A boy weighing 50 kg put on a backpack weighing 5 kg. What is the force of gravity acting on it?

The innate property of a person is not to notice his weight, but to noticeably react to the weight of the objects he carries.

In this case, the boy, having put on a backpack, will immediately feel its weight, which is equal to –

p = mg, where m is the mass of the backpack, g is the acceleration of gravity,
(let’s round it up to 10 m / s² for convenience).

Backpack weight – 5 kg * 10 m / s² = 50 N.

It is this force (one might say, the gravity (weight) of the backpack), directed down to the ground, stretching the straps of the backpack, will exert noticeable pressure on the boy (on his shoulders).