FORCES
forces cause: change in shpe + size, change in motion, moment

Types of forces
-Gravitational F: exists bet any 2 masses, always attractive F (F exerted by Earth on mass)
-Electric F: bet 2 charges
-Magnetic F: bet 2 poles
-Frictional F: bet 2 surfaces
-Viscous F: in fluids, resists motion
-Upthrust: upwards F exerted on mass in liquids due to diff in pressure at top and bottom of mass
Archimedes principle: upthrust acting on an obj = weight of fluid displaced

Centre of gravity,cg
-rigid body (doesn't change shape appreciable under acc) considered made up of many molecules
-each molecule pulled downwards by Earth > each molecule has weight
-weight forces of each molecule act parallelly downwards, when all parallel F added > sing F_R = weight of obj
CG = pt through which whole weight of obj seems to act, regardless of body orientation
Stability = ability of obj to return to ori. position of equilibrium after external forces displace it
obj design depends on CG (position of CG determines stability)
stable: low cg + wide base

Moment of a force = turning effect of a force abt an axis
moment (Nm) = F × perpendicular dist (from line of action of F to axis of rotation)
-vector dirⁿ of moment: given by right-hand grip rule (anti-clockwise: +ve, clockwise: -ve)

Couple: pair of equal & opposite parallel F whose line of action don't coincide (torque: moment of a couple)
-F_R = 0, couples > rotation
torque/moment of a couple = mag. of 1 F × perpendicular dist. bet the 2 forces
[F-F = Fd]
moment: can be supplied by single forces, may result in non-zero F_R of obj
torque: always implies only turning effect (of a couple) considered

Coplanar forces: forces acting on same plane
for obj acted on >1 coplanar forces to be in equilibrium, 2 conditions MUST be satisfied
1: translational equilibrium- vector sum of all forces = 0, Σ F = 0, F_R = 0
2: rotational equilibrium- vector sum of all torque = 0, Σ T = 0, T_R = 0

rotational equilibrium = Principle of moments: when an obj is in equilibrium, the sum of the clockwise moments abt any pt is equal to the sum of the anit-clockwise moment abt the same pt


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