definitions and concepts

• breaking stress

  the maximum stress that an object can withstand before failure occurs
• brittle

  a brittle object will show very little strain before reaching its breaking stress
• centre of mass

  the single point through which all the mass of the object can be said to act
• conservation of energy

  energy cannot be created or destroyed - it can only be transferred into different forms
• conservation of momentum

  the total momentum of a system before an event, must be equal to the total momentum of the system after the event, assuming no external forces act
• couple

  two equal and opposite parallel forces that act on an object through different lines of action

  • it has the effect of causing a rotations without translation
• density

  the mass per unit volume of a material
• efficiency

  the ratio of useful output to total input for a given system
• elastic behaviour

  if a material deforms with elastic behaviour, it will return to its original shape when the deforming forces are removed

  • the object will not be permanently deformed
• elastic collision

  a collision in which the total kinetic energy of the system is conserved before and after the collision
• elastic limit

  the force beyond which an object will no longer deform elastically, and instead deform plastically

  • beyond the elastic limit, when the deforming forces are removed, the object will not return to its original shape
• elastic strain energy

  the energy stored in an object when it is stretched

  • equal to the work done to stretch the object
  • can be determined from the area under a force-extension graph
• equilibrium

  both the resultant force and resultant moment acting on the object must be equal to zero
• hooke's law

  the extension of an elastic object will be directly proportional to the force applied to it up to the object's limit of proportionality
• impulse

  the change of momentum of an object when a force acts on it

  • equal to the product of the force acting on the object and the length of time over which it acts
• inelastic collision

  a collision where the total kinetic energy of the system before the collision is not equal to the kinetic energy of the system after the collision
• moment

  the product of a force and the perpendicular distance from the line of action of the force to the pivot
• momentum

  the product of an object's mass and velocity
• newton's first law

  an object will remain in its current state of motion, unless acted on by a resultant force

  • an object requires a resultant force to accelerate
• newton's second law

  the sum of the forces acting on an object is equal to the rate of change of momentum of the object
• newton's third law

  every action has an equal and opposite reaction

  • if an object exerts a force on another object, then the other object must exert a force back that is opposite in direction and equal in magnitude
• plastic behaviour

  if a material deforms with plastic behaviour, it will not return to its original shape when the deforming forces are removed

  • the object will be permanently deformed
• principle of moments

  for an object to be in equilibrium, the sum of the clockwise moments acting about a point must be equal to the sum of the anticlockwise moments acting about the point
• scalar

  a scalar quanitity only has magnitude

  • e.g. length, mass, temperature
• spring constant

  the constant of proportionality for the extension of a spring under a force

  • the higher the spring constant, the greater the force needed to achieve a given extension
• stiffness

  a measure of how difficult it is to stretch a given object
• tensile strain

  the ratio of an object's extension to its original length

  • it is a ratio of two lengths so has no units
• tensile stress

  the amount of force acting per unit area

  • unit is pascals, Pa
• vector

  a vector quanitity has both magnitude and direction

  • e.g. velocity, displacement, acceleration
• young modulus

  the ratio of stress to strain for a given material

  • unit is pascals, Pa