motion, forces and energy IGCSE Cambridge extended physics

• An object at rest stays at rest and an object in motion stays in motion with the same speed and in the same direction unless acted upon by an unbalanced force.
• A scalar quantity only has magnitude.
• A vector quantity has both magnitude (size) and direction.
• The acceleration is directly proportional to the net force acting on it and inversely proportional to its mass.
• Scalars are quantities that have only a magnitude (size or extent of something). For example, mass and speed

  Vector quantities have both a magnitude and a direction. For example velocity
• Speed is how fast an object moves. It is measured as distance travelled per unit time. Speed does not tell us about the direction of movement. The symbol used for speed is s.
• Velocity is how far an object travels in a given amount of time and also tells us which way it's moving. Velocity is calculated using the formula v = d/t where v is the velocity, d is the displacement and t is the time taken.
• Acceleration is the rate of change of velocity. Acceleration can be positive if the velocity increases, negative if the velocity decreases and zero when there is no change in velocity.
• • The speed of an object is the distance it travels per unit time


  speed is a scalar quantity
• speed = distance travelled/ time taken
• • Speed is measured in metres per second (m/s)
  • Distance travelled is measured in metres (m)
  • Time taken is measured in seconds (s)
• average speed = distance travelled / time taken
• average speed

  
• Planes fly at typical speeds of around 250 m/s. Calculate the total distance travelled by a plane moving at this average speed for 2 hours.

  1 800 000m
• • The velocity of a moving object is similar to its speed, except it also describes the object’s direction
  • The speed of an object only contains a magnitude - it’s a scalar quantity
• velocity is a vector quantity because it describes both magnitude and direction
• velocity = displacement / time
• v = s / t
• velocity (m/s)
• displacement (m)
• time (s)
• • Acceleration is defined as the rate of change of velocity
  • In other words, it describes how much an object's velocity changes every second
• acceleration = change in velocity / change in time
• a = change in v / change in t
• acceleration (m/s^2)
• change in velocity (m/s)
• time taken (s)
• The change in velocity is found by the differencebetween the initial and final velocity
• change in velocity = final velocity - initial velocity
• change in v = v - u

  v = final velocity (m/s). u = initial velocity (m/s)
• • An object that speeds up is accelerating
•  object that slows down is decelerating
• • The acceleration of an object can be positive or negative, depending on whether the object is speeding up or slowing down
  • If an object is speeding up, its acceleration is positive
  • If an object is slowing down, its acceleration is negative (sometimes called deceleration)
• Remember the units for acceleration are metres per second squared, m/s2. In other words, acceleration measures how much the velocity (in m/s) changes every second, m/s/s.
• A distance-time graph shows how the distance of an object moving in a straight line (from a starting position) varies over time
• Constant Speed on a Distance-Time Graph

  • Distance-time graphs also show the following information:
  • If the object is moving at a constant speed
  • How large or small the speed is
  • A straight line represents constant speed
  • The slope of the straight line represents the magnitude of the speed:
  • A very steep slope means the object is moving at a large speed
  • A shallow slope means the object is moving at a smallspeed
  • A flat, horizontal line means the object is stationary(not moving)
• Changing Speed on a Distance-Time Graph

  • Objects might be moving at a changing speed
  • This is represented by a curve
  • In this case, the slope of the line will be changing
  • If the slope is increasing, the speed is increasing(accelerating)
  • If the slope is decreasing, the speed is decreasing(decelerating)
• the speed of a moving object can be calculated from the gradient of the line on a distance-time graph
• garden = rise / run
• gradient = rise / run


  rise = change in y (distance) values and the run = change in x (time) values