Grade 9 · Physics · Forces on Objects
Nothing
touches it.
🔩 A nail sits on a table. A magnet moves close — the nail leaps toward it across empty space. No hand. No string. No contact whatsoever.
Is this really a force? And if so — where exactly does it act on the nail? At the surface? The centre? Everywhere at once? Your answer determines how you draw physics.
E2 · Explore
Contact or at a Distance?
Before any definitions — tap each scenario below and classify it. Does the force require physical contact, or does it act across a gap?
Contact force — objects touch
Force at a distance — no contact needed
E3 · Forces & Their Properties
Understanding Forces
A force is a push or pull. Every force has four characteristics that completely describe it.
Four Characteristics of Every Force
| Characteristic | What it means | On a diagram | Example |
|---|---|---|---|
| Point of Application | The exact point where the force acts on the body | The tail of the force arrow | Contact force → contact point. Weight → centre of gravity G |
| Line of Action | The straight line along which the force acts (extended in both directions) | The line containing the arrow | Weight acts along a vertical line through G |
| Direction | Which way along the line of action the force pushes/pulls | The arrowhead | Weight acts downward. Normal reaction acts upward |
| Magnitude | How strong the force is | The length of the arrow (to scale) | Weight = mg (in Newtons) |
Force Classification
Contact Forces
Require direct physical contact between the two objects.
⬆ Normal reaction of ground/table on object
↙ Friction force on sliding/stationary object
⬆ Buoyant force by liquid on floating object
↑ Tension in a string/spring on an object
Point of application: at the contact point
Forces at a Distance
Act across empty space without any physical contact.
⬇ Weight (gravitational force) on all objects
↔ Electric force between charged objects
→ Magnetic force by magnet on iron/nail
Point of application: at centre of gravity G
How to Name a Force
Always name a force using: [Type of force] by [source] on [object]
✓ Weight of the box
✓ Normal reaction of table on box
✓ Magnetic force by magnet on nail
✓ Electric force by charge A on charge B
✓ Buoyant force by mercury on ball
✓ Tension of spring on mass
E3 · Interactive Force Diagram Tool
Draw the Forces
Select a scenario, choose a force, then tap the correct point of application on the diagram. Drag the arrow to rotate it. The tool checks if your point of application is correct.
Centre of gravity G — distance forces applied here
Contact points — contact forces applied here
Your force arrow
Select a force to draw
Forces placed
None yet
E4 · Practice Problems
Apply Your Knowledge
Six scenarios. Name the forces, classify them, identify the correct point of application.
Problem 01 · Force Identification
🏋️ A metal ball is suspended from the free end of a vertical spring. The ball is at rest.
How many forces act on the ball? Select the complete and correct list:
Problem 02 · Point of Application
📦 A wooden box rests on a rough inclined plane. Points are labelled: G (centre of box), A (bottom surface of box), B (side surface of box).
Match each force to its correct point of application:
⚠ Misconception Trap
Problem 03 · True or False
🔮 Two positively charged spheres A and B repel each other. They are not touching.
A student says: "The electric force by A on B should be drawn from the surface of B closest to A." Is this correct?
Problem 04 · Force Classification
⚽ A ball floats on the surface of liquid mercury. Three forces act on it.
Classify each force correctly:
Problem 05 · Direction of Forces
🧲 An iron nail rests on a table. A magnet is held above the nail. The nail does not move yet.
What are the correct directions of all forces acting on the nail?
Open Question · AI Tutor
A uniform rectangular box rests on a rough horizontal surface. Label four points on the box: G (centre), A (bottom-left corner), B (bottom-right corner), C (top-centre).
List ALL forces acting on the box, name each one correctly, classify it as contact or distance, state its point of application, and describe its direction. Then explain why knowing the point of application matters for engineering and construction.
List ALL forces acting on the box, name each one correctly, classify it as contact or distance, state its point of application, and describe its direction. Then explain why knowing the point of application matters for engineering and construction.
E5 · Submit
Submit Your Work
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