G3 Physics and O-Level Forces Hub
G3 Physics and O-Level forces hub: Newton's laws, free-body diagrams, friction, terminal velocity, moments and stability.
Learning goals
- Distinguish contact forces from non-contact forces
- State that mass measures the amount of matter in a body
- Describe a gravitational field as a region where a mass experiences gravitational force
- Define gravitational field strength as gravitational force per unit mass
- Apply weight = mass × gravitational field strength
- Distinguish mass from weight
- Describe the effect of balanced and unbalanced forces on a body
- Describe ways a force may change motion
- Identify action–reaction pairs on interacting bodies
- Draw free-body diagrams for force systems in at most two dimensions
- Solve three-force static equilibrium graphically
- Apply resultant force = mass × acceleration
- Relate mass to resistance to change in motion
- Explain the effects of friction on motion
- Describe falling with and without air resistance, including terminal velocity
- Describe a moment as a force's turning effect in everyday examples
- Apply moment = force × perpendicular distance from the pivot
- State the principle of moments for a body in equilibrium
- apply the principle of moments to new situations or to solve related problems
- show an understanding that the weight of a body may be taken as acting at a single point known as its centre of gravity
- Explain qualitatively how centre-of-gravity position affects stability
This hub moves from identifying forces to explaining motion, adding forces graphically and analysing turning effects. Use it to choose the correct model before calculating.
First revise scalars and vectors and kinematics. Then work through the lesson groups below in order: identify forces → analyse motion → add forces graphically → analyse turning.
Start with What is a Force?. After free-body diagrams, use the Forces & Motion Explorer and predict the arrows before revealing the motion result.
What you will learn
The lessons cover the complete Dynamics and Turning Effects sequence:
- distinguish contact and non-contact forces; relate mass, weight and gravitational field strength;
- apply Newton’s laws, draw free-body diagrams, use Fᵣₑₛᵤₗₜₐₙₜ = ma, and explain inertia, friction, drag and terminal velocity;
- solve a static point mass under three forces by a two-dimensional graphical method; and
- calculate moments, apply the principle of moments, and explain centre of gravity and stability.
The syllabus requires you to apply Newton’s laws; it does not require their statements to be recalled word for word. Three-force graphical equilibrium is a Dynamics outcome, while the principle of moments belongs to Turning Effect of Forces.
Lessons
Dynamics (Newton’s Laws)
What is a Force?
Contact vs non-contact forces.
Inertia
How mass resists change in motion.
Weight & Gravity
Gravitational field strength and weight.
Free Body Diagrams
Drawing forces correctly (arrows from the body).
Balanced Forces and Newton's First Law
One-body resultants, rest, and constant velocity.
Newton's Third-Law Interaction Pairs
One interaction, equal and opposite forces on different bodies.
Newton's 2nd Law
Calculating acceleration from net force (F = ma).
Friction
Effects of friction and how to reduce it.
Terminal Velocity
Motion with air resistance (zero net force).
Turning Effects (Moments)
Moment of a Force
Turning effect = force × perpendicular distance.
Equilibrium
Conditions for static equilibrium (net force and net moment are zero).
Centre of Gravity
Finding CoG and its role in stability.
Stability
Stable, unstable, and neutral equilibrium.
Graphical force skills
Adding Forces
Finding the resultant force (parallelogram/triangle method).
Forces in Equilibrium
Closed vector triangles for 3 balanced forces.
Revision
The force-analysis workflow
Use the same order in every dynamics question: isolate one body → draw external forces → choose a positive direction → find the resultant → apply Fₙₑₜ = ma.
Quick Reference
| Term | Formula / Rule | Unit |
|---|---|---|
| Newton’s 2nd Law | Fₙₑₜ = ma | N |
| Weight | W = mg | N |
| Gravitational field strength | g ≈ 10 N kg⁻¹ | N kg⁻¹ |
| Resultant (net) force | Vector sum of all forces on the object | N |
| Moment (turning effect) | M = F × d | N m |
| Equilibrium | Fₙₑₜ = 0 AND ∑ M = 0 | - |
Relationships and ideas to apply
- Balanced motion: zero resultant force means zero acceleration, so velocity stays constant (including rest).
- Unbalanced motion: for constant mass, use Fᵣₑₛᵤₗₜₐₙₜ = ma.
- Action-reaction pairs: if body A exerts a force on body B, body B exerts an equal and opposite force on body A. The forces act on different bodies.
- Principle of Moments: For an object in equilibrium, the sum of clockwise moments equals the sum of anticlockwise moments about the same pivot.
- Centre of Gravity: The point through which the entire weight of the object appears to act.
- Resultant force: The single force that has the same effect as all the forces combined (vector sum).
Free Body Diagram (FBD) Checklist
- Draw only forces on the object (not forces the object exerts on something else).
- Draw arrows from the object, with clear labels (e.g., weight W, normal reaction N, friction f, tension T).
- If motion is involved, show the direction of motion so friction/drag directions make sense.
- Don’t include Newton’s 3rd-law partner forces on the same diagram (they act on a different body).
Common Forces (name them correctly)
- Weight (W): always acts vertically downward.
- Normal reaction (N; sometimes written R for reaction): contact force perpendicular to the surface.
- Friction (f): acts parallel to the surface and opposes relative motion (or tendency to move).
- Tension (T): pull in a stretched string/rope (along the string).
- Air resistance / drag: opposes motion through air.
- Upthrust: upward force on an object in a fluid.
Exam Trigger Phrases (What To Do)
Use the phrase in the question to pick the correct method quickly.
| Trigger phrase(s) | What it really means | What to do |
|---|---|---|
| “at rest” / “constant velocity” / “terminal velocity” | acceleration is zero | Write Fₙₑₜ = 0 and balance forces. |
| “accelerating” / “slowing down” / “speeding up” | acceleration is not zero | Draw an FBD, choose a positive direction, find Fₙₑₜ, then use Fₙₑₜ = ma. |
| “smooth surface” | friction is negligible | Do not include friction in Fₙₑₜ unless stated. |
| “rough surface” | friction is present | Draw friction opposite motion/tendency of motion and include it in Fₙₑₜ. |
| “in equilibrium” / “balanced” / “level” / “not turning” | no linear and no turning acceleration | Use both: Fₙₑₜ = 0 and ∑ M = 0 (about the same pivot). |
| “uniform rule/beam” | weight acts at the middle | Put the weight at the centre of the object when taking moments. |
| “three forces in equilibrium” | triangle of forces applies | Use the graphical method: head-to-tail vectors form a closed triangle (state a scale). |
| “about to topple” | tipping point | The line of action of weight passes through the edge of the base (take moments about that edge). |
Visual Snapshots (graphs)
These are schematic. Use them to connect equations to shapes quickly.
Data table
| Force on the object (up is +, down is −) | Book at rest on a table |
|---|---|
| Normal reaction | 10 |
| Weight | -10 |
Balanced forces means resultant force is zero. The forces do not have to be “one up, one down” or even in a pair (e.g., on a slope you can have several forces with a zero vector sum).
Newton’s 2nd Law (constant mass): a is proportional to Fₙₑₜ
A straight-line graph of acceleration against resultant force for a constant-mass object; the line passes through the origin.
Scroll across the graph to read all labels.
View figure data
| Resultant force (N) | a vs F |
|---|---|
| 0 | 0 |
| 2 | 1 |
| 4 | 2 |
| 6 | 3 |
| 8 | 4 |
Top Exam Traps
- Mass vs Weight: Mass (kg) is constant; Weight (N) depends on gravity (g). They are NOT the same.
- Newton’s 3rd Law pairs: Action and reaction forces act on different bodies. They never cancel out on a Free Body Diagram of a single object.
- Normal reaction is not always weight: On slopes or when accelerating, N is often not equal to W.
- Tension is not “always equal to weight”: It depends on acceleration and the situation (use Fₙₑₜ = ma).
- Moment distance: The distance d must be the perpendicular distance from the pivot to the line of action of the force.
- Inertia: Inertia is not a force; it’s a property of mass (resistance to change in motion).
- “No Force” vs “Balanced Force”: An object moving at constant velocity has balanced forces (Fₙₑₜ = 0), not “no forces”.
Check your understanding
Worked example: A 6.0 kg trolley is pulled right with 18 N while friction acts left with 6.0 N. The resultant force is 18-6 = 12 N to the right, so
a = Fᵣₑₛᵤₗₜₐₙₜ/m = 12/6.0 = 2.0 m s⁻² to the right.
The free-body diagram should show the two horizontal forces plus weight and normal contact force vertically. Do not draw acceleration as another force.
Practise this: The same trolley now moves at constant velocity while the pull remains 18 N. Find the friction force and explain why the trolley can move even though the resultant force is zero.
Practice
Use the Dynamics check for force, mass and gravity, Newton’s laws, free-body diagrams, inertia, friction and terminal velocity, returning to the lesson it identifies whenever your explanation is incomplete.
Then complete the Forces structured questions for longer calculations and explanations.
Continue learning
After the quiz and structured practice, continue to Work, Energy and Power.
Continue with the next resource in this course.
Course and syllabus information
- Course
- SEC G3 Physics
- Edition
- SEC G3 Physics 2027