Exercise 6.8
How are glass items packed for transportation using a bubble wrap or hay protected from damage?
Not cross-checked
NCERT prints no numerical answer for this exercise, so this working has not been cross-checked against the book.
Bubble wrap and hay protect glass by increasing the time over which a jolt changes the item's velocity, which reduces the force on it.
During transport a glass item that is jolted or dropped has to be brought to rest.
A soft, compressible packing squashes as the item pushes into it, so the velocity falls to zero over a longer time interval instead of instantly.
A longer stopping time means a smaller magnitude of acceleration, and by \(\displaystyle F = ma \) a smaller force acts on the glass.
Hitting a hard crate wall directly would stop the item in a very short time, needing a very large force — enough to crack it, just as the ground's large force cracks a coconut (Fig. $\displaystyle 6.20$).
This is the same principle as a cricketer drawing their hands back while catching, and as a vehicle airbag.
