Contact & Non-Contact Forces (OCR GCSE Physics A (Gateway))

Contact & Non-Contact Forces

• A force is defined as:

  A push or a pull that acts on an object due to the interaction with another object

• Forces can have an effect on bodies in several different ways:

  • They can change their speed

  • They can change their direction

  • They can change their shape

• Some examples of these changes are:

  • An engine can increase the speed of a vehicle due to a force called thrust

  • A comet’s direction can be affected by gravitational attraction

  • A spring can have its shape changed by the force from a heavy load

Diagram showing the effects of forces on different objects

• Important forces to be familiar with:

  • Weight – the name given to the force of gravity on a mass

  • Electrostatic force – the force between two charged objects

  • Thrust – the force causing an object to move (such as the force from a rocket engine)

  • Air resistance (or drag) – the friction of the air on a moving object

  • Upthrust – the force of a fluid (such as water) pushing an object upwards (making it float)

  • Compression – forces that act inward on an object, squeezing it

  • Tension – force transmitted through a cable or a string when pulled on by forces acting on its opposite ends

  • Reaction force (or the normal force)– a force due to contact between two objects

Diagram showing several different types of forces acting on objects

• All forces can be categorised into one of two types:

  • Contact forces

  • Non-contact forces

Contact Forces

• A contact force is defined as:

  A force which acts between objects that are physically touching

• Examples of contact forces include:

  • Friction

  • Air resistance

  • Tension

  • Reaction force / Normal force

• Friction:

  • Is a force that opposes motion

  • Occurs when objects rub against each

• Air resistance:

  • Is a type of friction

  • Occurs when an object moves through air

• Tension:

  • Is a force that pulls two objects connected by a length, such as a string or rope

  • Occurs when a force is applied to the length

• Reaction force / Normal force:

  • Is a force that pushes touching objects apart

  • Occurs when objects are supported by a surface

Examples of contact forces

Non-Contact Forces

• A non-contact force is defined as:

  A force which acts at a distance, without any contact between bodies, due to the action of a field

• Examples of non-contact forces include:

  • Gravitational force

  • Electrostatic force

  • Magnetic force

• Gravitational attraction:

  • The attractive force experienced by two objects with mass

  • For example, the force between a planet and a comet

• Electrostatic force:

  • A force experienced by charged objects which can be attractive or repulsive

  • For example, the attraction between a proton and an electron

• Magnetic force:

  • A force experienced between magnetic poles that can be attractive or repulsive

  • For example, the attraction between the North and South poles of magnets

Examples of non-contact forces