Forces and Interactions

Orientation

Lesson goal: build accurate physics fluency for forces and interactions and use that fluency to support clear HSC-style scientific writing.

This page is materialised into the MentorMind course shell from existing teaching, textbook, and eduKG material. Use it as the main lesson surface; use the tutor for targeted repair, worked examples, and concise writing feedback.

Source Lesson Material

Syllabus inquiry question

How do interactions between objects produce changes in motion?

From The Feynman Lectures on Physics, Vol I, Chapter 9:

A force is not a property of a single object. It is a description of an interaction between two objects, and it always has a direction.

Learning Objectives

Identify contact and field forces.
Draw and interpret free-body diagrams.
Calculate weight and normal force in simple cases.
Determine net force and equilibrium conditions.

Content

Types of forces

Forces arise from interactions between objects. We classify them into two categories:

Contact forces require physical touching:

Normal force (N): Perpendicular push from a surface
Friction (f): Parallel to surface, opposes relative motion
Tension (T): Pull through a rope, string, or cable
Applied force (F): Push or pull from an external agent

Field forces act at a distance:

Gravitational force (Weight, W): Attraction toward Earth's centre
Electric force: Between charged objects
Magnetic force: Between magnets or moving charges

The SI unit of force is the newton (N), where 1 N = 1 kg·m/s^2.

Interactive: Force Categories

Explore different types of forces acting on objects:

Interpreting the diagram:

Red vector (W): Weight pulling downward
Blue vector (N): Normal force pushing upward from surface
Green vector (F): Applied force pushing right
Purple vector (f): Friction opposing the motion

Weight and normal force

Weight is the gravitational force on a mass:

$$W = mg$$

where $g = 9.8$ m/s^2 near Earth's surface.

Normal force is the surface's response to being compressed. On a horizontal surface at rest, the normal force balances weight:

$$N = W = mg$$

Mass (m) is the amount of matter, measured in kilograms (kg)
Weight (W) is a force, measured in newtons (N)

An astronaut's mass is the same on Earth and the Moon, but their weight differs because $g$ differs.

Free-body diagrams

A free-body diagram (FBD) shows only the forces acting on a single object. Rules for drawing:

Represent the object as a dot or simple shape
Draw all forces as arrows starting from the object
Label each force with its name and magnitude
Include only forces acting on the object, not forces it exerts

Interactive: Building a Free-Body Diagram

Consider a box being pushed across a floor. The free-body diagram shows all forces on the box:

The net force is the vector sum of all forces. In this example:

Vertical: $N - W = 0$ (equilibrium vertically)
Horizontal: $F - f = 25 - 10 = 15$ N to the right

Equilibrium

When the net force is zero, the object is in equilibrium:

$$\vec{F}_{net} = \sum \vec{F} = 0$$

An object in equilibrium is either:

Static equilibrium: At rest
Dynamic equilibrium: Moving with constant velocity

An object in equilibrium will remain at rest or continue moving at constant velocity. This is Newton's First Law, covered in the next section.

Interactive: Equilibrium vs Acceleration

Compare the net force when forces are balanced versus unbalanced:

Worked Examples

Example 1: Weight and normal force

A 6.0 kg box rests on a horizontal floor. Find the weight and normal force.

Solution:

Weight: $W = mg = 6.0 \times 9.8 = 58.8$ N downward
At rest on a level surface, the net vertical force is zero
Therefore: $N = W = 58.8$ N upward

Example 2: Net force in one dimension

Two horizontal forces act on a trolley: 14 N east and 9 N west.

Solution:

Choose east as positive
Net force: $F_{net} = 14 - 9 = 5$ N east
The trolley accelerates east (in the direction of net force)

Example 3: Tension in a hanging mass

A 2.5 kg mass hangs at rest from a light rope. Find the tension.

Solution:

Net force is zero (equilibrium)
Forces: Weight down, Tension up
$T = W = mg = 2.5 \times 9.8 = 24.5$ N upward

Common Misconceptions

Misconception: Weight and mass are the same. Correction: Mass is measured in kg, weight in N. Weight depends on location; mass does not.
Misconception: The normal force always equals weight. Correction: This is only true on horizontal surfaces with no other vertical forces. On slopes or with additional forces, $N \neq W$.
Misconception: Opposing forces always cancel. Correction: Forces only cancel if they act on the same object with equal magnitudes and opposite directions.
Misconception: A free-body diagram should show all forces in the problem. Correction: Show only forces acting on the chosen object, not forces it exerts on others.

Practice Questions

Easy (2 marks)

A 4.0 kg object rests on a table. Calculate its weight.

Use $W = mg$ (1)
Correct value: $W = 4.0 \times 9.8 = 39.2$ N with units (1)

Answer: 39.2 N (or 39 N)

Medium (4 marks)

A box is pulled with 30 N east while friction acts 12 N west. Determine the net force and describe the motion.

Correct net force calculation: $F_{net} = 30 - 12 = 18$ N (2)
Direction of net force: east (1)
Statement that box accelerates east (1)

Answer: Net force is 18 N east. The box accelerates to the east.

Hard (5 marks)

A 10 kg crate is pulled upward by a cable with 140 N tension. Determine the net force and state whether the crate accelerates or is in equilibrium.

Weight calculation: $W = 10 \times 9.8 = 98$ N (1)
Net force calculation: $F_{net} = 140 - 98 = 42$ N upward (2)
Correct statement: crate accelerates upward (1)
Optional: Calculate acceleration $a = 42/10 = 4.2$ m/s^2 (1)

Answer:

Weight = 98 N down
Net force = 140 - 98 = 42 N upward
The crate accelerates upward at 4.2 m/s^2

Multiple Choice Questions

Test your understanding with these interactive questions:

Summary

Forces describe interactions between objects and always have direction
Contact forces (normal, friction, tension) require physical contact
Field forces (gravity, electric, magnetic) act at a distance
Weight is $W = mg$ where $g = 9.8$ m/s^2
Free-body diagrams show all forces acting on one object only
Equilibrium means $\vec{F}_{net} = 0$ (at rest or constant velocity)

Self-Assessment

Check your understanding:

After studying this section, you should be able to:

Distinguish between contact and field forces
Calculate weight using $W = mg$
Draw a complete free-body diagram
Find the net force from multiple forces
Identify equilibrium conditions

Scientific Writing And Exam Support

When answering questions from this lesson, separate:

the physical quantity being discussed,
the model or law being applied,
the mathematical relationship, including units,
the conclusion in words.

For explanation questions, write in the pattern: claim -> physics reason -> consequence. For calculation questions, state the formula, substitute with units, calculate, then interpret the answer.

Tutor Context

Use this lesson context when the student asks about forces and interactions, related calculations, representations, or scientific writing. Prefer a short diagnostic before re-teaching. Check whether the student is confusing closely related categories such as force, velocity, acceleration, field, energy, momentum, model evidence, or mathematical representation.

Useful tutor diagnostic:

Which quantity is changing here, what causes that change, and what unit should the final answer use?

Maintenance Loop

One-minute retrieval:

State the key law, model, or relationship used in this lesson.
Identify one common misconception that would lead to a wrong answer.
Write one sentence that links the calculation or evidence back to the physical meaning.

Source Trace

This content record was materialised from:

edu/physics-prep/hsc/physics/textbook/module-2/sections/m2-1-forces-interactions.qmd
edu/physics-prep/hsc/physics/eduKG/lessons/program/year-11/module-2/T2W1L1-m2-1-forces-interactions.qmd
/Users/philiphaynes/devel/teaching/teaching/cromer/2026/classes/11PHY5/lessons/mod-2/lesson-1-forces-newtons-laws.qmd