Forces & Motion – Quick Review

Forces – General Principles

  • Force: push, pull, twist, squash, stretch, support.
  • Unit: newton (N)\text{newton (N)}, measured with a newton meter.
  • Effects: start/stop motion, change speed/direction, change shape.

Balanced vs Unbalanced Forces

  • Balanced (resultant =0=0): object stays at rest OR moves in a straight line at constant speed.
  • Unbalanced (resultant ≠0\neq0): produces acceleration in direction of larger force.

Mass, Weight & Gravity

  • Mass: amount of matter (kg); unchanged by location.
  • Weight: downward force due to gravity W=mgW = mg.
  • Earth’s gravitational field strength g≈10 N/kgg \approx 10\, \text{N/kg}.
    • 1 kg⇒10 N1\,\text{kg} \Rightarrow 10\,\text{N} ; 80 kg⇒800 N80\,\text{kg} \Rightarrow 800\,\text{N}.
  • In space: mass same, weight =0=0 (no gravitational field).

Friction, Drag & Air Resistance

  • Friction: contact force opposing motion; creates heat, wear.
  • Reduced by smooth surfaces, oil, wax; useful for grip & braking.
  • Drag: friction in fluids (liquids/gases); in air ⇒ air resistance.
  • Faster motion ⇒ larger drag.
  • Streamlining reduces drag → higher speed & better fuel efficiency.

Free Fall & Terminal Velocity

  • Initial fall: only weight acts ⇒ acceleration downward.
  • Speed ↑ ⇒ air resistance ↑.
  • Terminal velocity: point where air resistance=weight\text{air resistance} = \text{weight} ⇒ constant speed.
    • Approximate values: head-first 290 km/h290\,\text{km/h}; horizontal 190 km/h190\,\text{km/h}; parachute open 8 km/h8\,\text{km/h}.
  • Parachute increases area ⇒ greater drag ⇒ lower terminal velocity for safe landing.

Hooke’s Law (Springs & Wires)

  • Extension ∝\propto applied force until elastic limit reached.
  • Formula: F=kxF = kx (FF force, xx extension, kk spring constant).
  • Beyond elastic limit: permanent deformation, law no longer holds.

Moments & Levers

  • Moment (turning effect): M=F×dM = F \times d (force FF perpendicular to pivot at distance dd).
  • Longer lever ⇒ larger moment with same force ⇒ easier to turn/lift.

Buoyancy (Upthrust)

  • Upthrust equals weight of displaced fluid.
  • Floating object: upthrust=weight\text{upthrust} = \text{weight} (forces balanced).
  • Add weight ⇒ object sinks until new equilibrium or until \text{weight} > \text{max upthrust} (sinks completely).

Quick Facts Recap

  • 11 balanced = no acceleration.
  • Fnet≠0F_{\text{net}} \neq 0 → acceleration per Newton’s laws.
  • Streamlined rears of vehicles prevent low-pressure drag.
  • Lubricants reduce friction inside machines; brakes & tyres rely on friction.
  • Longer spanners/bike pedals exploit moments to reduce required effort.