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This lesson covers the different types of forces, weight and gravitational field strength, resultant forces, and free body diagrams — as required by the Edexcel GCSE Physics specification (1PH0), Topic 1: Key Concepts of Physics. You need to understand how forces are classified, how to calculate weight, and how to determine the effect of resultant forces on motion.
A force is a push or a pull that acts on an object due to the interaction with another object. Forces can:
Forces are measured in newtons (N) and are vector quantities — they have both magnitude and direction.
Forces are classified into two categories based on whether the objects need to be touching.
Contact forces act between objects that are physically touching.
| Force | Description |
|---|---|
| Friction | Opposes the motion of surfaces sliding past each other |
| Air resistance (drag) | A type of friction between an object and the air |
| Tension | The pulling force in a stretched string, rope, or cable |
| Normal contact force | The support force from a surface, acting perpendicular to the surface |
| Upthrust | The upward force from a fluid on a submerged object |
Non-contact forces act between objects that are not physically touching — they act over a distance through a field.
| Force | Description |
|---|---|
| Gravitational force (weight) | Attraction between any two masses |
| Electrostatic force | Attraction or repulsion between charged objects |
| Magnetic force | Attraction or repulsion between magnets or magnetic materials |
Exam Tip: A common exam question asks you to identify whether a force is a contact or non-contact force. Gravity is the most commonly asked example of a non-contact force. If the objects are touching, it is a contact force. If they are not touching, it is a non-contact force.
Weight is the force of gravity acting on an object's mass. It is a vector quantity that always acts downward (toward the centre of the Earth).
weight = mass × gravitational field strength
W=mg
Where:
On the surface of the Earth, g ≈ 9.8 N/kg.
| Feature | Mass | Weight |
|---|---|---|
| Definition | Amount of matter in an object | Force of gravity on an object |
| Type | Scalar | Vector |
| Unit | kg | N |
| Depends on location? | No — same everywhere | Yes — depends on gravitational field strength |
| Measured with | Balance | Newton meter (spring balance) |
Example 1: A person has a mass of 70 kg. Calculate their weight on Earth (g = 9.8 N/kg).
W = mg = 70 × 9.8 = 686 N
Example 2: An object weighs 45 N on Earth. What is its mass?
m = W ÷ g = 45 ÷ 9.8 = 4.6 kg (to 2 s.f.)
Example 3: The same 70 kg person goes to the Moon where g = 1.6 N/kg. What is their weight on the Moon?
W = mg = 70 × 1.6 = 112 N
Note: The mass is still 70 kg — mass does not change with location.
Exam Tip: Never confuse mass and weight. Mass is in kg and does not change. Weight is in N and changes depending on where you are (Earth, Moon, Jupiter, etc.). If a question asks for weight, your answer must be in newtons.
The resultant force is the single force that has the same effect as all the individual forces acting on an object combined.
When the forces on an object are balanced, the resultant force is zero. The object will either:
When the forces are unbalanced, there is a non-zero resultant force. The object will accelerate in the direction of the resultant force. This means:
graph TD
A["Forces on an Object"] --> B["Balanced<br/>(Resultant = 0 N)"]
A --> C["Unbalanced<br/>(Resultant ≠ 0 N)"]
B --> D["Stationary<br/>or<br/>Constant Velocity"]
C --> E["Object Accelerates<br/>(speeds up, slows down,<br/>or changes direction)"]
style A fill:#2c3e50,color:#fff
style B fill:#2980b9,color:#fff
style C fill:#c0392b,color:#fff
style D fill:#27ae60,color:#fff
style E fill:#e67e22,color:#fff
A car has a driving force of 4000 N and friction/air resistance of 1500 N acting in the opposite direction. What is the resultant force?
Resultant = 4000 − 1500 = 2500 N (in the direction of the driving force, i.e. forward)
Since the resultant force is not zero, the car will accelerate forward.
A free body diagram shows all the forces acting on a single object as arrows. Each arrow starts on the object and points in the direction of the force. The length of the arrow represents the magnitude of the force.
Forces acting on the book:
If the book is stationary, these forces are balanced: W = N.
graph TD
N["Normal contact force (N)<br/>↑ upward"] --> Book["Book"]
Book --> W["Weight (W)<br/>↓ downward"]
style N fill:#2980b9,color:#fff
style Book fill:#2c3e50,color:#fff
style W fill:#c0392b,color:#fff
Forces acting on a skydiver falling through the air:
At the start of the fall, W > D, so the resultant force is downward and the skydiver accelerates. As speed increases, air resistance increases until D = W. At this point, the forces are balanced and the skydiver falls at a constant velocity called terminal velocity.
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