Revise: Forces
Newton's three laws of motion, gravitation, contact forces and equilibrium, mass and weight, freely falling bodies, and momentum and its conservation.
An object keeps its state of motion unless a net force acts (inertia).
A tablecloth yanked fast leaves the dishes almost still.
Every action has an equal, opposite reaction on the OTHER object.
Swimmer pushes water back; water pushes swimmer forward.
W = mg. Mass is constant; weight depends on local gravity.
80 kg on the Moon (g=1.6) → 128 N.
Zero net force means constant velocity, not necessarily rest.
A frictionless ball keeps rolling forever.
Action-reaction pairs act on two different objects, never the same one.
Rocket exhaust pushed back; rocket pushed forward.
F = Gm₁m₂/r² — inverse-square law.
50kg & 80kg, 2m apart → F ≈ 6.67×10⁻⁸ N.
F(friction) = μN. Normal reaction and tension are also contact forces.
μ=0.4, N=300N → friction max = 120 N.
Resultant = vector sum of all forces. Equilibrium: resultant = 0.
40N right, 15N friction left → resultant 25N right.
F=ma and W=mg together give a=g, independent of mass.
Apollo 15's hammer and feather land together on the Moon.
A longer contact time reduces the force for the same momentum change.
400N for 0.5s → impulse 200 N·s.
Total momentum before = total momentum after (no external force).
4kg@6m/s hits stationary 2kg, stick together → 4 m/s.