Resolving Vectors (AQA AS Physics): Revision Note

Resolving vectors

• Two vectors can be represented by a single resultant vector

  • Resolving a vector is the opposite of adding vectors

• A single resultant vector can be resolved

  • This means it can be represented by two vectors, which in combination have the same effect as the original one

• When a single resultant vector is broken down into its parts, those parts are called components

• For example, a force vector of magnitude F and an angle of θ to the horizontal is shown below

• It is possible to resolve this vector into its horizontal and vertical components using trigonometry

• For the horizontal component, F_x = F cos θ

• For the vertical component, F_y = F sin θ

Forces on an inclined plane

• Objects on an inclined plane is a common scenario in which vectors need to be resolved

  • An inclined plane, or a slope, is a flat surface tilted at an angle, θ

• Instead of thinking of the component of the forces as horizontal and vertical, it is easier to think of them as parallel or perpendicular to the slope

• The weight of the object is vertically downwards and the normal (or reaction) force, R is always vertically up from the object

• The weight W is a vector and can be split into the following components:

  • W cos (θ) perpendicular to the slope

  • W sin (θ) parallel to the slope

• If there is no friction, the force W sin (θ) causes the object to move down the slope

• The object is not moving perpendicular to the slope, therefore, the normal force R = W cos (θ)

Equilibrium

• Coplanar forces can be represented by vector triangles

• Forces are in equilibrium if an object is either:

  • at rest

  • moving at constant velocity

• In equilibrium, coplanar forces are represented by closed vector triangles

  • The vectors, when joined together, form a closed path

• The most common forces on objects are:

  • weight

  • normal reaction force

  • tension (from cords and strings)

  • friction

• The forces on a body in equilibrium are demonstrated below: