Topic 5: Forces

• Scalar quantities...


  have magnitude (size) only.
• Examples of scalar quantities...


  include mass, temperature, speed, energy, distance and time.
• Examples of vector quantities...

  include displacement, weight, force, velocity, acceleration and momentum.
• Vector quantities...


  have both magnitude and direction.
• A vector quantity may be...

  represented by an arrow.
• The length of the arrow...

  represents the magnitude.
• The direction of the arrow...

  represents he direction of the vector quantity.
• A force...


  is a push or pull that acts on an object due to the interaction with another object.
• In a contact force...

  the objects are physically touching.
• Examples of contact forces...

  include friction, air resistance, tension and normal contact force.
• In a non-contact force...

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

  include gravitational force, electrostatic force and magnetic force.
• The mass of an object...

  is how much matter the object has in it.
• The mass of an object does not depend on...


  where the object is.
• Weight...


  is the downward force acting on an object due to gravity.
• The weight of an object depends on...

  the mass of the object and the gravitational field strength at the point where the object is.
• If an object is placed on the surface of the Earth...

  then it experiences a gravitational force towards the centre of the Earth.
• The force of gravity close to the Earth...

  is due to the gravitational field around the Earth.
• The gravitational field strength...


  is the measure of the force of gravity in a particular location.
• The weight of an object...


  is directly proportional to the mass of the object.
• Weight is measured...


  using a calibrated spring-balance (newtonmeter).
• The centre of mass of an object


  is the single point where the weight of the object (the force due to gravity) can be considered to act.
• The resultant force...


  is a single force that has the same effect as all of the original forces acting together.
• To work out the resultant force...

  subtract the smaller force from the larger force
• In a free-body diagram...

  the object is shown as a point and the forces are drawn as arrows starting at the point.
• For an aeroplane flying at a constant altitude...

  there must be a force the same magnitude as the weight but acting in the opposite direction (lift).
• For an aeroplane flying at a constant velocity...

  the forward (thrust) and backward (air resistance or drag) forces must be balanced.
• Whenever a force is used to move an object...

  energy is transferred or work is done on the object.
• A force does work on an object...

  when the force causes a displacement of the object.
• 1 joule (J) is equal to...

  1 newton-metre (Nm)
• Applying a force on an elastic material...

  causes it to stretch, compress or bend.
• Elastic materials will always...

  return to their original length or shape if the forces acting on them are taken away.
• Elastic deformation is...


  when the forces are removed, the material returns to its original length and shape.
• In order to change an object's length or shape...


  more than one force has to be applied.
• If only one force is applied to a stationary object...

  the forces are not balanced, so the object would move rather than changing length or shape.
• When inelastic materials are stretched...


  they do not return to their original length and shape when the forces are removed.
• Inelastic deformation is...


  when the forces are removed, the material does not return to its original length and shape.
• When an elastic object is stretched or compressed...

  a force is used to do work and elastic potential energy is stored in the object.
• The work done to stretch or compress an elastic object...

  is equal to the elastic potential energy stored in the object, only if the object is not inelastically deformed.
• The turning effect of a force is called...

  the moment of the force.