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What Is A Force?

Key idea: Learn what a force is, why it is a vector, and how to classify common forces as contact or non-contact (O Level Physics).

  • SEC G3 Physics 2027
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Learning objectives

  • 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

1. Definition

A. Force

A force is a push or pull that can change an object’s motion (speed or direction) or change its shape.

Unit and type
  • SI unit: newton (N)
  • Force is a vector (it has magnitude and direction).

2. Key Ideas

  • A force is drawn as an arrow: longer arrow = larger force; arrow direction = force direction.
  • Forces can be classified as:
    • contact forces (need contact): normal reaction, friction, tension, air resistance
    • non-contact forces (act at a distance): gravitational, electrostatic, magnetic
  • The resultant force is the vector sum of all forces on the object.
  • If the resultant force is zero, forces are balanced:
    • object is at rest, or
    • object moves with constant velocity (straight line, constant speed)
  • If the resultant force is not zero, forces are unbalanced and the object accelerates.

3. Detailed Explanations

What Can A Force Do?

A force can:

  • make a stationary body move
  • change the speed of a body
  • change the direction of motion of a body
  • change the size or shape of the body

A. Contact vs non-contact forces (6091)

This syllabus skill is to identify forces and classify them:

  • Contact forces (need contact): normal reaction, friction, tension, air resistance
  • Non-contact forces (act at a distance): gravitational, electrostatic, magnetic

B. Common forces in O Level questions

Here are the forces you see most often and how to draw them:

  • Weight, W (non-contact): gravitational force on the object, acts downwards (see Gravitational Field Strength & Weight).
  • Normal reaction, N (sometimes written R for reaction) (contact): acts perpendicular to the surface (e.g. table pushes up on a book).
  • Friction, f (contact): acts parallel to the surface, opposes motion (or attempted motion) (see Friction).
  • Tension, T (contact): a pulling force in a string/rope, acts along the string, away from the object.
  • Air resistance / drag (contact): acts opposite to the direction of motion through air.

C. Resultant force (balanced vs unbalanced)

Forces add as vectors. The resultant force is the single force that has the same effect as all the forces together.

  • If resultant force = 0, forces are balanced → no acceleration.
  • If resultant force ≠ 0, forces are unbalanced → the object accelerates in the direction of the resultant force.

You’ll use these ideas in:

4. Common Mistakes

  • “Object at rest means no forces.” False. It can have forces that balance (resultant = 0).
  • Treating force as a scalar (forgetting direction).
  • Calling any contact force “contact force” (be specific: normal reaction, friction, tension, etc.).
  • Drawing friction in the wrong direction (it always opposes relative motion or attempted motion).
  • Mixing up action–reaction pairs with “balanced forces” (action–reaction forces act on different objects).

5. Exam Tips

  • When asked to “state the forces”, name them clearly (e.g. weight, normal reaction, friction, tension, air resistance).
  • For diagram questions, draw forces as arrows starting from the object and label each force.
  • If the question says “constant speed” or “at rest”, write: resultant force is zero.
  • Always include units when giving a force value: N.

6. Worked Examples

Modelled example 1

Book on a table (identify + classify)

Core

Problem

A book is at rest on a horizontal table. State the forces on the book, classify each as contact or non-contact, and relate their magnitudes.
Study the worked solution

  1. Identify and classify

    Method

    Weight acts down and the normal reaction acts up.

    Reason

    Weight is gravitational and non-contact; the table’s normal reaction requires contact.

    Working

    W down (non-contact); N up (contact).

  2. Use the motion state

    Method

    Set the vertical resultant to zero.

    Reason

    The book is at rest and has no acceleration.

    Working

    N = W

Guided practice 2

Pulling a box at constant speed (direction of friction)

About 3 min

Problem

A box is pulled to the right across a rough floor at constant speed. State the direction of friction on the box and what constant speed implies horizontally.

Try this before viewing the solution

Friction direction

Hints

Hint 1: separate velocity from resultant
Constant speed does not remove friction; it constrains the resultant force.
View solution step by step

  1. Set the friction direction

    Method

    Place friction to the left.

    Reason

    It opposes the box’s motion relative to the floor.

    Working

    Motion right → friction left.

  2. Use constant velocity

    Method

    State that horizontal forces balance.

    Reason

    Constant velocity means zero acceleration and zero resultant force.

    Working

    Pull magnitude = friction magnitude.

Common misconception 3

Hanging mass (tension vs weight)

Find and correct the mistake

Learner claim

A mass hangs at rest from a light string. A learner draws only its weight because “gravity is the only force”. Locate and correct the error.

Try this before viewing the solution

Missing force

View solution step by step

  1. Restore the missing interaction

    Method

    State weight downward and tension upward.

    Reason

    The Earth and taut string both interact with the mass.

    Working

    W down; T up.

  2. Use equilibrium

    Method

    Equate their magnitudes.

    Reason

    At rest means zero acceleration and zero resultant force.

    Working

    T = W.

Examiner practice 4

Pushing a box that doesn’t move (balanced contact forces)

3 marks

Examination question

A box is pushed right on a rough floor but remains at rest. State the two horizontal forces and their relationship. [3 marks]

Try this before viewing the solution

View solution step by step

  1. Applied force

    1 mark

    Method

    State the push to the right.

    Reason

    This is the stated applied force.

    Working

    Push right.

  2. Static friction

    1 mark

    Method

    State static friction to the left.

    Reason

    It opposes attempted relative motion.

    Working

    fₛ left.

  3. Equilibrium

    1 mark

    Method

    Set the magnitudes equal.

    Reason

    The stationary box has zero horizontal resultant.

    Working

    fₛ = Fₚᵤₛₕ.

Challenge 5

Highest point of a throw (rest does not mean “no forces”)

Minimal support

Instantaneous-state transfer

A ball is thrown vertically upwards. At the highest point its speed is zero; ignore air resistance. State the force on it and the direction of acceleration.

Try this before viewing the solution

Hints

Hint 1: do not replace velocity with force
Zero velocity at one instant does not imply zero resultant force.
View solution step by step

  1. Identify interactions

    Method

    Keep only the ball’s weight downward.

    Reason

    Gravity still acts at the highest point, and air resistance is excluded.

    Working

    Resultant force: W downward.

  2. Infer acceleration

    Method

    Place acceleration downward.

    Reason

    Acceleration follows the nonzero resultant force, not the instantaneous velocity.

    Working

    a = g downward.

7. Mind Stretchers

Mind stretcher 1: “Balanced forces” vs “action–reaction”Extension

A book rests on a table. Many students say: “weight and normal reaction are an action–reaction pair.”

Is that correct? Explain briefly.

Show Answer

Not correct.

  • Weight and normal reaction act on the same object (the book), so they are a pair of forces that can be balanced.
  • An action–reaction pair acts on different objects.

The reaction to the book’s weight is the gravitational pull of the book on the Earth. The reaction to the normal reaction is the force of the book on the table.

See: Newton’s Third-Law Interaction Pairs.

Mind stretcher 2: Does “constant velocity” mean “no forces”?Extension

A car moves at constant velocity along a straight, level road.

Does this mean there are no forces on the car? Explain.

Show Answer

No. It means the resultant force is zero.

For example, the driving force can balance air resistance and friction, and the normal reaction can balance the weight.

Continue with the next resource in this course.

Course and syllabus information
Course
SEC G3 Physics
Edition
SEC G3 Physics 2027