Forces, Density and Pressure: notes

Forces

• Newton's first law: an object keeps its state of rest or steady motion unless an unbalanced force acts on it.
• Newton's second law: F = ma. A 4 kg trolley pushed by a 12 N net force accelerates at 3 m/s².
• Weight: W = mg, the pull of gravity. A 5 kg mass weighs 50 N where g is 10 N/kg. Mass stays the same everywhere; weight does not.
• Friction: a force that opposes motion between two surfaces.
• Hooke's law: a spring's extension is proportional to the load, up to the limit of proportionality.

Turning effects

• Moment: force x perpendicular distance from the pivot. 20 N at 0.5 m gives 10 N m.
• Equilibrium: the clockwise moments equal the anticlockwise moments, and the forces balance.

Pressure

• Solid pressure: p = F / A. 600 N on 0.03 m² gives 20 000 Pa. A smaller area gives a larger pressure.
• Liquid pressure: p = hρg. It grows with depth and density, and acts equally in all directions.
• Atmospheric pressure: measured with a barometer. It falls as you go higher.