Topic 5 – Forces

• Quantities of Measure


  • Scalar (Magnitude)
  • Vector (Magnitude + Direction)
  • Speed (m/s)
  • Distance (m)
  • Time (s)
  • Mass (kg)
  • Velocity (m/s)
  • Displacement (m)
  • Force (N) e.g. Weight (N)
  • Momentum (kgms-1)
• Types of Forces


  • Contact
  • Non-Contact
  • Friction
  • Air resistance
  • Tension in ropes
  • Normal contact force
  • Electrostatic force
  • Gravitational force
  • Magnetic force
• A pair of forces acting on SAME object = Equal and opposite (N's 3rd Law)
• Mass


  Amount of matter
• Weight


  Force due to gravity
• Mass never changes, Weight changes based on planet
• Balance


  Newton meter
• Centre of Mass


  The point at which the mass is thought to be concentrated
• Objects with higher centre of mass/smaller base = more likely to fall over
• Weight


  Mass (kg) x Gravitational Field Strength (N/kg)
• Newton's 1st Law


  Resultant (overall) forces
• Resultant force effects


  • No change to object: STOPPED → STOPPED, MOVING → MOVING (At same speed)
  • A change occurs (Acceleration): START, SPEED UP, CHANGE DIRECTION, SLOW DOWN, STOP
• Work done


  When a FORCE (N) is used to move and object = Energy is transferred
• Scale drawings


  Put forces end-to-end until you get an OVERALL force
• A car has driving force of 40N north and wind 30N east – what is overall?
• Elastically deformed


  Can return to original shape, Energy is transferred to elastic potential store, When released, Elastic potential → Kinetic (as it moves to original shape)
• Inelastically deformed


  Can not return to original shape
• Spring constant (k)


  • Large = Higher gradient on graph, STIFFER
  • Small = Lower gradient on graph, FLEXIBLE

  
• Practical - Relationship Force (N) vs. extension (m) in spring


  1. Set up spring on clamp beside ruler

  2. Measure natural length of spring (no masses)

  3. Add mass to spring (force)

  4. Record extension (change in length)

  5. Repeat step 3-4 until you have enough results to plot F vs. e graph

  
• Elastic energy


  E = 1/2Ke2
• Moments


  A Turning effect created by a FORCE
• Pressure


  Force / Area
• The lower you go the more (height) increases = ^Pressure, More pressure from the bottom spouts
• Factors affecting pressure


  • ^ Height (below surface)
  • ^Density
  • Gravitational field
• Upthrust


  Resultant force (UP) when object is submerged in fluid
• Float vs Sink


  • Weight ≤ Upthrust
  • Weight > Upthrust
• How do Submarines rise and dive?

  1. DIVE: Fills up with water, Weight > Upthrust

  2. RISE: Fills up with air, Weight < Upthrust
• Atmospheric pressure reduces as you go higher (less air particles)
• Speed


  Distance / Time
• Acceleration


  v - u / t
• Uniform/Constant Acceleration


  v2 - u2 = 2ad
• Terminal velocity


  The maximum speed of a moving object (forces balance out)

  
• Sky-diving person reaching terminal velocity

  1. Initially Weight > Air resistance

  2. Resultant force → Acceleration

  3. As speed^ → Air resistance^ but weight stays same

  4. Eventually Weight = Air resistance

  5. No overall force = no acceleration

  6. Speed stays constant (Terminal Velocity)
• Car speeding up to terminal velocity

  1. Initially driving force > Friction + AR

  2. Resultant force → Acceleration

  3. As speed^ → Friction + AR^ but driving force stays same

  4. Eventually driving force = Friction + AR

  5. No overall force = no further acceleration

  6. Constant speed (Terminal Velocity)

  
• Terminal Velocity depends on surface area, Larger SA = Large Air Resistance, AR catches up with driving force/weight quicker, Less time to accelerate, Reach terminal velocity sooner (lower)
• Inertia


  How difficult it is to change the velocity of an object (accelerate it), A larger mass = more Force to accelerate = more inertia
• Newton's 3rd Law scenarios

  • Forces are equal and opposite in all scenarios
  • Same TYPE of force (push (contact) vs. normal contact)
  • Different types of forces
  • Different objects
  • Both on same/one object
• Forces are equal and Earth has less mass so shouldn't it be pulled into the Sun? Yes, BUT, Earth is also trying to moving away at the same time. Gravity (IN) and Earth's velocity (AWAY) cancel out and that's why it stays in the same place.
• Investigating motion (Newton's 2nd Law)

  1. Trolley (w/Card) attached to hanging mass on a ramp

  2. Ensure the table is high enough and line is taut (not loose)

  3. Release trolley and allow it to accelerate down ramp

  4. When card interrupts light gate à acceleration calculated

  5. Repeat and calculate an average
• Accelerating Force


  Weight = gravity acting on hanging masses