Classroom Activity for 11-14
- Activity time 20 mins
Students balance a ruler and a wooden spoon. An introduction to the idea of the centre of gravity of an object.
Each group of students will need:
- Wooden spoon
- 30 cm ruler
- Electronic balance (capable of measuring to nearest g or better)
You will also need:
- A saw
- Masking tape
Preparation & safety
Choose identical spoons with an oval-shaped bowl and cylindrical handle. Check that each spoon balances at a point on its handle.
Cut through each spoon at its balance point. Connect the handle and bowl together again using masking tape to make a spoon once more.
Ask students to:
- Use their outstretched finger as a pivot to find the balance point of the ruler.
- Repeat for the spoon.
- Take the masking tape off the spoon and balance each part of the spoon separately.
- Measure distances from balance positions to the cut edge for both handle and bowl sections.
- Why does the ruler balance at its midpoint, but not the spoon?
- How does the mass left and right of the pivot compare?
- Where should I draw gravity force arrows on a diagram?
Students will accept that a ruler balances at its midpoint because it has a uniform shape. The mass left and right of the pivot is equal. Gravity pulls downs on each side with equal effect.
Students may suggest drawing one, two or many arrows to represent the pull of gravity on the spoon or the ruler. Encourage the discussion. The number of arrows depends on how we think of an object: a single entity, two sections or many stuck together. In principle we could draw small arrows for each of the many atoms that make it up. Introduce the term ‘centre of gravity’ as the point where we should draw a single arrow to represent the gravitational pull on the whole.
Experimentally, the centre of gravity position can be found by finding the point at which an object balances. If the handle has half the mass of the bowl, its centre of gravity must be twice as far from the balance point. More generally, for both bowl and handle, multiply mass by distance to show that these are equal either side of the pivot for the spoon.
Students explain why the centre of gravity of an irregular object is not half way along its length.
This experiment was safety-checked in March 2020.