Topic 15 - Forces and Matter

• Stretching, bending, or compressing an object

  Requires more than one force
• Suspended spring experiment


  1. Suspend spring from clamp stand

  2. Add 50 gram mass to hanger

  3. Record extension

  4. Add another 50 gram mass

  5. Record extension

  6. Repeat with additional masses
• The extension recorded for each added mass is: 2.1 cm, 4.4 cm, 6.5 cm, 8.5 cm, 10.6 cm, 12.8 cm
• The experiment could be improved by repeating the readings as the masses are unloaded
• Force


  Plotted up the side of the graph
• Extension


  Plotted along the bottom of the graph
• The graph produces a straight line going through the origin, showing a linear, directly proportional relationship
• Spring constant (k)


  Force divided by extension (f/x)
• Energy transferred in stretching the spring (e)

  1/2 k x^2
• Elastic behaviour


  • Spring bounces back to original shape under small loads
  • Inelastic distortion can occur under very large loads
• Elastic band experiment


  1. Load elastic band with 50 gram masses

  2. Unload elastic band

  3. Plot graph of loading and unloading
• Elastic band graph


  • Shows hysteresis curve
  • Area between loading and unloading curves represents energy used up in heating the band
• Air pressure decreases with increasing height, just like water pressure increases with depth
• Pressure


  Force per unit area (F/A)
• Pressure in a column of liquid = height x density x gravitational field strength
• Upthrust


  Upwards force on an object immersed or floating in a liquid or gas, equal to the weight of the fluid displaced
• Whether an object sinks or floats depends on upthrust, weight, and density of the fluid
• Cartesian diver


  • Sachet of tomato ketchup
  • Lava lamp