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.
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.
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.