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Theme A — Space, Time and Motion

A.2.1 Free-Body Diagrams — IB Physics HL Revision Notes & Topical Questions

5 min read 10 parts Lesson 12 of 63 1 quiz

📚 IB Physics HL Hub / Topical Revision
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Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
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Lesson summary

A.2.1 Free-Body Diagrams — IB PHYSICS SL notes: Draw and interpret free-body diagrams, identify the forces on a body and find the resultant force.Force = a push or pull arising from the interaction between objects; it is a vector.Point particle — the object is drawn as a point at…

  • Test for each force
  • Balanced forces
  • One dimension
  • Two dimensions
  • Three forces in equilibrium
  • Equal and opposite arrows

Part 1 of 8What a free-body diagram shows

What a free-body diagram shows
Force
Force a push or pull arising from the interaction between objects; it is a vector.
Point particle
  • Point particle the object is drawn as a point at its centre of mass.
Free-body diagram
  • Free-body diagram it shows the forces acting on one object only, with the surroundings left out.
Arrows
  • Arrows the length shows the magnitude and the direction shows the direction of the force.
Uses
  • Uses to identify which forces act in which plane and to find the resultant force.

Part 2 of 8Rules for drawing

One body
  • One body the object is a point mass and only the forces acting on it are included.
Direction
  • Direction each force is drawn in its correct direction.
Scale
  • Scale the arrows have proportional lengths.
Labels
  • Labels each arrow is labelled with a name or standard symbol such as weight, Fg or mg.
  • Unconventional notation can lose marks.
Test for each force
  • Test for each force name the object that exerts it; if none exists, the force does not belong.
  • A "force of motion" after release does not exist.

Part 3 of 8Common forces on a diagram

ForceSymbolDirection
WeightFg or mgTowards the surface of the planet (down)
TensionFTAlong the string, away from the mass
Normal reactionFNPerpendicular to the surface
FrictionFfOpposite to the motion or its tendency
DragFdOpposite to the velocity through a fluid
UpthrustFbUpwards from a fluid on a submerged body

Part 4 of 8Resultant force

Resultant force
  • Resultant force the vector sum of the forces on a body; it is also called the net force.
Effect
  • Effect it gives the direction of the change of motion and the magnitude of the total force.
Balanced forces
  • Balanced forces they cancel, the resultant is zero and the motion does not change.
  • A book on a desk has weight balanced by the normal force of the desk.
Unbalanced forces
  • Unbalanced forces they do not cancel completely, so there is a non-zero resultant.
  • In a tug of war, pulls of 80 N and 100 N in opposite directions leave a resultant of 20 N towards the larger pull.

Part 5 of 8Finding the resultant

1One dimension
  • One dimension choose a positive direction and add the forces with their signs.
  • Equal forces in opposite directions give no resultant.
2Two dimensions
  • Two dimensions resolve each force into perpendicular components and combine them with Pythagoras and trigonometry.
3Slope
  • Slope a weight mg on a slope at angle θ has a component mg sin θ down the slope and mg cos θ into the slope.
  • With no acceleration perpendicular to the surface, FN = mg cos θ.
4Three forces in equilibrium
  • Three forces in equilibrium they form a closed triangle when drawn head to tail.

Part 6 of 8Drawing a diagram step by step

1
Choose the object and draw it as a dot or box.
2
Draw the weight first.
3
Add an arrow for each surface or string that touches the object.
4
Add friction or drag only where relevant.
5
Label every arrow and scale the lengths if the sizes are known.
6
For a body at constant velocity, check that the arrows sum to zero in every direction.

Part 7 of 8Reading motion from a diagram

1Equal and opposite arrows
  • Equal and opposite arrows no acceleration.
2Unequal arrows
  • Unequal arrows the body accelerates in the direction of the resultant, with a = F/m.
3Skydiver at terminal speed
  • Skydiver at terminal speed weight down and drag up, equal in size.
4Ball at the top of its flight
  • Ball at the top of its flight only weight acts when air resistance is ignored.
5Car accelerating
  • Car accelerating the driving force forwards is larger than the resistive forces backwards.
6Book at rest on a table
  • Book at rest on a table weight and normal force are equal and opposite, but they are not a third-law pair.

Part 8 of 8More from the specification

More from the specification
Free-Body Diagrams and Resultant Force
  • A free-body diagram isolates an object to show all external vector forces acting upon it;
    • according to Newton's second law, an unbalanced resultant force produces linear acceleration in the direction of the net force (Sigma F = ma).
Momentum and Impulse in Collisions
  • Linear momentum is defined as p = mv; the impulse of a force during a collision is the integral of force over time, representing the change in momentum.
    • (J = F * Delta t = Delta p)
Newton's Third Law Pairs
  • Whenever body A exerts a contact or field force on body B, body B simultaneously exerts an equal and opposite force on body A;
    • these action-reaction pairs never act on the same single body.
Centripetal Force in Circular Motion
  • When an object traces a circle of radius r at constant angular velocity omega, the free-body diagram shows a net inward centripetal force perpendicular to velocity, calculated by F = m * v2 / r = m * omega2 * r.
Buoyancy
  • Another name for upthrust.
Centripetal force
  • Net force toward the centre, mv2/r.
Static friction
  • Friction between surfaces at rest relative to each other; fs ≤ μsFN.
Kinetic energy
  • K = ½mv2, energy of motion.
Angular speed
  • Magnitude of the angular velocity.
Centripetal acceleration
  • Ac = v2/r toward the centre.
Contact force
  • Force requiring touching objects.
Momentum
  • P = mv, a vector.
Newton
  • Unit of force, kg m s-2.
Collision
  • Short interaction with large internal forces.
Linear
  • A graph that is a straight line through the origin, so resistance is constant.
Impulse
  • J = FΔt = Δp.
    What the symbols mean: F = force (N) · Δt = time the force acts (s) · Δp = change in momentum (kg m s−1); FΔt is the impulse (N s)
Radius
  • Distance from the centre to the circular path.

Key terms

TermMeaning
Free-body diagramDiagram of one body showing all forces acting on it as labelled arrows
Resultant forceVector sum of all forces acting on a body
Balanced forcesForces whose resultant is zero
WeightGravitational force on a mass, Fg = mg
TensionForce within a string or rope pulling along its length
Normal forceContact force perpendicular to a surface

Avoid these mistakes

1
Including a force the object exerts on something else — draw only forces acting on the object.
2
Drawing a force of motion or force of the hand after contact has ended — no such force exists.
3
Treating weight and normal reaction as a third-law pair — they act on the same body and are different types of force.
4
Taking the normal reaction as always equal to mg — on a slope or with other vertical forces it differs.
5
Drawing tension toward the string — tension pulls away from the body along the string.

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