Lecture Notes on Floating and Sinking

Introduction

• Thought Experiment: Olympic athlete lifting two people underwater.
  • Objects feel lighter in water compared to on land.
  • Question posed: What would a scale read when weighing an object in and out of water?

Experiment

• Predictions and Measurements:
  • Weighed objects in water using a spring scale.
  • Objects appear to weigh less in water.

Buoyant Force

• Explanation:
  • Something pushes upward on the object in water.
  • Upward force is called the buoyant force.
  • Weight acts downward; buoyant force acts upward.
  • For every action, there's an equal and opposite reaction (Newton's Laws).

Representing Forces

• Free Body Diagrams:
  • Arrows show direction and magnitude of forces.
  • Draw arrows from the object's center of mass.

Quantitative Analysis of Buoyancy

• Weight and Mass:

  • Weight of an object = mass x acceleration due to gravity.
  • Real weight: Same in air and water on Earth.
  • Apparent weight: Mass in water times gravity.

• Buoyant Force Calculation:

  • Buoyant force = Real weight - Apparent weight.
  • Example: Real weight 5 kg, apparent weight 3 kg, buoyant force = 2 kg.

Displacement and Buoyancy

• Water Displacement:

  • Special Beaker captures water displaced by the object.
  • Weight of displaced water = Real weight - Apparent weight.

• Equations:

  • Buoyant force = Weight of displaced water.

Conditions for Floating and Sinking

• Floating:

  • Buoyant force > Weight of object.
  • Weight of displaced water > Weight of object.

• Sinking:

  • Buoyant force < Weight of object.
  • Weight of displaced water < Weight of object.

Conclusion

• Archimedes' Principle:

  • Recommended viewing Khan Academy video for reinforcement.

• End of Unit: Floating and Sinking.

  • Open for questions.
  • Take care and have a good night.