How to find weight of a meter rule using a fulcrum and a 100g mass?

Answer 1

Finding the weight of a ruler using moments

Apparatus:

Metre ruler, knife edge, set of 100g masses [M20C]

Diagram:

Procedure:

1. Place the metre ruler (without the weight shown in the above diagram) on the knife edge so that it is balanced.

The knife edge should be roughly below the 50cm mark on the ruler. Note down the precise position of the knife edge below the ruler to the nearest mm. This is the position of the ruler's centre of gravity.

2. Place a 100g mass, a weight of 1N, near the left-hand end of the ruler as shown in the diagram above.

The ruler will tip down on this side.

3. Move the knife edge towards the weight until the ruler again balances.

Measure the distance p (in mm) between the centre of the weight and the new knife edge position and the

distance q (in mm) between the new knife edge position and the position of the centre of gravity of the ruler

(see diagram above).

4. Record your measurements in a table with the headings shown below.

5. Repeat stages 2 to 4 for weights of 2N (using 200g of mass) to 9N (900g mass).

Calculations:

1. When the ruler is balanced the anticlockwise moment caused by the weight of the masses is equal to the clockwise moment caused by the weight of the ruler.

Therefore:

weight of mass, W (N) x distance p (mm) = ruler weight R (N) x distance q (mm)

2. Calculate the value distance, q divided by weight of mass , W [q / W] for each of your results in the appropriate column in the table.

Weight of mass W

in newtons

Distance p

(mm)

Distance q

(mm)

q divided by W

(mm/N)

3. Draw a graph of distance p (mm) [Y-AXIS] against q / W (mm/N) [X-AXIS]

4. Measure the gradient of this graph.

The gradient is equal to the weight of the ruler in newtons.

Write out clearly your answer.

5. Explain why the gradient of your graph equals the weight of the ruler