Uniform Laminas (Edexcel International A Level Maths: Mechanics 2)

Revision Note

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Uniform Laminas

A brief reminder of centre of mass …

  • The centre of mass is the point at which the total mass of a body (or a system of bodies) can be considered to act as one

What is a lamina?

  • A lamina is a (3D) body that has one very small dimension compared to the other two dimensions which means the third dimension is negligible and the body can be modelled as a (2D) plane
    • Common examples include
      • a sheet of paper, card, wood, metal, plastic, cloth, etc
      • a wall

What modelling assumptions are used with laminas?

  • (For this Revision Note) all laminas are uniform
    • uniform means the mass per unit area (m kg) is equal at every point on the lamina
      • In the real world a uniform lamina is very difficult to achieve – particularly with natural materials such as timber where knots and other imperfections occur
      • The cloth on a professional snooker table needs to be as close as possible to a uniform lamina - it should have no imperfections (such as a small tear) if the table and balls are to play as a professional would expect
  • In Mechanics 2 both uniform (this Revision Note) and non-uniform laminas (Revision Note 2.1.7) are considered

How do I find the centre of mass of a (standard) uniform lamina?

  • If the shape of a lamina has an axis of symmetry, the position of the centre of mass will be somewhere on that axis
  • If the shape of a lamina has more than one axes of symmetry, the position of the centre of mass will be the intersection of those axes
  • The most common shapes – rectangles (including squares), triangles, circles and sectors (including semi-circles) are referred to as standard uniform laminas

2-1-2-fig1-standard-laminas

How do I find the centre of mass of a triangular lamina?

  • Triangles are a little more complicated
  • In any triangle a median is a line joining a vertex to the midpoint of the opposite side
  • In any uniform triangle the position of the centre of mass
    • lies begin mathsize 16px style 2 over 3 end styleof the way along a median when measured from its vertex.
      This is stated in the formulae booklet
    • is the point of intersection of the medians

2-1-2-fig2-tri-2-3rds

  • The coordinates of the centre of mass of any uniform triangle can be found if the coordinates of its three vertices are known
    • The coordinates of the centre of mass of any uniform triangular lamina is the mean of the coordinates of its vertex
    • This only works for triangles and is not true for all other polygons

2-1-2-fig3-tri-coords

  • In addition to the above, for equilateral and isosceles triangles only
    • In an equilateral triangle all three medians are also axes of symmetry
    • In an isosceles triangle one of the medians is an axis of symmetry
    • The position of the centre of mass will always lie on an axis of symmetry so for both equilateral and isosceles triangles only one median need be used
    • An axis of symmetry is perpendicular to the side they bisect so any missing lengths can be found using Pythagoras’ theorem and/or basic trigonometry

2-1-2-fig4-equ-iso

Worked example

A shop sign is made from a sheet of wood in the shape of an isosceles triangle with its two equal sides of length 60 cm meeting at the vertex O with an angle of  fraction numerator 2 straight pi over denominator 3 end fraction radians.

(a)
State any modelling assumptions regarding the sheet of wood being used to make the shop sign.

 

(b)
Describe the position of the centre of mass of the shop sign in relation to vertex O.

 

The shop sign is adapted slightly to make the shape of a sector of a circle with the two equal sides of the isosceles triangle forming two radii of the sector and vertex O becoming the centre of the circle
(c)
Describe the new position of the centre of mass of the shop sign in relation to vertex O.
(a)
 State any modelling assumptions regarding the sheet of wood being used to make the shop sign.

2-1-2-fig5-we-solution_a

(b)
 Describe the position of the centre of mass of the shop sign in relation to vertex O.

2-1-2-fig5-we-solution_b

The shop sign is adapted slightly to make the shape of a sector of a circle with the two equal sides of the isosceles triangle forming two radii of the sector and vertex O becoming the centre of the circle
(c)
Describe the new position of the centre of mass of the shop sign in relation to vertex O.

2-1-2-fig5-we-solution_c

Examiner Tip

  • Sketch diagram(s) or add to any given in a question – this may help you to see an axis of symmetry.
  • If a triangle is involved, be clear about the type of triangle (and the information about it) you have been given.
  • Remember that a semi-circle is a sector of a circle with angle π radians at the centre.
  • Beware!  The formula booklet lists the formulae for laminas and frameworks together.

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Paul

Author: Paul

Expertise: Maths

Paul has taught mathematics for 20 years and has been an examiner for Edexcel for over a decade. GCSE, A level, pure, mechanics, statistics, discrete – if it’s in a Maths exam, Paul will know about it. Paul is a passionate fan of clear and colourful notes with fascinating diagrams – one of the many reasons he is excited to be a member of the SME team.