Topic 15 ~ Forces and Matter

• Stretching, bending, and compressing

  More than one force has to be applied
• Deformation

  Changing shape
• Elastic Deformation

  • The object returns to its original shape when the load has been removed
  • Example: An elastic band
• Plastic Deformation (distortions)

  • The object does not return to its original shape when the load has been removed
  • Example: A spring when pulled too far
• Linear Elastic Distortion, Hooke's Law

  • F = kx, where F is the force applied to the spring, N, k is the spring constant, Nm−1, and x is the extension, m
  • Hooke's Law: The extension of a spring is directly proportional to the force applied
• Linear line section on a Force/Extension Graph

  • This is elastic
  • Following Hooke's Law
  • Gradient is k
  • Elastic deformation
  • The point it stops being linear is the elastic limit
  • From then on, it does not obey Hooke's Law
• Non-Linear line section

  • Not elastic
  • Does not follow Hooke's Law
  • Plastic Deformation
  • If shallow, lots of extension for not a lot of force, easy to stretch
  • If graph is just linear, with no "non-linear end section", the material is "brittle", so snaps instead of stretches after the elastic limit
• Work Done

  • Area under the Force/Extension graph
  • Work Done = 1/2 kx^2
• Atmospheric Pressure

  • The total weight of the air above a unit area at a certain altitude
  • Atmospheric pressure decreases with increasing height above the Earth's Surface
• Pressure in a fluid

  • Pressure of a gas (for example held in a balloon) depends on atmospheric pressure
  • Pressure in a fluid causes a force perpendicular to any surface
  • Pressure = force/area
• Pressure in Liquids

  • Deeper in a fluid you are, the greater the pressure
  • Denser the fluid is, the greater the pressure
• Floating and Sinking

  • An object floats if its weight is less than the weight of the water it displaces
  • The buoyancy force is the upwards force that counteracts the weight of the floating object
  • This force is equal to the weight of the fluid displaced by the object
• Pressure due to a column of liquid = height of column × density of liquid × g