Question

why a sharp needle pierece the skin easily but not a blunt needle altugh the force applied on both nedle is same

Answer 1

As we know pressure = force/area
And cross area of sharp needle is less then blunt
So we need more pressure for that

Answer 2

Surface area matters. As,
$pressure = \frac{force}{area}$
so less area provides more pressure which pierce the skin easily in case of sharp needle.
but in blunt needle, area is more which means less pressure hence skin doesn't gets pierced when the force is equal.