Question

Explain why?

The size of the middle of a syringe controls flow rate better than the thumb pressure exerted by a doctor while administering an injection

Answer 1

Here, size of the needle controls velocity of flow and thumb pressure controls pressure. According to Bernoulli's theorem.

ρ + ρgh + 1/2ρv² = a constant, shows that ρ occurs with powers one and v occurs with power two, hence the velocity has more influence. That is why needle has a better control over flow.