Stretching, bending or compressing an object requires more than one force

Elastic distortion: object returns to original shape after removing force.

Inelastic distortion: object permanently deforms.

F = kx

E = 0.5kx^{2}

F: force (N), k: spring constant (N/m), x: extension (m), E: energy (J).

Hooke's Law: extension is directly proportional to force until elastic limit (straight line).

Work done = energy transferred = area under the graph.

Atmospheric pressure is the total weight of the air above a unit area at a certain altitude. It decreases as height above the Earth increases because there are fewer air molecules when higher, so there is less weight, so less force, so less pressure on the same given area.

Fluids: liquids or gases.

Pressure in a fluid causes a force on any surface, equal to force/ area.

Pressure in a fluid is due to the fluid and atmospheric pressure e.g. a balloon can expand as you pump more particles inside it, so there is outward pressure. This is greater than the inward force caused by the lower atmospheric pressure.

P = F/A

P: pressure (Pa), F: Force (N), A: area (m^{2}).

Pressure in a fluid increases due to depth or due to density, due to a greater weight so a greater force per unit area.

P = h× ρ × g

h: height of column (m), ρ: density (kg/m^{3}), g: gravitational field strength (N/m).

Factors of floating/ sinking: upthrust, weight, density of fluid.

An object will float after it has it displaced the same weight of water as its own weight.