Interacting forces (AQA GCSE Physics Combined Science): Revision Notes
Interacting forces
What are interacting forces?
Forces happen when objects interact with each other. A force creates a push or pull on an object. This interaction can happen in two different ways - either when objects touch each other directly, or when they affect each other from a distance.
When forces act, they always work in pairs. These pairs act in opposite directions on different objects. Scientists call these interaction pairs and they help explain how objects affect each other in our world.
Interaction pairs are fundamental to understanding forces - whenever one object exerts a force on another, the second object always exerts an equal and opposite force back on the first object.
Non-contact forces
Some forces can work without objects actually touching each other. There are three main types of non-contact forces you need to know about for your GCSE exam.
Gravitational force
This force acts between all objects that have mass. It always pulls objects towards each other - it never pushes them apart.
The Earth's gravitational force pulls everything downwards towards the ground. This is why objects fall when you drop them. The bigger the mass of an object, the stronger its gravitational pull becomes.
Key fact: Gravity always attracts - it never repels.
Magnetic force
This force works between magnetic materials and magnets. Magnets have two ends called poles - a north pole and a south pole.
The important rule to remember is that unlike poles attract each other, whilst like poles repel each other. This means:
Magnetic pole interactions:
- North pole attracts south pole
- North pole repels north pole
- South pole repels south pole
Electrostatic force
This force acts between objects that have electric charge. Just like magnets, there are two types of electric charge - positive and negative.
The rule works the same way as magnetic poles:
Electric charge interactions:
- Unlike charges attract each other (positive attracts negative)
- Like charges repel each other (positive repels positive, negative repels negative)
Contact forces
These forces only work when objects are actually touching each other. When objects are in contact, there are interaction pairs of forces that act in opposite directions.
Normal contact force
When an object sits on a surface, the surface pushes back upwards against the object's weight. This upward push is called the normal contact force.
For example, when you place a book on a table, the book pushes down on the table due to its weight. At the same time, the table pushes back up on the book with exactly the same strength. This upward force from the table is the normal contact force.
Key fact: The normal force always acts at right angles (perpendicular) to the surface.
Friction force
Friction always works to oppose motion or attempted motion. When you try to push an object along a surface, friction pushes back in the opposite direction.
This force happens because surfaces are never perfectly smooth - they have tiny bumps and grooves that catch against each other. The rougher the surfaces, the stronger the friction becomes.
Walking example: When you walk, your feet push backwards against the ground. Friction pushes forwards on your feet, helping you move forwards.
Other contact forces
There are several other contact forces you might encounter:
- Air resistance - opposes objects moving through air
- Tension - the pulling force in ropes, strings, or cables
- Drag - opposes objects moving through liquids like water
Key example - firing an arrow
Worked Example: Forces in Archery
When an archer fires an arrow from a bow, several forces are involved:
Contact forces:
- Normal force where the arrow touches the bow string
- Friction as the arrow moves through the air
- Tension in the bow string
Non-contact forces:
- Gravity pulls the arrow downwards once it's released
This example shows how both types of forces work together in real situations.
Key Points to Remember:
- Forces always come in interaction pairs that act in opposite directions
- Non-contact forces work at a distance: gravitational (always attracts), magnetic (poles attract/repel), electrostatic (charges attract/repel)
- Contact forces need objects to touch: normal force pushes perpendicular to surfaces, friction opposes motion
- Gravity affects all objects with mass and always pulls them together
- Friction always opposes motion - it acts in the opposite direction to movement