Forces & Motion
Types of Force
1st Year · 2nd Year · 3rd Year
- ✓By the end of this lesson students will be able to define force and identify its SI unit.
- ✓By the end of this lesson students will be able to distinguish between contact and non-contact forces, providing examples of each.
- ✓By the end of this lesson students will be able to describe and give examples of gravity, friction, normal force, and tension.
- ✓By the end of this lesson students will be able to identify and represent the forces acting on an object in various scenarios.
Key concepts
A force is a push or a pull that can change an object's motion (speed or direction), shape, or size. Forces are vector quantities, meaning they have both magnitude (how strong they are) and direction. The SI unit of force is the newton (N).
A contact force is a force that acts on an object by direct physical contact with another object. The two objects must be touching for the force to be exerted.
A non-contact force is a force that acts on an object without direct physical contact. The objects do not need to touch each other for the force to be exerted. These forces act over a distance.
Gravity is the non-contact force of attraction between any two objects with mass. On Earth, it is the force that pulls objects towards the centre of the Earth. The force of gravity acting on an object is also known as its weight (W). Weight is directly proportional to an object's mass (m) and the acceleration due to gravity (g). Gravity always acts downwards.
Friction is a contact force that opposes the relative motion or tendency of motion between two surfaces in contact. It acts parallel to the surfaces and in the opposite direction to the motion or attempted motion. Friction can be useful (e.g., allowing us to walk) or unhelpful (e.g., making it harder to push a heavy box).
The normal force is a contact force exerted by a surface on an object resting on it or in contact with it. It always acts perpendicular (at a 90-degree angle) to the surface and away from the surface. On a flat horizontal surface, the normal force is often equal in magnitude and opposite in direction to the weight of the object.
Tension is a contact force transmitted through a string, rope, cable, or wire when it is pulled tight by forces acting from opposite ends. Tension acts along the length of the string/rope and away from the object it is pulling.
Key facts to remember
- 1A force is a push or a pull that can change an object's motion, shape, or direction.
- 2The SI unit of force is the newton (N).
- 3Contact forces require direct physical touch between objects.
- 4Non-contact forces act over a distance without physical touch.
- 5Gravity (weight) is a non-contact force that always acts downwards towards the centre of the Earth.
- 6Friction is a contact force that opposes motion between surfaces in contact.
- 7Normal force is a contact force exerted by a surface, acting perpendicular to it.
- 8Tension is a contact force transmitted through a taut string, rope, or cable.
Worked examples
Example 1
A book is resting on a horizontal table. Identify the forces acting on the book and classify each as a contact or non-contact force.
Answer
The forces acting on the book are: Gravity (non-contact force) and Normal Force (contact force).
Always remember gravity acts on all objects with mass, even if they are at rest.
Example 2
A student pulls a toy car across a rough floor using a string. Identify all the forces acting on the toy car.
Answer
The forces acting on the toy car are: Gravity (downwards), Normal Force (upwards), Tension (in the direction of the pull), and Friction (opposite to the direction of motion).
It's helpful to imagine the direction of each force when identifying them.
Example 3
Classify the following forces as either contact or non-contact: (a) The force pushing a swing, (b) The force holding a magnet to a fridge, (c) The force pulling an apple towards the ground.
Answer
(a) Contact force, (b) Non-contact force, (c) Non-contact force.
Common mistakes
- ✗Confusing mass with weight: Mass is a measure of the amount of matter in an object, while weight is the force of gravity acting on that mass.
- ✗Forgetting to include gravity as a force, especially when an object is at rest on a surface.
- ✗Incorrectly identifying the direction of friction; it always opposes the motion or attempted motion.
- ✗Assuming normal force is always equal to weight; this is only true for objects on a horizontal surface with no other vertical forces.
- ✗Not clearly distinguishing between contact and non-contact forces based on whether physical touch is required.
Exam tips
- ★Always draw a clear diagram (often called a free-body diagram) to represent the forces acting on an object. Use arrows to show the direction of each force.
- ★Label all forces clearly with their correct names (e.g., 'Weight' or 'Gravity', 'Normal Force', 'Tension', 'Friction').
- ★Remember that forces are vector quantities, so indicating their direction is just as important as their magnitude.
- ★When asked to classify forces, think carefully: 'Does this force require the objects to be touching?' If yes, it's contact; if no, it's non-contact.
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