

Explanation
(a) PROCEDURE
Make at least 3 well-spaced pin holes round the edge of the cardboard. Clamp the pin horizontally and suspend the cardboard on it through one of the pin-holes such that the cardboard can swing freely
Hang the simple pendulum on the same pin and let its string be very close to the cardboard.  
When the whole system is at rest (or in equilibrium) trace the plumbline on the cardboard. Repeat the procedure for each of the two other pin holes. 
 CONCLUSION
The point at which the (three) traced lines intersect is the centre of gravity of the cardboard. 
 PRECAUTIONS 
-  Repeat procedure
- Pin rigidly and firmly held by retort stand and clamp
- Allow the simple pendulum to rest before tracing the shadow of
 the plumbline on the cardboard.
-  The string to be close to the cardboard
b)  Diagram: 

  Both axes correctly labelled  
  Any correct shape of graph showing acceleration segment 

Acceleration = gradient of AB  
Total distance covered = area under the graph  

(c) Diagram

- At least one axis labelled 
- correct shape of graph  
Let V\(_1\) = maximum velocity after 20 sec.
 V\(_2\) = maximum velocity after 30 sec.
(i) Then  \(\frac{V_1}{20}\)  = 6
 
 