Resultant Forces

    OCR
    GCSE

    Resultant forces represent the vector sum of all individual forces acting on an object, effectively replacing multiple interactions with a single force that dictates the object's subsequent motion. This concept is foundational to Newton's First and Second Laws, distinguishing between equilibrium states where forces balance and dynamic states where a non-zero resultant causes acceleration. Candidates must demonstrate proficiency in calculating these forces through algebraic addition for collinear vectors and geometric or trigonometric methods for non-collinear vectors.

    0
    Objectives
    3
    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 calculating the magnitude of the resultant force by correctly subtracting forces acting in opposite directions
    • Credit scale drawings that employ a stated, consistent scale and clearly indicate the direction of the resultant vector with an arrowhead
    • Award 1 mark for linking a non-zero resultant force to a change in velocity (acceleration or deceleration), not just movement
    • For Higher Tier, award marks for the correct application of Pythagoras' theorem to determine the magnitude of two forces acting at right angles
    • Credit responses that identify zero resultant force as the condition for terminal velocity or stationary equilibrium

    Example Examiner Feedback

    Real feedback patterns examiners use when marking

    • "You correctly calculated the magnitude, but you must also state the direction to fully describe the vector"
    • "Be careful with 'stationary' vs 'constant velocity' — both mean zero resultant force. Check the context of the question"
    • "Your scale drawing is neat, but you forgot the arrowheads. In Physics, a line without an arrow is not a vector"
    • "Excellent use of Pythagoras here; to secure the top marks, ensure you also calculate the angle for the direction"

    Marking Points

    Key points examiners look for in your answers

    • Award 1 mark for calculating the magnitude of the resultant force by correctly subtracting forces acting in opposite directions
    • Credit scale drawings that employ a stated, consistent scale and clearly indicate the direction of the resultant vector with an arrowhead
    • Award 1 mark for linking a non-zero resultant force to a change in velocity (acceleration or deceleration), not just movement
    • For Higher Tier, award marks for the correct application of Pythagoras' theorem to determine the magnitude of two forces acting at right angles
    • Credit responses that identify zero resultant force as the condition for terminal velocity or stationary equilibrium

    Examiner Tips

    Expert advice for maximising your marks

    • 💡When the command word is 'Determine' in the context of vectors, a scale drawing is often expected; ensure angles are accurate to within ±2°
    • 💡Always explicitly state 'resultant force is zero' if the question describes motion as 'constant speed in a straight line'
    • 💡For Higher Tier calculations of perpendicular forces, use SOHCAHTOA to find the direction (angle) relative to the horizontal or vertical, not just the magnitude

    Common Mistakes

    Pitfalls to avoid in your exam answers

    • Assuming that an object with zero resultant force must be stationary, failing to recognize it could be moving at a constant velocity
    • Adding forces that act in opposite directions instead of finding the difference
    • Omitting arrowheads on vector diagrams, thereby failing to define the vector nature of the force
    • Confusing Newton's Third Law interaction pairs with balanced forces acting on a single object

    Study Guide Available

    Comprehensive revision notes & examples

    Read Study Guide

    Key Terminology

    Essential terms to know

    Vector addition and subtraction of parallel forces
    Newton's First Law and equilibrium conditions
    Newton's Second Law (F = ma) and acceleration
    Parallelogram of forces and scale drawings
    Resolution of forces into perpendicular components

    Likely Command Words

    How questions on this topic are typically asked

    Calculate
    Draw
    Explain
    Determine
    Describe

    Practical Links

    Related required practicals

    • •{"code":"P2.3","title":"Investigation of Acceleration (Newton's Second Law)","relevance":"Demonstrates relationship between resultant force, mass, and acceleration"}

    Ready to test yourself?

    Practice questions tailored to this topic