Few students understand how conservation of momentum follows from Newton’s Third Law

Forces and Motion


Students may also struggle to understand that the time interval during which the forces act is necessarily the same for both objects.

Resources to Address This

  • Skateboard forces (11-16)

    Source - Practical physics/ Force and motion/ Action and reaction/ Newton's third law

    This resource demonstrates the forces on a student and skateboard are equal and opposite.

    Also - Action and reaction with a metre rule in the same group of experiments.

    View Resource
  • Episode 212: Newton's third law of motion

    Source - TAP/ Mechanics/ Newton's laws

    There are a number of difficulties in teaching this concept: .....  recognising these difficulties is half the battle; the other half is ensuring that your own understanding is sound!

    View Resource


The following studies have documented this misconception:

  • Watts, D. M. () A study of schoolchildren's alternative frameworks of the concept of force. International Journal of Science Education, 5 (2),


    This study used an interview approach to identify the conceptions of force of 12 students aged 11-17. Students were drawn from a range of schools in the Greater London area, from both junior science classes and advanced-level physics classes.
    Review sheet

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