### Question 1B

You are provided with two metre rules, two knife edges, optical
pin, a known mass and other necessary apparatus.

(i)      Affix the optical pin at the 50 cm mark of one of the
metre rules supplied, with a sellotape / plasticine.

(ii)     Place the rule horizontally on the knife edges such that
the knife edges are at the 17.5 cm and 82.5 cm marks.
Record the distance D between knife edges.

(iii)    Mount the other metre rule vertically with the retort
stand and clamp and place it close to the pin to measure
the depressions of the metre rule when the weight M is
suspended at its mid-point.

(iv)    Read and record the position ho of the pointer on the
vertically mounted metre rule, with no weight suspended
on the horizontal metre rule.

(v)     Suspend M at the midpoint of the metre rule.

(vi)    Read and record the new position of the pointer hi'

(vii)   Determine the depression of the metre rule H =  hj- ho

(viii)  Evaluate log H and log D.

(ix)     Repeat the procedure with the knife edges set at
20.0cm and 80.0 cm, 22.5 cm and 77.5 cm and 25.0 cm
and 75.0 cm marks. Each time read and record the
distance D and h1

(x)     Evaluate the depressions H in each case and log H and
1og D.

(xii)   Plot a graph with log H on the vertical axis and log D on
the horizontal axis.

### Observation

Part (a).  This question was popular among the candidates and performance was fairly good.  Many candidates obtained good readings though for some candidates, it was negative because they disregarded the instruction.  Some did not record log H and log D to the required 3 d.p.  Graph was simple and well plotted though some candidates still matched points.  Precautions were well stated.  Part (b) was fairly attempted.

Candidates were expected to:

-       Read and record ho to at least 1 d.p  in cm
-       Read and record four values of D to at least 1 d.p in cm
-       Read and record four values of  h1 to at least 1 d.p in cm and in trend

Trend:

-  as D decreases h1 decreases
-       Determined four values of H =| h1 – ho|
-       Evaluate four values of log H   to at least 3 d.p.
-       Evaluated four values of log D to at least 3 d.p.
-       List D, h1,H, log H and log D composite table
-       Distinguish both axes
-       Plot graph using reasonable scales
-       Draw line of best fit.
-       Evaluate slope
-       State any two of the following precautions

e.g.

-     Avoided parallax error in reading meter rule
-           Avoided draught

Repeated readings shown on table

Ensured firm clamping of vertical metre rule

Ensured knife edges are rigid / firm

Ensured that mass was not oscillating / mass was steady

b(i)      Moment of a force about a point is the product of the force and the perpendicular
distance  of its line of action from the point.

(ii)    Using the principle of moments
m(50 – 38)  =  60 ( 38 – 16)
12m   =   1320

m=  110g