5. Forces

• Scalar


  A quantity with magnitude but no direction
• Vector


  A quantity with both magnitude and direction
• A force is a push or pull upon an object resulting from its interaction with another object
• Contact forces


  • Friction
  • Air resistance
• Non-contact forces


  • Gravitational force
  • Magnetic force
  • Electrostatic force
• Direction of arrow


  Direction of vector
• Increase magnitude of velocity to 60m/s


  Length of arrow gets longer
• Change direction of velocity to 60° to the horizontal


  Direction of arrow changes and points in that direction
• Force


  A push or pull upon an object resulting from its interaction with another object
• Two main types of forces


  • Contact forces
  • Non-contact forces
• Contact forces


  • Objects must touch each other
  • Examples: friction, air resistance, tension in ropes, normal contact force
• Non-contact forces


  • Can act at a distance without physical contact
  • Examples: magnetic force, gravitational force, electrostatic force
• Mass


  Amount of matter in an object
• Weight


  Force due to gravity acting on an object with mass
• Weight is different from mass as it is dependent on the gravitational field strength that the Earth (or a planet) exerts on the object</b>
• Gravitational field strength


  The weight of an object is influenced by the gravitational field strength where the object is located
• The gravitational field strength on Earth is 9.8 N/kg
• Calculating weight


  W = m x g
• Weight is directly proportional to the mass of an object
• Centre of mass


  The weight of an object acts through a single point known as the centre of mass
• Free body diagram


  Helps visualise all the forces acting upon a single object or system
• Resultant force


  The overall force resulting from the combination of all forces acting on an object
• Finding resultant force when multiple forces act along the same line


  Add forces moving in the same direction, subtract those in the opposite direction
• Finding resultant force when forces are not on the same line

  1. Step 1: Draw forces on a scale grid from 'tip to toe'

  2. Step 2: Draw a resultant arrow from start of first force to end of second force

  3. Step 3: Measure length of resultant force

  4. Step 4: Measure angle of resultant force
• Components of a force

  Horizontal component, vertical component
• Work


  Done on an object when it is moved through a distance by a particular force
• Calculating work done


  W = F x s
• One joule of work is done when a force of one newton causes an object to move a distance of one metre</b>
• Energy transfer


  When work is done, energy is transferred from one form to another
• If work is done against frictional forces, the kinetic energy is often transformed into thermal energy, which can cause a rise in the temperature of the object involved
• Deformation


  When more than one force is applied to a stationary object, it can cause the object to change shape
• Types of deformation


  • Elastic deformation
  • Inelastic deformation
• Hooke's law


  The extension of an elastic object like a spring is directly proportional to the force applied, given the limit of proportionality is not exceeded
• Hooke's law equation


  F = k x e
• Hooke's law graph


  • Linear relationship until limit of proportionality, then becomes non-linear
  • Elastic limit is the point where object deforms inelastically
• Elastic potential energy


  The work done in stretching (or compressing) an elastic object is stored as elastic potential energy within the object
• Calculating elastic potential energy


  EPE = 1/2 x k x e^2
• Distance


  How far an object has travelled, regardless of starting point or final position
• Displacement


  How far an object is from its starting point and in what direction
• Speed


  Measure of how fast an object is moving, defined as distance travelled in a given time