P1.5.3P1. Motion, Forces and Energy
Centre of gravity — Combined Science 0653 - Physics Revision Notes & Topical Questions
5 min read 7 parts Lesson 8 of 15 1 quiz
Lesson summary
P1.5.3 Centre of gravity — COMBINED SCIENCE notes: Define centre of gravity, find it for a plane lamina, and explain how its position affects stability.Centre of gravity = the point through which the weight of an object appears to act.Symmetrical uniform objects — the centre of g…
- Suspended objects
- Why it works
- Stable object
- Most stable design
- Least stable design
- Wider base
Section 1 of 7Centre of gravity
Centre of gravity
Centre of gravity
Centre of gravity the point through which the weight of an object appears to act.
Symmetrical uniform objects
- Symmetrical uniform objects the centre of gravity is at the point of symmetry.
- The centre of gravity of a sphere is at its centre.
Suspended objects
- Suspended objects a freely hanging object settles with its centre of gravity directly below the point of suspension.
Section 2 of 7Finding the centre of gravity of a plane lamina
1
Punch three holes near the edge of the lamina in different places.
2
Hang a cork horizontally in a clamp and hang the lamina from a pin through one hole so that it swings freely.
3
Hang a plumb line (a weighted thread) from the same pin.
4
Mark the line of the thread on the lamina with a pencil once it has settled.
5
Repeat with the other holes to draw three lines.
6
Read off the centre of gravity at the point where the lines cross.
7
Check by balancing the lamina on a pin tip at that point.
Why it works
- Why it works the plumb line passes through the centre of gravity, so two lines are enough to find it.
Accuracy
- Accuracy punch the holes first, let the lamina settle, view the plumb line straight on and use a sharp pencil.
Safety
- Safety secure the clamp stand and push the pin into a cork when it is not in use.
Section 3 of 7Stability
Stable object
- Stable object its centre of gravity lies above its base.
Toppling
- Toppling when the line of action of the weight falls outside the base, the weight produces a resultant moment that tips the object over.
Most stable design
- Most stable design a low centre of gravity and a wide base.
- A car is more stable than a tall bus because its centre of gravity is lower.
Least stable design
- Least stable design a high centre of gravity and a narrow base, as in a bowling pin that is meant to topple.
Section 4 of 7Increasing stability
Wider base
- Wider base a wide base keeps the line of action of the weight inside the base for larger tilts, as in a Bunsen burner or a child's high chair.
Lower centre of gravity
- Lower centre of gravity placing the mass low down means the object must be tilted further before it topples.
Tall, narrow vehicles
- Tall, narrow vehicles high-sided lorries have a high centre of gravity and are advised to avoid exposed bridges in strong wind.
Section 5 of 7More from the specification
More from the specification
Moments
- Moments the moment of a force is force times perpendicular distance from the pivot, in newton metres.
Principle of moments
- Principle of moments in equilibrium the clockwise moments equal the anticlockwise moments about any pivot.
Centre of gravity
- Centre of gravity the point where the whole weight seems to act;
- a freely suspended object hangs with it directly below the pivot.
Experiment
- Experiment balance a ruler on a pivot with masses at measured distances to verify the principle,
- or hang a plumb line to find the centre of gravity of an irregular card.
Equilibrium
- Equilibrium the resultant force and resultant moment are both zero, so no acceleration or rotation occurs.
Moments
- Moments the moment of a force is force times perpendicular distance from the pivot, in newton metres.
Principle of moments
- Principle of moments in equilibrium the clockwise moments equal the anticlockwise moments about any pivot.
Centre of gravity
- Centre of gravity the point where the whole weight seems to act;
- a freely suspended object hangs with it directly below the pivot.
Experiment
- Experiment balance a ruler on a pivot with masses at measured distances to verify the principle,
- or hang a plumb line to find the centre of gravity of an irregular card.
Equilibrium
- Equilibrium the resultant force and resultant moment are both zero, so no acceleration or rotation occurs.
Perpendicular distance
- The shortest distance from the pivot to the line of action of the force.
Turning effect
- The tendency of the equal and opposite forces on the coil to rotate it about the axle.
Clockwise
- Turning in the direction of the hands of a clock.
Equilibrium
- No resultant force and no resultant moment on an object.
Force
- A push or pull on an object caused by an interaction with another object.
Section 6 of 7Key terms
| Term | Meaning |
|---|---|
| Centre of gravity | The point through which the weight of an object acts. |
| Plane lamina | A flat sheet of uniform thickness. |
| Plumb line | A weighted thread that hangs vertically. |
| Stability | The ability of an object to return to its position instead of toppling. |
| Line of action | The line along which a force acts. |
Section 7 of 7Exam technique
1
Saying the centre of gravity is always at the geometric centre — it is only there for symmetrical objects of uniform density.
2
Punching the holes after finding the centre — this changes the position of the centre of gravity.
3
Thinking an object topples when it is tilted at all — it topples when the weight acts outside the base.
4
Drawing only one plumb line — at least two are needed and a third checks the result.
Marked practiceQuizzes for this lesson
Last updated: