force and inversely proportional to the mass. If the net force is non-zero, it means that the particle has a non-zero acceleration, therefore its velocity is changing (it can be changing either in magnitude, if the force has same direction as the velocity, or also in direction, if the force has a different direction).