forces and their interactions

• Scalar quantities 


  have magnitude only
• Vector quantities 


  have magnitude and an associated direction.
• A vector quantity...


  may be represented by an arrow.

  The length of arrow represents the magnitude

  the direction of the arrow represents the direction of the vector quantity
• A force


  a push or pull that acts on an object due to the interaction

  with another object.
• All forces between objects are either:

  • contact forces – the objects are physically touching

  • non-contact forces – the objects are physically separated.
• Examples of contact forces

  friction

  air resistance

  tension

  normal contact force
• Examples of non-contact forces

  gravitational force

  electrostatic force

  magnetic force.
• Force 


  is a vector quantity.
• Weight 


  the force acting on an object due to gravity
• The force of gravity close to the Earth is due to the gravitational field strength around the Earth.
• The weight of an object

  depends on the gravitational field strength at the point where the object is.
• The weight of an object can be calculated using the equation:

  weight = mass × gravitational field strength

  W = m g

  weight, W, in newtons, N

  mass, m, in kilograms, kg

  gravitational field strength, g, in newtons per kilogram, N/kg
• centre of mass


  The weight of an object may be considered to act at a single point
• The weight of an object and the mass of an object are directly

  proportional
• Weight is measured using a calibrated spring-balance (a

  newtonmeter).
• resultant force

  A number of forces acting on an object may be replaced by a single

  force that has the same effect as all the original forces acting

  together.

  to work it out : "larger force - smaller force"
• Vector diagrams

  draw out the forces (eg 10N = 10cm etc) as lines connecting according to the angle given

  • then draw those line in the opposing position
  • measure the middle line from both corners and that's the answer (resultant force)
• vector diagrams

  • draw faint lines to show the horizontal and vertical axes
  • using a protractor measure angle from the object and use a ruler to draw the vector showing the force.
  • draw a dotted line from the head of the vector to the horizontal and vertical axes
  • measure the vertical and horizontal lines and use the scale to find the results