Topic 15 – Forces and matter

• Using springs and other elastic objects, stretching, bending or compressing an object requires more than one force
• What is the difference between elastic and inelastic distortion?

  Elastic distortion: reversible deformation. Inelastic distortion: irreversible deformation.
• What is the formula for linear elastic distortion?

  $F =$$k *$$x$ where $F$ is the applied force (in newtons), $k$is the spring constant (in n/m) and $x$ is the extension in meters.
• How do you calculate work done in stretching a spring?

  $E =$$0.5 *$$k *$$x^2$
• What is the difference between linear and non-linear relationships between force and extension?

  • Linear relationships: Straight line on graph, proportional changes.
  • Non-linear relationships: Not a straight line, changes not directly proportional.
  • Linear: Constant proportionality between force and extension.
  • Non-linear: Proportionality may vary between force and extension.
• How do you investigate the link between Force and Extension?

  1. Measure natural length of spring.
  2. Add mass to spring, record mass, measure new length for extension.
  3. Repeat process for multiple measurements (at least 6).
  4. Plot force-extension graph; curve indicates limit of proportionality.
  5. Ensure measurements taken at eye level; use markers for accuracy.

  
• • Atmospheric pressure decreases with height above Earth's surface.
  • Due to weight of air above, pressure is highest at Earth's surface.
  • As altitude increases, air density decreases, leading to lower pressure.
  • At higher altitudes, fewer air molecules exert force per unit area.
  • Simple model: Earth's atmosphere behaves like layers of gas, denser at lower altitudes.
  • Greater weight of air above at lower altitudes results in higher pressure.
• The pressure in fluids causes a force normal to any surface.
• • Pressure is force divided by area.
  • Higher force or smaller area increases pressure.
  • Example: Pushing a thumbtack - higher pressure with smaller area.
  • Example: Standing on sand vs. concrete - higher pressure on concrete due to smaller area.
• What is the formula for pressure?

  $P =$$F / A$
• What is the unit for pressure?

  Pascal.
• • Pressure in fluids increases with depth.
  • This increase is due to the weight of the fluid above.
  • The deeper you go, the more fluid is pressing down.
  • Pressure also increases with density of the fluid.
  • Denser fluids have more mass per unit volume.
  • More mass means more weight, leading to higher pressure.
• Explain why the pressure in liquids varies with density and depth:

  • Pressure in liquids increases with depth.
  • More liquid above exerts more downward force.
  • Density affects pressure - denser liquids have higher pressure.
  • Denser liquids have more mass per unit volume.
  • More mass means more weight, leading to higher pressure.
• How do you calculate fluid pressure depending on Depth and density?

  $P =$$h × ρ × g$

  
• Why is an object in a fluid subjected to upthrust?

  • Objects in fluids experience an upward force called upthrust or buoyancy.
  • This is due to pressure difference between top and bottom of object.
  • Upthrust equals weight of fluid displaced by object.
  • Fully immersed objects displace their own volume of fluid.
  • Partially immersed objects displace volume equal to submerged part.
  • Upthrust opposes object's weight, making it feel lighter or partially lifted.
• The upthrust is equal to the weight of fluid displaced.
• Factors influencing whether an object will float or sink:

  • Upthrust: The upward force exerted by a fluid on an object. If upthrust is greater than the object's weight, it floats.
  • Weight: The force exerted by gravity on the object. If weight is greater than upthrust, the object sinks.
  • Density of fluid: Objects with greater density than the fluid sink, while those with lower density float.