Newton's laws of motion
1st: object at rest stays at rest. 2nd: F = ma. 3rd: action-reaction equal and opposite.
Y12 Mechanics. F is the resultant force in Newtons; m in kg; a in m/s². Always identify all forces, find resultant, apply F = ma.
Worked examples
5 kg block; 20 N horizontal force; smooth surface: a = 4 m/s².
Lift accelerating up at 2 m/s²; person mass 60 kg: tension in floor = 60(g+2) = 60×11.8 = 708 N.
Action-reaction: book on table; gravity pulls book down; table pushes up equally.
Frequently asked questions
Weight vs mass?
Mass is the amount of matter (kg); weight is the gravitational force on it (N) = mg.
g = 9.8 or 10?
9.8 (or 9.81) for accuracy; 10 for quick mental work in some questions.