1.5.3Motion, Forces and Energy
Centre OF Gravity — Cambridge IGCSE Physics 0625 Revision Notes & Topical Questions
5 min read 8 parts Lesson 50 of 91 1 quiz
Lesson summary
1.5.3 Centre of Gravity — IGCSE PHYSICS notes: Define centre of gravity, find it for a plane lamina and explain how it affects stability.Centre of gravity = the point through which the whole weight of an object appears to act.A symmetrical object of uniform density has its centre…
- It shifts as the shape changes
- Balance test
- Tilting limit
- Lower the centre of gravity
- Widen the base
- Plumb line
Part 1 of 6Centre of gravity
1Centre of gravity
Centre of gravity the point through which the whole weight of an object appears to act.
2
A symmetrical object of uniform density has its centre of gravity at its point of symmetry.
- A sphere has it at the centre, a uniform rod at its midpoint and a rectangle at its geometric centre.
3
It is a hypothetical point, so it can lie outside the material of the object, as in a ring or an L-shape.
4It shifts as the shape changes
- It shifts as the shape changes a person's centre of gravity is lower when leaning forward than when upright.
Part 2 of 6Finding the centre of gravity of a lamina
1
Make three small holes near the edge of the card lamina.
2
Hang the card freely from a pin through one hole, with a plumb line from the same pin.
3
When still, mark the plumb line's position on the card and draw a line.
4
Repeat from a second hole.
5
The centre of gravity is where the lines cross; check with the third hole.
A freely hanging object rests with its centre of gravity directly below the point of suspension.
Balance test
- Balance test the lamina balances on a pin tip at the marked point.
Part 3 of 6Stability
An object is stable when its centre of gravity lies above its base.
If tilted slightly, the weight gives a moment that returns the object to its original position.
An object topples when its centre of gravity moves outside its base.
- The line of action of the weight then lies outside the base and gives a resultant moment that turns it over.
Tilting limit
- Tilting limit a car can tilt to about 60° without toppling but a bus topples at about 45°.
Part 4 of 6Increasing stability
Increasing stability
Lower the centre of gravity
- Lower the centre of gravity the object must tilt further before it topples.
Widen the base
- Widen the base the weight line stays inside the base for larger tilts.
Design examples:
- Racing cars and cars have a low centre of gravity and a wide base.
- Buses carry heavy parts low down.
- Bunsen burners have a wide base.
- Traffic cones have a wide base.
Tall objects with a narrow base topple easily, and ten-pin bowling pins are designed to do so.
High-sided lorries are advised to avoid exposed bridges on windy days.
Part 5 of 6Stability compared
| Factor | More stable when | Less stable when |
|---|---|---|
| Centre of gravity | Low | High |
| Base | Wide | Narrow |
| Weight line | Inside the base | Outside the base |
Part 6 of 6More from the specification
More from the specification
Suspended plumb lines and gravitational forces
- Suspended plumb lines and gravitational forces plumb line cords aligned along gravitational forces form reference loops passing through the centre of gravity.
Principle of moments
- Principle of moments for a body in equilibrium the sum of clockwise moments about any pivot equals the sum of anticlockwise moments
Moment of a force
- Moment of a force force multiplied by the perpendicular distance from the pivot, measured in newton metres
Centre of gravity
- Centre of gravity the single point through which the whole weight of an object appears to act
Equilibrium of a ruler
- Equilibrium of a ruler a uniform ruler balances at its midpoint, and adding masses requires adjusting distances so moments balance
Finding centre of gravity experimentally
- Finding centre of gravity experimentally hang an irregular lamina from a pin at different points and draw plumb lines;
- the centre of gravity is where they cross
Principle of moments
- Principle of moments for a body in equilibrium the sum of clockwise moments about any pivot equals the sum of anticlockwise moments
Moment of a force
- Moment of a force force multiplied by the perpendicular distance from the pivot, measured in newton metres
Centre of gravity
- Centre of gravity the single point through which the whole weight of an object appears to act
Equilibrium of a ruler
- Equilibrium of a ruler a uniform ruler balances at its midpoint, and adding masses requires adjusting distances so moments balance
Finding centre of gravity experimentally
- Finding centre of gravity experimentally hang an irregular lamina from a pin at different points and draw plumb lines;
- the centre of gravity is where they cross
Turning effect
- Moment that rotates the coil.
M = Fd
What the symbols mean: moment (N m) · F = force (N) · d = perpendicular distance from the pivot to the line of the force (m)
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.
Perpendicular
- At right angles.
Key terms
| Term | Meaning |
|---|---|
| Centre of gravity | Point where the whole weight seems to act. |
| Stability | Resistance to toppling. |
| Base | Area in contact with the ground. |
| Toppling | Occurs when the weight acts outside the base. |
| Plumb line | Weight on a string showing the vertical. |
| Equilibrium | No resultant force or moment. |
Avoid these mistakes
1
Saying the centre of gravity is always inside the object — it can lie outside, as in a ring.
2
Saying an object topples when it is tilted at all — it topples only when the weight line passes outside the base.
3
Forgetting to link stability to a moment — the weight outside the base produces a turning moment.
4
Using one plumb line only to find the centre of gravity — the intersection of two or more lines is needed.
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