Topic 15 - Forces and Matter

• Springs

  • Stretching, bending, and compressing
  • More than one force has to be applied
  • Deformation means changing shape
  • Elastic Deformation: Object returns to original shape when load is removed
  • Plastic Deformation: Object does not return to original shape when load is removed
• Linear Elastic Distortion, Hooke’s Law

  1. Spring constant k = Force required to extend a spring per metre

  2. F = kx

  3. Hooke’s Law: The extension of a spring is directly proportional to the force applied
• Linear line section on a Force/Extension Graph

  • Elastic
  • Following Hooke’s Law
  • Gradient is K
  • Elastic deformation
  • Elastic limit marks the point where it stops being linear
• Non-Linear line section on a Force/Extension Graph

  • 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”
• Work Done

  1. Area under the graph

  2. Work Done = 1/2 kx^2
• Atmospheric Pressure is 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
• The weight of the air

  Is the force which causes the pressure
• With higher elevation

  There are fewer air molecules above the unit area than at lower heights, resulting in less pressure
• Pressure in a fluid is a liquid or a gas
• Pressure of a gas

  Depends on atmospheric pressure
• The air exerts an inward force on the outer surface of the balloon

  If this force is greater than the outward force exerted by the gas
• Pressure in a fluid

  A fluid is a liquid or a gas
• Pressure of a gas in a balloon

  1. The air exerts an inward force on the outer surface of the balloon, and if this force is greater than the outward force exerted by the gas particles inside, the balloon will collapse

  2. In space, the gas inside exerts a greater force outward force on the balloon surface compared to the thinner air from the outside, so the balloon can expand

  3. Increasing the amount of particles in the balloon (blowing into it) increases the amount of collisions the balloon surface experiences per second, so there is a greater pressure on the inside and the balloon will expand
• Pressure in liquids

  • Deeper in a fluid, the greater the pressure due to the greater weight of fluid above the object
  • Denser the fluid, the greater the pressure as density × volume means greater weight of fluid on the object, resulting in greater force and pressure
• Factors influencing floating and sinking

  • An object floats if its weight is less than the weight of the water it displaces
  • Buoyancy force is the upwards force that counteracts the weight of the floating object, equal to the weight of the fluid displaced by the object
  • A ping pong ball floats on water as its density is less than the density of the water, resulting in buoyancy force
• Increasing the depth

  The greater the weight of the water above you, resulting in greater force felt and greater pressure
• Pressure due to a column of liquid = height of column × density of liquid × g