OCR AS Physics

Revision Notes

2.3.3 Resolving Vectors

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

Representing Vectors, downloadable AS & A Level Physics revision notes

The resultant force F at an angle θ to the horizontal

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

Resolving Vectors, downloadable AS & A Level Physics revision notes

The resultant force F can be split into its horizontal and vertical components

  • For the horizontal component, Fx = F cos θ
  • For the vertical component, Fy = F sin θ

Example: 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)

Vectors On an Inclined Plane, downloadable AS & A Level Physics revision notes

The weight vector of an object on an inclined plane can be split into its components parallel and perpendicular to the slope

Worked Example

A helicopter provides a lift of 250 kN when the blades are tilted at 15º from the vertical.

Resolving Forces Worked Example, downloadable AS & A Level Physics revision notes

Calculate the horizontal and vertical components of the lift force.

Step 1: Draw a vector triangle of the resolved forces

4.1.2 Resolving Forces Worked Example Answer

Step 2: Calculate the vertical component of the lift force

Vertical = 250 × cos(15) = 242 kN

Step 3: Calculate the horizontal component of the lift force

Horizontal = 250 × sin(15) = 64.7 kN

Exam Tip

If you’re unsure as to which component of the force is cos θ or sin θ, just remember that the cos θ is always the adjacent side of the right-angled triangle AKA, making a ‘cos sandwich’

Resolving Vectors Exam Tip, downloadable AS & A Level Physics revision notes

Author: Ashika

Ashika graduated with a first-class Physics degree from Manchester University and, having worked as a software engineer, focused on Physics education, creating engaging content to help students across all levels. Now an experienced GCSE and A Level Physics and Maths tutor, Ashika helps to grow and improve our Physics resources.

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