Types of Forces Notes
Edexcel GCSE Physics: Revision notes
Key facts
- A force is a push or pull from the interaction between two objects, and it always acts in an equal and opposite pair.
- Contact forces (friction, normal contact force) need touching; non-contact forces (gravitational, electrostatic, magnetic) act through a field.
- A vector has size and direction; a scalar has size only. Force is a vector.
- The resultant force is the single force with the same effect as all the forces together. Equilibrium means it is zero (Higher tier).
- Lubrication reduces friction, so less energy is wasted as heat.
Contact and non-contact forces
A force is a push or pull; it acts either by touching (contact) or at a distance through a field (non-contact).
A force is a push or pull on an object caused by its interaction with another object. Forces always come in interaction pairs: if A pushes B, B pushes back on A with an equal and opposite force.
Contact forces need touching: the normal contact force is a surface pushing back on an object, and friction opposes relative motion between surfaces. Non-contact forces act through a field.
| Contact | Non-contact | |
|---|---|---|
| Objects touching? | Yes | No: acts through a field |
| Examples | Normal contact force, friction | Gravitational, electrostatic, magnetic |
Contact
- Objects touching?:
- Yes
- Examples:
- Normal contact force, friction
Non-contact
- Objects touching?:
- No: acts through a field
- Examples:
- Gravitational, electrostatic, magnetic
Which of these is a non-contact force?
Vectors and scalars
Vectors have a size and a direction; scalars have a size only. Force is a vector.
A scalar has magnitude (size) only. A vector has magnitude and direction. A vector is drawn as an arrow: the length shows the size and the arrow points in the direction.
A force must always be described with both a size and a direction.
Scalars (size only)
- speed
- distance
- mass, energy
Vectors (size and direction)
- velocity
- displacement
- force, acceleration, momentum
Which pair contains only vector quantities?
Combining forces (Higher tier)
Forces add to a single resultant; when the resultant is zero the object is in equilibrium.
The resultant force is the single force with the same effect as all the forces acting together. Scale vector diagrams add forces at different angles, and one force can be resolved into two components at right angles.
Equilibrium is when the resultant force is zero: the object is stationary or moving at constant velocity. A free body force diagram shows only the object and the forces on it, as labelled arrows.
- 1
Isolate the object
Draw it alone, as a box or dot.
- 2
Add each force
One labelled arrow per force on it.
- 3
Check lengths
A longer arrow means a larger force.
- 4
Find the resultant
Combine the arrows, allowing for direction.
An object moves at constant velocity. What is the resultant force on it?
Reducing friction
Friction between moving surfaces wastes energy as heat and causes wear; lubrication reduces it.
Friction between moving surfaces transfers energy as heat. This is often unwanted: it wastes energy and wears out parts.
Lubrication means putting a substance such as oil or grease between the surfaces so they slide past each other more easily.
Without lubrication
- More friction
- More energy wasted as heat
- More wear
With lubrication
- Less friction
- Less energy wasted
- Less wear
Why is oil put on a bicycle chain?
Try an exam question
Describe the difference between a contact force and a non-contact force, and give one example of each.
[4 marks]
- [1]A contact force acts only when the objects are touching.
- [1]Example of a contact force: friction (or normal contact force).
- [1]A non-contact force acts between objects that are not touching, through a field.
- [1]Example of a non-contact force: gravity (or magnetic or electrostatic force).
That's the notes covered.
Carry on to the next subtopic.