Physics · Both
Density & Pressure
Calculating density and pressure, floating and sinking, and pressure in liquids.
Density tells you how much mass is packed into a volume. Pressure is force per unit area – important for floating, hydraulics and atmospheric effects.
Density
Density ρ = mass / volume
ρ = m / V
Units: kg/m³ or g/cm³
An object denser than the fluid it is in will tend to sink; less dense will float.
Worked example
A block has mass 200 g and volume 80 cm³. Density?
Solution
ρ = 200 / 80 = **2.5 g/cm³**
Pressure
Pressure P = force / area
P = F / A
Units: pascals (Pa) = N/m²
Same force on a smaller area → higher pressure (e.g. sharp knife, high heels).
Pressure in a liquid increases with depth:
P = ρ g h (Higher)
Hydraulics (idea)
Liquids are (almost) incompressible.
Force applied to a small piston can create the same pressure on a larger piston → larger output force (used in car brakes, jacks).
Practice questions
Try these without looking at the answers first.
Q1.Density formula?
Show answer
ρ = m / V
Q2.Pressure formula?
Show answer
P = F / A
Q3.Why does a sharp knife cut more easily?
Show answer
Smaller area → higher pressure