It is called a backpack or, less often now, a rucsack.
rucksack
You are describing a backpack.
A knapsack is a bag with shoulder straps that is carried on the back. It is usually made out of canvas o other weatherproof material.
Yes, the backpack straps on this bag are detachable.
Generally, a backpack is used for school. Make sure that it is large enough for everything with extra padding and support at the back and on the shoulder straps.
uhh backpack you idiot..
Larger the area smaller the pressure. School bags usually have broad straps so that the weight of the bag may fall over a large area on the shoulder producing less pressure and less pressure, it is comfortable to carry heavy bag.
That would be a backpack.
SHOULDER or MESSENGER BAG
a bag
The boy carried a capacious(meaning : of large capacity) bag.
A rucksack or (backpack, haversack, knapsack, packsack) is a bag carried by a strap on your back or shoulder.