Forces and motion

• Speed


  Distance travelled per unit time
• Acceleration


  Change in velocity per unit time
• Velocity


  Speed in a given direction
• Units


  • Distance: metres (m), Time: seconds (s), Speed/Velocity: metres per second (m/s), Acceleration: metres per second squared (m/s^2)
• Distance-time graph


  Gradient is velocity, Negative gradient is returning to starting point, Horizontal line is stationary, Distance zero is back at starting point, Curved line is accelerating
• Velocity-time graph


  Gradient is acceleration, Negative gradient is deceleration, Speed zero is at rest, Horizontal line is constant speed, Area under line is distance travelled, Curved line is changing acceleration
• Vector


  Has magnitude and direction
• Scalar


  Has just a magnitude
• Examples of scalars and vectors

  • Scalars: Distance, Speed, Time, Energy
  • Vectors: Displacement, Velocity, Acceleration, Force
• Forces


  • Can change speed, shape or direction of a body, Measured in Newtons (N), Different types include gravitational and electrostatic
• Friction


  Force between two surfaces that impedes motion and results in heating, Air resistance is a form of friction
• Finding resultant of forces

  Add if in same direction, Subtract if in opposite direction
• Newton's first law


  An object has a constant velocity unless acted on by a resultant force
• Newton's third law


  Every action force has an equal and opposite reaction force
• Mass


  Measure of how much matter is in an object, Measured in kilograms (kg)
• Weight


  Gravitational force, Effect of gravitational field on mass, Calculated as Mass x Gravitational field strength
• Gravitational field strength on Earth is 10N/kg
• Weight of an object acts through its centre of gravity
• Motion of a body falling in uniform gravitational field


  1. Initially no air resistance, only weight force

  2. As it falls, speed increases so air resistance increases

  3. Resultant force downwards decreases

  4. Acceleration decreases, not speeding up as quickly

  5. Eventually air resistance and weight balance, so no resultant force and terminal velocity reached
• Thinking distance


  Distance travelled between driver realising need to brake and actually pressing brakes, Increased by greater speed or slower reaction time
• Braking distance


  Distance travelled between pressing brakes and vehicle stopping, Increased by greater speed, mass, or poor road/car conditions
• Stopping distance


  Sum of thinking distance and braking distance
• Deformation


  Change in size and shape of a body due to a force
• Elastic deformation


  Object returns to original shape when load removed, e.g. stretching a spring
• Force-extension graph


  Linear graph is elastic deformation following Hooke's law, Gradient is spring constant, Non-linear graph is deformation not following Hooke's law