Contact and Non-Contact Forces

    OCR
    GCSE

    Forces are vector quantities characterized by both magnitude and direction, acting as interactions between objects. These are strictly categorized into contact forces, such as friction, air resistance, tension, and normal contact force, which require physical touching, and non-contact forces like gravitational, electrostatic, and magnetic forces which act through fields. Mastery involves applying Newton's Third Law to identify interaction pairs and utilizing free body diagrams to resolve resultant forces affecting motion and deformation.

    0
    Objectives
    4
    Exam Tips
    4
    Pitfalls
    5
    Key Terms
    5
    Mark Points

    What You Need to Demonstrate

    Key skills and knowledge for this topic

    • Award 1 mark for correctly categorising friction, air resistance, tension, and normal contact force as contact forces
    • Award 1 mark for identifying gravitational, electrostatic, and magnetic forces as non-contact forces acting at a distance
    • Credit responses that draw force arrows starting from the centre of mass, with length proportional to magnitude
    • Award 1 mark for stating that interaction pairs are equal in magnitude, opposite in direction, and act on different objects (Newton's Third Law)
    • Award 1 mark for calculating the resultant force by subtracting opposing forces along the same line of action

    Example Examiner Feedback

    Real feedback patterns examiners use when marking

    • "You have correctly identified the forces, but your arrows must start from the centre of mass to gain full credit"
    • "Be careful with terminology: 'gravity' is a field, 'weight' is the force. Use 'weight' in your labels"
    • "You have drawn the forces on a single object. Remember, Newton's Third Law pairs always act on *different* objects"
    • "Excellent use of a scale drawing. To secure the top mark, ensure you state the bearing or angle for the direction of the resultant force"

    Marking Points

    Key points examiners look for in your answers

    • Award 1 mark for correctly categorising friction, air resistance, tension, and normal contact force as contact forces
    • Award 1 mark for identifying gravitational, electrostatic, and magnetic forces as non-contact forces acting at a distance
    • Credit responses that draw force arrows starting from the centre of mass, with length proportional to magnitude
    • Award 1 mark for stating that interaction pairs are equal in magnitude, opposite in direction, and act on different objects (Newton's Third Law)
    • Award 1 mark for calculating the resultant force by subtracting opposing forces along the same line of action

    Examiner Tips

    Expert advice for maximising your marks

    • 💡When drawing free-body diagrams, ensure arrows touch the object and point away from the point of application; do not leave 'floating' arrows
    • 💡Always specify the *type* of force (e.g., 'electrostatic force') rather than just describing the effect (e.g., 'repulsion')
    • 💡For Newton's Third Law questions, explicitly name both objects involved in the format: 'Force of Object A on Object B is equal to Force of Object B on Object A'
    • 💡In vector scale drawings, use a sharp pencil and a large scale to minimise percentage uncertainty in your final measurement

    Common Mistakes

    Pitfalls to avoid in your exam answers

    • Labeling the downward force simply as 'gravity' rather than the specific term 'weight' or 'gravitational force'
    • Drawing Newton's Third Law pairs acting on the same object, rather than on the two interacting bodies
    • Assuming the normal contact force is always equal and opposite to weight, failing to account for slopes or additional vertical components
    • Neglecting to include a scale or direction when asked to determine a resultant force using a vector diagram

    Study Guide Available

    Comprehensive revision notes & examples

    Read Study Guide

    Key Terminology

    Essential terms to know

    Vector nature of forces (magnitude and direction)
    Differentiation between contact and non-contact interactions
    Newton's Third Law interaction pairs
    Free body diagrams and resultant force calculation
    Field concepts (gravitational, magnetic, electrostatic)

    Likely Command Words

    How questions on this topic are typically asked

    State
    Describe
    Explain
    Draw
    Calculate
    Estimate

    Practical Links

    Related required practicals

    • •{"code":"PAG P2","title":"Investigation of force and extension","relevance":"Demonstrates tension (contact force) and calibration of spring scales"}

    Ready to test yourself?

    Practice questions tailored to this topic