# Newton’s law

Newton’s first law (the law of inertia) The body keeps its speed constant (uniform rectilinear motion v=const, a=0) or is at rest while the body is not affected by external forces.

F=0

F — force, N.

Newton’s second law

The force acting on a body is equal to the product of mass and acceleration.

F=ma

m — mass, kg;
a — acceleration, m/s2. Newton’s second law applies only to terrestrial bodies moving at a low speed. An example of Newton’s second law is the motion of a body along an inclined plane discovered by Galileo Galilei. N+Ffriction+mg=ma

N — the reaction force of the support.
Ffriction — friction force, determined by the formula Ffriction=µN
μ is the coefficient of rolling friction.
X-аxis:

Ffriction−mgsinθ=ma

Y-аxis:

N−mgcosθ=0

Newton’s third law The forces acting on each other are opposite in direction and equal in modulus

F12=−F21

m12a12=−m21a21

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