4PH1

Forces, Movement & Changing Shape

Forces & Motion · 1 question type

Practise

Exam Frequency Analysis

Past paper frequency (2018 to 2024)

This topic accounts for approximately 14% of your exam marks.

stable
High
Stable14%

F = ma, resultant forces and Hooke's Law calculations are high-frequency multi-mark questions.

What the resultant force is

  • The on an object is the single force that has the same effect as all the individual forces added together, direction included
  • It determines:
    • the direction the object will move (or accelerate) in
    • the magnitude of the net push or pull the object actually experiences
  • The resultant alone matters for the motion; once you have it, you can ignore the individual forces

Adding forces along a line

  • Forces along the same straight line combine by simple addition, taking sign into account:
    • Forces pointing the same way are added together
    • Forces pointing in opposite directions are subtracted
  • A final answer is only complete when both magnitude and direction are stated, in newtons and as a compass-style direction or left/right/up/down
Three free-body diagrams adding forces along a line: two equal 5 N forces in opposite directions give a resultant of 0 N so the forces are balanced; a 3 N and a 7 N force in the same direction add to give 10 N to the right; a 7 N force opposed by a 3 N force subtract to give 4 N to the left
Source: Resultant Forces by Save My Exams

Friction inside the resultant

  • Friction is the force that opposes motion of an object across or through another material
  • On a free-body diagram, a friction arrow points opposite to whichever way the object is sliding
  • Drag and air resistance are both special cases of friction: drag inside any fluid, air resistance inside air specifically