When a skateboarder rides down a hill, the normal force exerted on the skater by
the hill is

Newton's second law allows us to find that the correct answer is:
Newton's second law states that the net force and is propositional to the mass and acceleration of the body
For these problems it is essential to set a reference system, with respect to which to carry out the measurements, in this case we set a coordinate system with the x axis parallel to the plane and positive in the direction of movement and the y axis perpendicular to the plane.
In the attachment we can see a free-body diagram of the problem, let's work each axis separately
x-axis
Wₓ -fr = m a
y-axis
N - [tex]W_y[/tex] = 0
N = [tex]W_y[/tex]
Where Wₓ and W_y are the components of the weight, fr the friction force that opposes the movement, m the mass and the acceleration of the body
let's use trigonometry to find the components of the weight
cos θ = [tex]\frac{W_y}{W}[/tex]
sin θ = [tex]\frac{W_x}{W}[/tex]
[tex]W_y[/tex] = W cos θ
Wₓ = W sin θ
we substitute
N = mg cos θ
From this equation we can see that the normal is less than the weight of the body.
In conclusion using Newton's second law we find that the correct answer is:
Learn more about Newton's second law here:
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