Force Diagrams and Buoyancy

All | Design & Technology | Physics | Science | 11-16

Learning Objectives

  • Know how forces act on a stationary object
  • Describe what happens when forces are balanced
  • Show the forces acting on an object using a force diagram

Skills focus:

  • Collecting accurate data

Lesson Description

In this lesson we explore balanced forces and how buoyancy force is affected by the salinity of water and its relevance to INEOS TEAM UK’s race boat.

Students carry out a practical investigation into how forces affect an object in water, and one factor that can change the size of the force. Students apply their learning to answer the question of which type of water they think would be best for performance of the race boat.

This lesson is one in a series of five exploring forces through practical investigations linked to the real-life exciting context of the British America’s Cup Team.

Each lesson focuses on a series of key questions and is aimed at the knowledge and skills requirements of KS3 Science; it also has relevance for both Engineering and Design Technology students at KS4.



For immediate access to all of our free Key stage 3 and 4 STEM teaching resources.


Learn about buoyancy, and the forces acting on an object in water, and why some things float in water whereas others sink.

2.Forces Lesson 3 Video Transcript
3.Teaching presentation Editable teaching PowerPoint presentation using the context of the cutting-edge INEOS TEAM UK race boat and historic information from the Lloyd's Register, to investigate balanced forces and the impact of forces on buoyancy.

KS3 Curriculum Links England

Motion and Forces

Describing motion

  • Speed and the quantitative relationship between average speed, distance, and time (speed = distance ÷ time)
  • The representation of a journey on a distance-time graph


  • Forces as pushes or pulls, arising from the interaction between 2 objects
  • Using force arrows in diagrams, adding forces in 1 dimension, balanced and unbalanced forces
  • Moment as the turning effect of a force
  • Forces: associated with deforming objects; stretching and squashing – springs; with rubbing and friction between surfaces, with pushing things out of the way; resistance to motion of air and water
  • Forces measured in newtons, measurements of stretch or compression, as force is changed

Pressure in fluids

  • Pressure in liquids, increasing with depth; upthrust effects, floating and sinking

Balanced forces

  • Opposing forces and equilibrium: weight held by stretched spring or supported on a compressed surface

Forces and motion

  • Forces being needed to cause objects to stop or start moving, or to change their speed or direction of motion (qualitative only)
  • Change depending on direction of force and its size

KS3 Curriculum Links Scotland

Forces: developing an understanding of how forces can change the shape or motion of an object and the forces acting on it, and developing an understanding of the concept of buoyancy, force and density.

Working scientifically

  • Through the content across all five lessons students will develop:

Scientific attitudes

  • Pay attention to objectivity and concern for accuracy, precision, repeatability and reproducibility
  • Evaluate risks

Experimental skills and investigations

  • Ask questions and develop a line of enquiry based on observations of the real world, alongside prior knowledge and experience
  • Make predictions using scientific knowledge and understanding
  • Select, plan, and carry out the most appropriate types of scientific enquiries to test predictions, including identifying independent, dependent and control variables, where appropriate
  • Use appropriate techniques, apparatus, and materials during laboratory work, paying attention to health and safety
  • Make and record observations and measurements using a range of methods for different investigations; and evaluate the reliability of methods and suggest possible improvements

Analysis and evaluation  

  • Apply mathematical concepts and calculate results
  • Present observations and data using appropriate methods, including tables and graphs
  • Interpret observations and data, including identifying patterns and using observations, measurements and data to draw conclusions
  • Present reasoned explanations, including explaining data in relation to predictions and hypotheses
  • Evaluate data, showing awareness of potential sources of random and systematic error
  • Identify further questions arising from their results


All, Design & Technology, Physics, Science

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Film, Practical Investigation, Teacher Presentation, Teachers' notes


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