Moments (CIE IGCSE Physics)

A student is determining the weight of a metre rule using a balancing method.

 
Fig. 1.1 shows the apparatus.

The student places the metre rule on the pivot. He places the load P on the metre rule at the 90.0 cm mark. Keeping load P at the 90.0 cm mark, he adjusts the position of the metre rule on the pivot so that the metre rule is as near as possible to being balanced.

 
He records the distance a from the 90.0 cm mark to the pivot.

 
He records the distance b from the pivot to the 50.0 cm mark.

 
He repeats the steps, placing the load P at the 85.0 cm, the 80.0 cm, the 75.0 cm and the 70.0 cm marks.

 
The readings are shown in Table 1.1.

Table 1.1

Plot a graph of a/cm (y-axis) against b/cm (x-axis). You do not need to begin your axes at the origin (0,0).

Determine the gradient G of the graph. Show clearly on the graph how you obtained the necessary information.

Calculate the weight W₁ of the metre rule using the equation W₁ = G × P, where P = 1.0 N.

Suggest one practical reason why it is difficult to obtain accurate readings for a and b in this type of experiment.

The student measures the mass of the rule on a balance. Write down the mass m shown on the balance in Fig. 1.2 to the nearest gram.

 m = ..................................................... g

A student is determining the mass of a metre rule by a balancing method.

He is using the apparatus shown in Fig. 1.1.

He places the metre rule on the pivot and then places block Q with its centre at the 95.0 cm mark.
The student stated that it is difficult to place the mass accurately at the 95.0 cm mark.

Explain how the student could overcome this. You may draw a diagram to help your explanation.

The student keeps block Q at the 95.0 cm mark and adjusts the position of the metre rule on the pivot until the metre rule is as near to being balanced as possible.

Describe a method to find the point at which the metre rule is as near to being balanced as possible.

The student determines the distance a between the centre of block Q and the 50.0 cm mark and also the distance b between the centre of block Q and the pivot.

He repeats the procedure for positions of block Q at the 90.0 cm, 85.0 cm, 80.0 cm and 75.0 cm marks. His results are shown in Table 1.1.

[4]

Two students carry out the experiment correctly but with different values for the mass of block Q. One student obtains values of b that are larger than those obtained by the other student.

State and explain whether the larger values of b are likely to produce a more accurate value for the mass of the metre rule.

A student is determining the mass of a load using a balancing method.

 
Fig. 1.1 shows the apparatus.

The load X has been taped to the metre rule so that its centre is exactly over the 90.0 cm mark.
It is not moved during the experiment.

 
A mass m of 40 g is placed on the rule and its position adjusted so that the rule is as near as possible to being balanced with the 50.0 cm mark exactly over the pivot. Fig. 1.2(a) shows part of the rule when it is balanced.

 
The procedure is repeated for a range of masses. Fig. 1.2(b) – (e) shows the rule when balanced for values of m of 50 g, 60 g, 70 g and 80 g.

m/g	d/cm
40
50
60
70
80

(ii)

For each value of d, calculate  and record it in the table.

[1]

Describe one difficulty the student might have when carrying out this experiment, and how he might overcome this difficulty.

Plot a graph of m/g (y-axis) against (x-axis).

Determine the gradient G of the graph. Show clearly on the graph how you obtained the necessary information.

Determine the mass μ, in grams, of the load X. Use the equation