Interacting forces (AQA GCSE Physics): Revision Notes
Interacting forces
What is a force?
A force is a push or a pull that acts on an object. Forces happen when one object interacts with another object. This interaction can happen in two ways:
- At a distance - objects don't need to touch
- By direct contact - objects must be touching
Forces always work in pairs and act in opposite directions. We can show forces using arrows called vectors.
Non-contact forces
These forces can act between objects even when they're not touching. There are three main types you need to know:
Gravitational force
- Acts between any objects that have mass
- Always pulls objects towards each other (attractive)
- The Earth pulls everything downwards with gravity
Example: A stone falling to the ground demonstrates gravitational force - the Earth's gravity pulls the stone downwards towards the centre of the Earth.
Magnetic force
- Acts between magnetic materials and magnets
- Can be attractive (opposite poles) or repulsive (like poles)
- North and south poles attract each other
- Two north poles or two south poles push away from each other
Example: Fridge magnets sticking to metal doors show magnetic attraction. The magnet and the metal door are attracted to each other even though they start apart.
Electrostatic force
- Acts between objects that have electric charge
- Can be attractive or repulsive
- Opposite charges attract each other
- Like charges repel each other
Example: Hair standing up when you rub a balloon on it demonstrates electrostatic force. The balloon becomes charged and attracts the oppositely charged hair.
Contact forces
These forces only work when objects are actually touching each other. The most important ones are:
Normal contact force
- Acts upwards when an object rests on a surface
- Balances the weight of the object
- Prevents objects from falling through surfaces
Example: A book resting on a table - the table pushes up on the book with normal force, which balances the book's weight pulling it down.
Friction
- Acts when objects try to slide past each other
- Always opposes movement
- Slows moving objects down
- Acts between the surfaces that are in contact
Example: Brakes on a bicycle wheel create friction to slow you down. The brake pads press against the wheel, creating friction that opposes the wheel's rotation.
Other contact forces
- Air resistance - acts on objects moving through air
- Tension - acts in ropes, strings and cables
- Drag - acts on objects moving through liquids
Force pairs
Every force has a partner force that acts in the opposite direction.
Examples of Force Pairs:
- When you sit on a chair, you push down on the chair and the chair pushes up on you
- When you walk, you push backwards on the ground and the ground pushes forwards on you
- When an archer fires an arrow, the string pulls the arrow forwards and the arrow pulls the string backwards
Remember!
Key Points to Remember:
- Forces are pushes or pulls that happen when objects interact
- Non-contact forces work at a distance: gravity, magnetism, electrostatic
- Contact forces need objects to touch: friction, normal force, air resistance
- All forces come in pairs that act in opposite directions
- We can represent forces using arrows called vectors