Objects Suspended in Equilibrium (Edexcel International A Level Maths: Mechanics 2)

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Objects Suspended in Equilibrium

What is meant by objects suspended in equilibrium?

  • An object can be suspended at a fixed point by a string such that it is allowed to hang freely and be in equilibrium (at rest)
    • e.g. A sign hung from the ceiling of a shop by single rope (string)
    • An object will either be a uniform lamina or a uniform framework
  • Equilibrium means
    • there is zero resultant (net) force acting on the object
    • there is zero resultant (net) moments about the point of suspension
  • Objects suspended in equilibrium have two forces acting on them
    • tension in the string (acting at the fixed point)
    • weight (acting at the position of the centre of mass)
  • These forces act through a vertical line which passes through the point of suspension and the position of the centre of mass
  • Problems often require the angle that is created between this vertical line and an edge of the object to be found

2-1-5-fig1-suspended-objects

  • A lamina could be suspended at a fixed point from a vertex (as above) or along an edge, or, suspended by a point on its surface that it would then be able to pivot (rotate) around
  • A lamina suspended at a pivot will have two forces acting on its
    • reaction force (acting at the pivot)
    • weight (acting at the position of the centre of mass)
  • As with the fixed-point scenario above, these forces will act through a vertical line which passes through the point of suspension and the position of the centre of mass

2-1-5-fig2-lamina-pivot

  • The mathematics involved in finding the angle is the same for both the fixed-point and pivot scenarios but you should be aware of the different phrasing in questions

How do I solve problems involving freely suspended laminas and frameworks?

 

STEP 1     Sketch a diagram or use a given one and add to it as you make progress

STEP 2     Find the position of the centre of mass of the object

                            Ensure you know whether you are working with a

                           lamina or a framework and the differences in finding the

                           position of the centre of mass

STEP 3     On your diagram, add the position of the centre of mass

        Draw a line to represent the downward vertical

        This will be a straight line connecting the point at which the body is suspended to the point of the centre of mass

                           Identify the angle required

 STEP 4    Use trigonometry to find the required angle

       Interpret the result in the context of the problem and answer the question accordingly

Worked example

As part of the set in a theatre production, a large polystyrene letter E is to be produced and will hang above the stage by a single rope attached to the ceiling.

2-1-5---we-diagram--ial-mech

The polystyrene letter is modelled as a uniform lamina and the rope is modelled as a light inextensible string.

Find the angle the rope makes with the side PQ when the sign is at rest and suspended from the point P.

2-1-5-fig3-we-solution-1

Examiner Tip

  • Ensure you know whether you are working with a lamina or a framework.
  • Once you have drawn the downward vertical line on your diagram turn your page (rather than trying to sketch it) to get an idea of how the lamina or framework would look like when suspended from the given point.

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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.