Gravitational Force (Weight)
Forces and Motion

Classroom Activity for 11-14 14-16

In this activity students are introduced to the idea of the centre of gravity by comparing the balance points of a ruler and a wooden spoon.

## Equipment

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

## Preparation & safety

Choose spoons with cylindrical handles and oval-shaped bowls and check that they balance at a point on their handles. Cut them through their balance points and connect the handle and bowl back together using masking tape.

## Procedure

Introduce the term ‘centre of gravity’ as the point around which the weight of an object is evenly distributed and the point at which an object will balance.

Ask students to:work in pairs to:

1. Use their outstretched finger as a pivot to find the balance point of the ruler.
2. Repeat for the spoon.
3. Remove the masking tape and balance each part of the spoon separately.
4. Measure distances from balance positions to the cut edge and calculate the mass x distance for both handle and bowl.
5. Draw force diagrams to show where the gravitational forces act on ruler and spoon.

## Discussion prompts

• 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?

## Teaching notes

Students will accept that the centre of gravity for the ruler is at its midpoint because it has a uniform shape. The mass left and right of the pivot is equal and so gravity pulls downs on each side with equal effect. Their measurements for handle and bowl should illustrate the more general case for an irregularly shaped object: it is the mass x distance from the pivot that must be equal for an object to balance.

They may suggest drawing one, two or many arrows to represent the pull of gravity on a spoon (or ruler). All options are correct. It depends on many they view the object: a single object, two sections or many stuck together. But whichever they choose, they must always start their arrow(s) at the centre of gravity position(s).

## Learning outcome

Students can 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.

###### Gravitational Force (Weight)
is a feature of the Rigid Body Model
is a special case of Force