A student investigates the centre of gravity and stability of everyday objects. (a) State what is meant by the centre of gravity of an object. [1] (b) The student has a flat, irregularly shaped piece of card. Describe an experiment, using a plumb line, that the student could use…

Answer: ((a)) The centre of gravity of an object is the single point at which the whole weight of the object may be considered to act. ((b)) Make a small hole near one edge of the card and hang it from a pin so it can swing completely freely; hang a plumb line (a thread with a small weight tied to the end) from the same pin and mark the straight vertical line it makes down across the card; repeat this using a second… ((c)) Stacking the cargo low down keeps the centre of gravity of the ship (with its cargo) low. A lower centre of gravity means the ship can tilt through a larger angle before the vertical line through its centre of gravity moves outside the base of the hull, and only once that happen…. Part (a): Defining centre of gravity The centre of gravity of an object is the single point at which the whole weight of the object can be considered to act. For a symmetrical, uniform object this point is at its geometric centre, but for…

IGCSE Physics 0625: Finding Centre of Gravity and Explaining Stability

Part (a): Defining centre of gravity

The centre of gravity of an object is the single point at which the whole weight of the object can be considered to act. For a symmetrical, uniform object this point is at its geometric centre, but for an irregular shape it must be found experimentally.

Part (b): Finding the centre of gravity with a plumb line

Make a small hole near one edge of the card, and hang the card from a pin pushed through the hole so that the card is completely free to swing.
Once the card has stopped swinging, it will hang with its centre of gravity directly below the pin. Hang a plumb line (a thread with a small weight tied to the bottom) from the same pin, and mark the straight vertical line it traces down across the card.
Remove the card, make a second small hole near a different edge, and repeat the process: hang the card freely from this new hole and mark the second vertical line traced by the plumb line.
The centre of gravity of the card is the point where these two marked lines cross, since the centre of gravity must lie on the vertical line through the point of suspension every time.

Part (c): Explaining ship stability

The centre of gravity of the ship-plus-cargo system is lower when the cargo is stacked low down inside the hull, rather than high up on deck.

When a ship rolls or tilts to one side, it will only capsize once the vertical line through the centre of gravity moves outside the base of the hull (the ship’s line of support in the water). At that point, the weight produces a moment that continues to turn the ship over rather than restoring it to upright.

With a lower centre of gravity, the ship has to tilt through a much larger angle before this vertical line moves outside the base, compared with a ship whose centre of gravity is high up. This means a low centre of gravity makes the ship less likely to capsize, so loading the cargo low down makes the ship more stable.

Final answers

(a) Centre of gravity $=$ the point where an object’s whole weight can be considered to act
(b) Suspend the card freely from a pin at two different points in turn, marking the vertical plumb-line each time; the centre of gravity is where the two lines cross
(c) Loading cargo low down gives the ship a lower centre of gravity, so it can tilt through a larger angle before capsizing, making it more stable