Apply Newton's second law for an object at rest on Earth and analyse the result.

Explanation

The object is at rest with respect to Earth (inertial coordinate system). There are two forces that act on the object.
(i) Gravity acting downward (negative y-direction) (ii) Normal force by the surface of the Earth acting upward (positive y-direction)
The free body diagram for this object is
vec F_g = - mg hatj `
vecF = N hatj `
But there is no acceleration on the ball. So `veca = 0` . By applying Newton.s second law `(vecF_("net") = mvec a )`

Since `vec a = 0 , vecF_("net") =-mg hatj + N hatj `
`(-mg + N) hatj = 0`
By comparing the components on both sides of the equation, we get
` - mg + N = 0`
N = mg `
We can conclude that if the object is at rest, the magnitude of normal force is exactly equal to the magnitude of gravity.