1a,b,c Forces and motion

• Speed

  Distance travelled per unit time
• Acceleration

  Change in velocity per unit time
• Velocity

  Speed in a given direction
• 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

  • Distance, Speed, Time, Energy
• Examples of Vectors


  • Displacement, Velocity, Acceleration, Force
• Forces

  • Can change speed, shape or direction of a body, Measured in Newtons (N)
• Friction


  Force between two surfaces which 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 a gravitational field on a 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 a uniform gravitational field

  Initially no air resistance, only weight force, Acceleration increases speed and air resistance, Resultant force decreases, Acceleration decreases, Terminal velocity reached when air resistance equals weight
• Thinking distance

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

  Distance travelled between pressing brakes and vehicle stopping, Increased by greater speed, mass, 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, Example: Stretching a spring
• Limit of proportionality

  Point where force-extension graph stops being linear and Hooke's law no longer applies
• Moment of a force

  Measure of its turning effect, Calculated as Force x Perpendicular distance from pivot, Measured in Newton metres (Nm)
• Equilibrium

  Sum of clockwise moments equals sum of anticlockwise moments, No resultant force
• Conservation of momentum

  Total momentum before a collision equals total momentum afterwards
• A vector quantity has both magnitude and direction.