Alternative B
Question 1B
You are provided with a set of masses M, a metre rule, a knife edge and other necessary apparatus.
(i) Balance the metre rule on the knife edge and determine its centre of gravity G
(ii) Suspend the mass M = 50 g at the point A = 5 cm mark of the metre rule and balance the whole arrangement on the knife edge.
(iii) Read and record the balance point K and determine the distance X between A and K.
Observation
Part (a) Candidates performance was poor. Many candidates were not able to determine G and K to at least 1 d.p in cm.
Part (b) Candidates response was unsatisfactory.
The expected response:
- The value of G = 50.0 ± 1.0 cm
- Six values of M correctly recorded in g
- Six values of K recorded to at least 1dp in cm and in trend
- Six values of X = K - 5 correctly determined
- Six values of correctly evaluated to at least 3 d.p
- Composite table showing at least M, K, X and 1/X
- Both axes correctly distinguished
- Plot a graph using reasonable scales
- Draw line of best it
- Determine the slope and intercept of the graph
- State any two of the following precautions.
e.g.
- Avoided errors due to parallax in reading the metre rule
- Ensured rigid support
- Avoided draught
- Repeated readings shown on table
- Avoided/Noted/Corrected zero error on metre rule
- Ensured mass did not rest on/touch the table
In part b, the expected answers are:
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) Moment of couple = one force x perpendicular distance between the forces .
= 20 x 0.5 = 10 Nm