(a) You are provided with a uniform metre rule, a knife edge, masses and other necessary apparatus.
(i) Suspend the metre rule horizontally on the knife edge. Read and record the point of balance G of the metre rule. Keep the knife edge at this point throughout the experiment.
(ii) Using the thread provided, suspend the object labeled W at the 15 cm of the metre rule.
(iii) Suspend a mass M = 20 g on the other side of G Adjust the position of the mass until the metre rule balances horizontally again.
(iv) Read and record the position Y of the mass M on the metre rule.
(v) Determine and record the distance L between the mass and G. Also determine and record the distance D between W and G.
(vi) Repeat the procedure for four other values of M = 30, 40, 50 and 60 g. In
each case, ensure that W is kept constant at the 15 cm mark and the knife edge at G.
(vii) Evaluate L¹ in each case. Tabulate your readings.
(viii) Plot a graph of M on the vertical axis against L¹ on the horizontal axis.
(ix) Determine the slope s, of the graph.
(x) State two precautions taken to obtain accurate results. [21 marks]
(b) (i) State the principles of moments. [2 marks]
(ii) Define centre of gravity. [2 marks]
Candidates were required to:
(i) set up an experiment as shown in the diagram above;
(ii) measure and record five values of G and D;
(iii) measure and record five values of Y to at least 1 decimal place and in trend. (Trend: as M increases, Y decreases)
(iv) evaluate five values of L and L¹;
(v) record data in a composite table showing M. Y, L and L¹;
(vi) indicate the graph axes, select reasonable scales, plot the five points;
(vii) draw a large right angled triangle on the graph line;
(viii) read and record Δm and ΔL¹also evaluate Δm ;
(ix) correctly evaluate s
(x) state any two of the following precautions in acceptable form of speech.
- draught was avoided
- parallax error on reading metre rule was avoided
- repeated readings (must be shown on table)
- ensured suspended masses did not touch the table.
b(i)The principle of moment states that for a body in equilibrium, the sum of the clockwise moments about a point is equal to the sum of the anticlockwise moments about the same point.
(ii)The centre of gravity of a body is defined as the point through which its resultant weight acts.
This question was popular and well attempted by most candidates. However, some candidates failed to record the values of Y, D and G as demanded by the question thereby losing marks. The L¹ was not calculated to 3 decimal places by most candidates.
The part b was not properly answered by most candidates as they confused the principles of moment with that of conditions for a rigid body to be in equilibrium.