Physic revision 2

• Speed is a scalar quantity
• Average speed


  v = total distance / total time
• Units for speed


  m/s, km/h, cm/s, mph
• In the IGCSEs, we use metric units. Hence, always use m/s
• Distance-time graphs


  • Indicate how much a vehicle has travelled within a given interval
  • The gradient (slope) of distance-time graphs indicates speed
• Speed


  = distance / time
• The steeper the gradient


  The higher the speed
• Types of distance-time graphs

  • Graph (a) has zero speed
  • Graph (b), Object A has a higher speed than Object B
  • Graph (c), speed is increasing
  • Graph (d), speed is decreasing
  • Graph (e), speed is constant

  
• Vector quantities


  Have both magnitude as well as direction
• Displacement


  Distance but with a particular direction
• Velocity


  Speed with a particular direction
• Average velocity


  Increase in displacement / time taken
• Acceleration


  Rate of change of velocity
• Velocity-time graphs


  • Indicate the velocity of a particle within a given amount of time
  • The gradient of velocity-time graphs indicates acceleration
  • The area under the graph indicates distance as total distance s = v × t

  
• Force


  A vector quantity, examples include friction, upthrust, weight, normal reaction force, etc. Its unit is Newton (N)
• In most situations, more than one force will act on the object
• Unbalanced force


  Can be calculated by considering the magnitude and direction of the forces acting on a particular object
• If forces are unbalanced, it can cause the object to change the way it moves. If balanced, the forces will have no effect
• Friction


  The force that causes moving objects to slow down and finally stop. Friction occurs when solid objects rub against other solids and also when objects move through fluids
• Forces can change the shape of an object


  Changes can be both temporary or permanent
• Temporary change in shape

  • Elastic band
• Permanent change in shape

  • Glass
• Springs


  Stretch when force is applied and return to original position after force is removed
• Hooke's Law


  The extension of the spring is directly proportional to the force applied. The graph obtained by plotting force against extension is a straight line passing through the origin
• Hooke's Law is applicable only up to a certain point called the limit of proportionality. This is where the spring stops obeying Hooke's law and starts stretching more for each load force increase
• Once the elastic limit has been reached, the spring will not return to its original length
• Elastic bands


  • When stretched, graphs like this are achieved
• If unbalanced forces are acting on an object

  The object may accelerate or decelerate depending on the direction of the unbalanced force
• When mass is constant

  The graph of force against acceleration is a straight line passing through the origin. Therefore, F ∝ a

  
• Brakes on cars and bicycles

  Work by increasing the friction between the rotating wheels and the vehicle's body
• If the road is wet or the tires are in bad condition, the frictional force will be smaller. If the brakes are too hard, the tires will not grip the road surface, and the car will skid
• Reaction time


  The time a driver takes to react to a new object or situation in front of them
• Thinking distance


  The amount of distance travelled during the reaction time
• Braking distance


  The distance travelled within the time when brakes are first applied and the car stops
• Weight


  The force that acts on an object because of gravity
• Air resistance or drag


  The force that opposes the movement of an object through air
• Drag coefficient


  A measure of how easily an object moves through the air
• Velocity–time graph for a free-fall parachutist reaching terminal velocity

  
• Momentum


  Measures how difficult it is to stop something that is moving
• Rate of change of momentum


  Proportional to the force applied to that object