Skip to content
Dabour HubLEARN · PRACTICE · EXPLORESign inStudent

1.4.1Motion, Forces and Energy

Resultant Forces — Cambridge IGCSE Physics 0625 Revision Notes & Topical Questions

5 min read 8 parts Lesson 31 of 91 1 quiz

📚 Cambridge IGCSE Physics 0625 Hub / Topical Revision
🩺
100% Curriculum Specification Match Lesson Doctor Verified
The only platform with 100% curriculum specification match and verified interactive question banks.
0%read
Lesson summary

1.4.1 Resultant Forces — IGCSE PHYSICS notes: Combine forces acting on an object to find the resultant and relate balanced and unbalanced forces to motion.Force = a push or a pull, measured in newtons (N).Vector — a force has both size and direction.A force can change the speed, …

  • Book on a table
  • Car on a road
  • Forces in the same direction add
  • Forces in opposite directions subtract
  • Forces at right angles (Extended) combine using Pythagoras' theorem
  • Parachutist at terminal speed

Part 1 of 6Forces and their effects

Force
Force a push or a pull, measured in newtons (N).
Vector
  • Vector a force has both size and direction.
A force can change the speed, the direction of motion, or the shape and size of an object.
  • A spring is stretched and a sponge is squashed.
Common forces are weight, friction, air resistance, upthrust, tension, the normal contact force, and electrical and magnetic forces.

Part 2 of 6Free-body diagrams

Free-body diagrams
Draw the object as a simple box or dot.
Draw an arrow for every force acting on it, starting at the object.
Make the arrow length show the size and the arrow direction show the direction.
Book on a table
  • Book on a table weight acts downwards and the contact force acts upwards.
Car on a road
  • Car on a road weight down, contact force up, driving force forward, drag and friction backward.
Only forces acting on the object are drawn, not forces the object exerts on others.

Part 3 of 6Finding the resultant force

Finding the resultant force
Resultant force
Resultant force the single force with the same effect as all the forces acting together.
Forces in the same direction add
  • 8 N and 5 N in the same direction give 13 N.
Forces in opposite directions subtract
  • 8 N forward and 5 N backward give 3 N forward.
Assign positive and negative signs to show direction.
Forces at right angles (Extended) combine using Pythagoras' theorem
  • 3.0 N and 4.0 N at 90° give 5.0 N.

Part 4 of 6Balanced forces

1Balanced forces
  • Balanced forces the resultant force is zero and the object is in equilibrium.
2
The velocity does not change.
  • The object stays at rest or keeps moving at constant speed in a straight line.
3
Balanced forces do not mean no forces; large forces can cancel.
4Parachutist at terminal speed
  • Parachutist at terminal speed air resistance equals weight, so the speed is steady.

Part 5 of 6Unbalanced forces

Unbalanced forces
Unbalanced forces
  • Unbalanced forces the resultant is not zero and the object accelerates in the direction of the resultant.
Resultant in the direction of motion
  • Resultant in the direction of motion the object speeds up.
Resultant opposite to the motion
  • Resultant opposite to the motion the object slows down.
Resultant at an angle to the motion
  • Resultant at an angle to the motion the direction changes.
A lift starting to go up has tension slightly larger than the weight, giving a small upward resultant.

Part 6 of 6More from the specification

1Hooke's Law and Spring Extension
  • Hooke's law states that the extension of a spring is directly proportional to the applied load or force, provided the elastic limit is not exceeded.
    • (F = k * Delta x)
2Limit of Proportionality on a Force-Extension Graph
  • On a graph of applied force against extension, the initial linear section represents the region where Hooke's law holds;
    • beyond the limit of proportionality, the graph curves as plastic deformation begins.
3Resultant Force and Acceleration
  • According to Newton's second law equation (F = m * a), an unbalanced resultant force acting on an object causes an acceleration in the direction of the force.
4Centripetal Force in Circular Paths
  • A resultant force directed perpendicularly toward the center of a circular path causes continuous change in direction without changing speed,
  • producing circular motion.
5Extension
  • Increase in length over the original length.
6Proportional
  • Changing by the same factor as another quantity.
7Elastic
  • Returns to original shape when force is removed.
8Circular motion
  • Motion along a circular path with a force towards the centre.
9Original length
  • Length of the spring with no load.
10Directly proportional
  • Two quantities increase together in a constant ratio, giving a straight line through the origin.

Key terms

TermMeaning
ForceA push or pull; a vector measured in newtons.
Resultant forceSingle force with the same effect as all forces acting on an object.
Balanced forcesForces whose resultant is zero.
Unbalanced forcesForces with a non-zero resultant, causing acceleration.
EquilibriumState with zero resultant force.
Free-body diagramDiagram showing all forces on one object.

Avoid these mistakes

1
Saying a moving object must have a resultant force in its direction — at constant velocity the resultant is zero.
2
Leaving out forces such as friction or air resistance — include all forces on the object.
3
Adding forces without regard to direction — forces in opposite directions are subtracted.
4
Putting forces exerted by the object in the diagram — only forces on the object count.
5
Thinking balanced forces mean no forces — large forces can cancel.

Last updated:

\n