Forces and Their Effects

This lesson covers contact and non-contact forces, weight, free body diagrams and resultant forces, as required by the Edexcel GCSE Combined Science specification (1SC0). Forces are fundamental to understanding why objects start moving, stop, or change direction.

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What Is a Force?

A force is a push or pull that can:

• Change the speed of an object (speed it up or slow it down)
• Change the direction of motion
• Change the shape of an object (stretch, squash, bend or twist)

Forces are measured in newtons (N) and are vector quantities — they have both magnitude and direction.

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Contact and Non-Contact Forces

Contact Forces

Contact forces require physical touching between two objects.

Force	Description
Friction	Opposes motion between surfaces in contact
Air resistance	Friction caused by air on a moving object
Tension	Pulling force in a stretched string, rope or spring
Normal contact force	Perpendicular force from a surface supporting an object
Upthrust	Upward force on an object in a fluid

Non-Contact Forces

Non-contact forces act at a distance — the objects do not need to touch.

Force	Description
Gravitational force	Attraction between any objects with mass
Electrostatic force	Attraction or repulsion between charges
Magnetic force	Attraction or repulsion between magnets or magnetic materials

Exam Tip: The exam may ask you to classify forces as contact or non-contact. Gravity is the most commonly tested non-contact force. Remember: weight is the gravitational force acting on an object.

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Weight and Mass

Mass is the amount of matter in an object. It is measured in kilograms (kg) and does not change with location.

Weight is the force of gravity acting on an object. It is measured in newtons (N) and depends on where you are.

$W = mg$W=mg

where:

• W = weight (N)
• m = mass (kg)
• g = gravitational field strength (N/kg)

On Earth, g = 9.8 N/kg (often approximated as 10 N/kg in simpler calculations).

Worked Examples

Example 1: Calculate the weight of a 5 kg object on Earth (g = 9.8 N/kg).

$W = mg = 5 \times 9.8 = 49 \text{ N}$W=mg=5×9.8=49 N

Example 2: An object weighs 24.5 N on Earth. What is its mass?

$m = \frac{W}{g} = \frac{24.5}{9.8} = 2.5 \text{ kg}$m=gW​=9.824.5​=2.5 kg

Example 3: A 70 kg person stands on the Moon where g = 1.6 N/kg. What is their weight?

$W = 70 \times 1.6 = 112 \text{ N}$W=70×1.6=112 N

On Earth the same person would weigh: 70 × 9.8 = 686 N. Their mass (70 kg) is the same on both.

Exam Tip: Weight changes when gravitational field strength changes (e.g. on the Moon), but mass stays the same everywhere. Never write "mass = 49 N" — mass is always in kg.

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Free Body Diagrams

A free body diagram shows all the forces acting on a single object, represented by labelled arrows.

Rules for Drawing Free Body Diagrams

1. Represent the object as a simple shape (box or dot).
2. Draw arrows showing each force acting on the object.
3. The length of each arrow should be proportional to the magnitude of the force.
4. Label each arrow with the name and size of the force.
5. Only include forces acting on the chosen object — not forces it exerts on other objects.

Example: A Book on a Table

graph TB
    W["Weight (W) ↓"] --> Book["Book"]
    Book --> N["Normal contact force (N) ↑"]

The book has two forces: weight pulling it downward and the normal contact force from the table pushing it upward. These are balanced (equal and opposite), so the book remains stationary.

Example: A Car Accelerating

graph LR
    F["Driving force →"] --> Car["Car"]
    Car --> D["Friction + air resistance ←"]
    style Car fill:#f9f,stroke:#333

If the driving force is greater than friction, there is a net force forward and the car accelerates.

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Resultant Force

The resultant force is the single force that has the same effect as all forces acting on an object combined.

Calculating Resultant Force (Same Line)

Add forces in the same direction; subtract forces in opposite directions.

Worked Example

A rocket has a thrust of 15 000 N upward and a weight of 10 000 N downward. What is the resultant force?

$F_{resultant} = 15\,000 - 10\,000 = 5000 \text{ N upward}$Fresultant​=15000−10000=5000 N upward

The rocket accelerates upward.

Worked Example

A car has a driving force of 5000 N and friction of 2000 N and air resistance of 1000 N.

$F_{resultant} = 5000 - 2000 - 1000 = 2000 \text{ N forward}$Fresultant​=5000−2000−1000=2000 N forward

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Balanced and Unbalanced Forces

Condition	Resultant Force	Effect on Motion
Balanced	0 N	Stationary OR constant velocity
Unbalanced	≠ 0 N	Acceleration in the direction of the resultant

Exam Tip: Balanced forces do NOT mean an object must be stationary. An object moving at constant velocity also has balanced forces (zero resultant). This links to Newton's First Law.

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Summary

• A force is a push or pull measured in newtons (N) and is a vector.
• Contact forces require touching (friction, tension, normal, air resistance, upthrust).
• Non-contact forces act at a distance (gravity, electrostatic, magnetic).
• Weight = mass × gravitational field strength: $W = mg$W=mg. On Earth, g = 9.8 N/kg.
• Mass (kg) is constant everywhere; weight (N) depends on location.
• Free body diagrams show all forces on one object as labelled arrows.
• The resultant force is found by combining all forces. If zero, forces are balanced.
• Balanced forces → stationary or constant velocity. Unbalanced forces → acceleration.

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Extended Worked Examples

Example 4: Weight across the Solar System

A Mars rover has a mass of 899 kg. Find its weight on Earth (g = 9.8 N/kg) and on Mars (g = 3.7 N/kg).