# The skier glides down the mountain at a constant speed of 5 m / s. The skier’s movement is straightforward

**The skier glides down the mountain at a constant speed of 5 m / s. The skier’s movement is straightforward. Consider the frame of reference associated with the Earth inertial. In this case 1 …**

1 of Newton’s law says that a body is at rest (does not move) or uniform (with constant speed) rectilinear motion if it is not acted upon by forces from other bodies or the sum of all forces is zero.

When a skier moves down a mountain, 3 forces act on him: gravity m * g, friction force Ffr, reaction force of the mountain surface N. Since he moves rectilinearly with a constant speed of 5 m / s (uniformly), the vector sum of these three forces is equal to zero: m * g + Ftr + N = 0.

Answer: the sum of all the forces acting on the skier is constant and not equal to zero.