Forces and Motion

• Units


  • Kilogram (kg): Unit of mass
  • Metre (m): Unit of length
  • Metre/second (m/s): Unit of speed
  • Metre/second² (m/s²): Unit of acceleration
  • Newton (N): Unit of force
  • Second (s): Unit of time
  • Newton/kilogram (N/kg): Unit of gravitational field strength
• Additional Units


  • Newton metre (Nm): Unit of torque or moment
  • Kilogram metre/second (kg m/s): Unit of momentum
• Distance-time graph


  • Shows how distance changes over time
  • Slope indicates speed, steeper slope means higher speed
  • Horizontal line means the object is stationary
• Average speed


  Distance moved / Time taken
• Investigating motion of everyday objects

  Use toy cars, tennis balls, etc., to observe and record their motion
• Acceleration


  Change in velocity / Time taken
• Velocity-time graph


  • Shows how velocity changes over time
  • Slope indicates acceleration
  • Horizontal line means constant velocity
• Gradient of the line in a velocity-time graph equals acceleration
• The area under the curve in a velocity-time graph represents the distance travelled
• Relationship between final speed, initial speed, acceleration, and distance moved


  (v)^2 = (u)^2 + 2as
• Effects of forces between bodies

  • Changes in speed: Forces can accelerate or decelerate objects
  • Changes in shape: Forces can deform objects
  • Changes in direction: Forces can alter the direction of motion
• Types of force


  • Gravitational force: Attraction between masses
  • Electrostatic force: Attraction or repulsion between charged particles
• Vector vs. Scalar quantities

  • Vector quantities have both magnitude and direction (e.g., force, velocity)
  • Scalar quantities have only magnitude (e.g., mass, speed)
• Force is a vector quantity
• Resultant force


  Sum of all forces acting on an object along the same line
• Friction opposes motion
• Relationship between unbalanced force, mass, and acceleration

  Force = Mass × Acceleration
• Relationship between weight, mass, and gravitational field strength


  Weight = Mass × Gravitational field strength
• Stopping distance of a vehicle

  Sum of thinking distance and braking distance
• Factors affecting vehicle stopping distance

  • Speed: Higher speed increases stopping distance
  • Mass: Heavier vehicles take longer to stop
  • Road condition: Wet or icy roads increase stopping distance
  • Reaction time: Longer reaction time increases stopping distance
• Forces on falling objects and terminal velocity

  • Gravity: Pulls the object downward
  • Air resistance: Opposes the motion
  • Terminal velocity: Reached when the force of gravity equals air resistance, resulting in no further acceleration
• Investigating extension with applied force

  Use helical springs, metal wires, and rubber bands to study how they stretch with applied force
• Hooke's Law


  The extension of a spring is directly proportional to the applied force, within the elastic limit
• Elastic behavior


  Ability of a material to return to its original shape after the force is removed
• Relationship between momentum, mass, and velocity

  Momentum = Mass × Velocity
• Use momentum to explain safety features

  • Crumple zones: Absorb impact force, reducing momentum change
  • Seatbelts: Extend the time over which momentum changes, reducing force
• Total momentum before a collision equals total momentum after, in the absence of external forces
• Relationship between force, change in momentum, and time taken

  F = (Change in momentum) / (Time taken)
• Newton's third law: For every action, there is an equal and opposite reaction
• Relationship between moment of a force and its perpendicular distance from the pivot


  Moment = Force × Perpendicular distance from the pivot
• Weight acts through the center of gravity
• For equilibrium, the sum of clockwise moments equals the sum of anticlockwise moments
• The upward forces (reactions) depend on the position of a heavy object placed on the beam. The closer the object to a support, the greater the force on that support