UPCAT P6 - FORCE

Newton's First Law

Law of Inertia; object stays at rest or moving in straight line unless acted on by external unbalanced force

Newton's Second Law

F_net = ma

Newton's Third Law

For every action force, equal and opposite reaction force

Weight formula

W = mg

Fundamental forces (4)

Strong nuclear, weak nuclear, electromagnetic, gravitational

Strongest vs weakest fundamental force

Strong nuclear = strongest; gravitational = weakest

Universal Law of Gravitation

F_G = G(m1m2)/r²

Normal force

Reactive force from a surface, perpendicular to it; prevents object from passing through

Friction

Resistance to motion; opposes direction of (intended) motion

Friction force formula

F_friction = μF_N (coefficient of friction × normal force)

Static vs Kinetic friction

Static = acts on stationary object; Kinetic = acts on sliding object

Friction coefficient relationship

μk < μs (harder to start sliding than to keep sliding)

Tension

Pulling force transmitted through a rope/string/cable; ropes can only pull, never push

Hooke's Law

F = -kΔx; k = spring constant (stiffness), Δx = deformation length

Higher spring constant (k) means

Stiffer spring; more force needed to deform, snaps back harder

Momentum formula

p = mv (vector)

Impulse formula

Impulse = Δp = F·t

Impulse force-time tradeoff

Large force + short time = same impulse as small force + long time

Conservation of momentum

Total momentum of system before collision = total momentum after (always true)

What differs between collision types

Whether kinetic energy is conserved

Elastic collision

Objects bounce apart; KE conserved; m1v1+m2v2=m1v1'+m2v2'

Totally inelastic collision

Objects stick together; KE NOT conserved (momentum still is); m1v1+m2v2=(m1+m2)v'

Why masses add in inelastic collision

Objects become one combined object, so total mass = m1+m2, moving at one shared velocity

inelastic collision

go in the same direction after bumping; one slow, one fast