# Edexcel A Level Physics:复习笔记2.8 Forces as Vectors

### Free-body Force Diagrams

• Free body diagrams are useful for modeling the forces that are acting on an object
• Each force is represented as a vector arrow, where each arrow:
• Is scaled to the magnitude of the force it represents
• Points in the direction that the force acts
• Is labelled with the name of the force it represents or an appropriate symbol

• Free body diagrams can be used:
• To identify which forces act in which plane
• To resolve the net force in a particular direction

#### Simple Free Body Diagrams

• The rules for drawing a free-body diagram are the following:
• Rule 1: Draw a point in the centre of mass of the body
• Rule 2: Draw the body free from contact with any other object
• Rule 3: Draw the forces acting on that body using vectors with correct direction and proportional length

Free body diagrams can be used to show the various forces acting on objects

#### Forces on a Particle or an Extended but Rigid Body

• Free body diagrams simplify problems by reducing complex shapes into simple one
• Any object can be thought of as a particle
• Even something as large as the Earth can be modelled as a point mass for most calculations
• Objects can also be thought of as being extended but rigid bodies
• This simply means that all parts stay in the same position relative to each other when the object moves

Point particle representation of the forces acting on a moving object

• The below example shows an object sitting on a slope in equilibrium

Three forces on an object in equilibrium form a closed vector triangle

#### Worked Example

Draw free-body diagrams for the following scenarios:

a) A picture frame hanging from a nail

b) A box sliding down a slope

Part (a)

A picture frame hanging from a nail:

• The size of the arrows should be such that the 3 forces would make a closed triangle as they are balanced

Part (b)

A box sliding down a slope:

• There are three forces acting on the box:
• The normal contact force, R, acts perpendicular to the slope
• Friction, F, acts parallel to the slope and in the opposite direction to the direction of motion
• Weight, W, acts down towards the Earth

#### Exam Tip

When labeling force vectors, it is important to use conventional and appropriate naming or symbols such as:

• w or Weight force or mg
• N or R for normal reaction force (depending on your local context either of these could be acceptable)

Getting used to this notation will make labelling free-body diagrams automatic - leaving you time to focus on the Physics rather than the process. This is crucial as a good diagram will show you exactly what to include in your calculations.