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1.5.4Motion, Forces and Energy

Investigating Centre of Gravity — Cambridge IGCSE Physics 0625 Revision Notes & Topical Questions

5 min read 9 parts Lesson 54 of 91 1 quiz

📚 Cambridge IGCSE Physics 0625 Hub / Topical Revision
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Lesson summary

1.5.4 Investigating Centre of Gravity — IGCSE PHYSICS notes: Describe an experiment to find the centre of gravity of an irregular flat object and explain each step and source of error.Aim — find the centre of gravity of an irregularly shaped plane lamina, whose position cannot be…

  • Balance test
  • Friction at the pin
  • Plumb line not hanging freely
  • Thick pencil marks
  • Card not flat
  • Holes close together

Part 1 of 7Aim and key idea

Aim
  • Aim find the centre of gravity of an irregularly shaped plane lamina, whose position cannot be guessed.
Key fact
  • Key fact a freely suspended object comes to rest with its centre of gravity vertically below the point of suspension.
Reason
  • Reason otherwise the weight would have a moment about the pin and the card would swing until that moment is zero.
Variables:
  • Independent variable is the shape of the lamina.
  • Dependent variable is the position of the centre of gravity.

Part 2 of 7Apparatus

1
An irregularly shaped plane lamina of thin, stiff card.
2
A hole punch to make holes near the edge.
3
A pin or nail and a cork held horizontally in a retort stand, boss and clamp.
4
A plumb line, made of thread with a small mass or sticky tack.
5
A pencil and a ruler.

Part 3 of 7Method

1
Punch three holes near the edge of the lamina, well separated from each other.
2
Hang the lamina from the pin through one hole and check that it swings freely.
3
Hang the plumb line from the same pin so that it lies in front of the lamina and does not touch it.
4
When the lamina is at rest, mark the position of the plumb line on the card and draw the line.
5
Repeat with a different hole each time until three lines are drawn.
6
The point where the lines cross is the centre of gravity.

Part 4 of 7Checking the result

Two lines are enough to locate the point; the third line confirms it.
Each plumb line shows all points that could be the centre of gravity, so only the intersection can be correct.
If the three lines do not meet at one point, there is an error.
Balance test
  • Balance test the lamina should balance horizontally on a pin or pencil tip at the marked point.
Pinned at the centre of gravity, the lamina stays at rest in any position.

Part 5 of 7Sources of error and improvements

1Friction at the pin
  • Friction at the pin the card may stick, so use a loose hole and tap the card gently.
2Plumb line not hanging freely
  • Plumb line not hanging freely keep it clear of the card and wait until it stops swinging.
3Thick pencil marks
  • Thick pencil marks use a sharp pencil and mark precisely.
4Card not flat
  • Card not flat a warped card moves the true centre of gravity out of the plane.
5Holes close together
  • Holes close together lines cross at shallow angles, so choose holes well apart.
6
Using the middle of the small triangle of intersection from three holes reduces random error.

Part 6 of 7Safety

Safety
Pins are sharp, so hold the card carefully.
Keep the card away from the face when pushing the pin through.
Place the retort stand where it cannot be knocked over.

Part 7 of 7More from the specification

More from the specification
Rotational equilibrium around a pivot
  • Rotational equilibrium around a pivot balanced resultant forces about a support pivot eliminate turning moments in suspended plumb line loops.
Turning effect
  • Moment that rotates the coil.
Clockwise
  • Turning in the direction of a clock's hands.
Anticlockwise
  • Turning against the direction of a clock's hands.
Distance
  • Total length of path travelled, a scalar.
Equilibrium
  • State with zero net force.
Force
  • A push or pull measured in newtons.
Resultant
  • Vector sum of two or more vectors.
Topple
  • Tip over because the weight acts outside the base.
Perpendicular
  • At right angles.
Centre of gravity
  • Centre of gravity its position depends on the shape of the body, so it shifts as the shape changes.

Key terms

TermMeaning
Plumb lineA string with a weight that hangs exactly vertically.
SuspendTo hang an object so it can swing freely.
Centre of gravityThe point where the weight of the object appears to act.
IntersectionThe point where lines cross.
Irregular objectAn object with no symmetry, where the centre of gravity cannot be guessed.

Avoid these mistakes

1
Forgetting that each plumb-line mark must start from the pin hole used.
2
Using one hole only — two or more lines are needed to find a point.
3
Letting the plumb line touch the card — it then hangs wrongly.
4
Reading before the card has stopped swinging.
5
Using holes close together — the lines cross at shallow angles and the intersection is inaccurate.

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