Many candidates attempted this question. The response of the candidates was poor. In part (a), it demanded explanation of upthrust’ as well as Archimedes’ principle and law of floctation. Here performance was generally fair although the explanation of upthrust was not correctly given by many. Part (b0, was poorly handled by candidates. They had no clear picture of what the required diagram should be. It was an application of knowledge which majority of candidates failed to see.
Both upthryust on and weight of the immersed body were correctly indicated by few candidates while tension was either omitted or wrongly indicated by the majority.
The expected answers are:
(a) (i) Upthrust is the upward force exerted by a fluid/liquid on a body (wholly or partially) immersed in the fluid/liquid.
(ii)
Archimedes’ principle states that A body wholly or partially immersed in a fluid experiences an upthrust (an apparent loss of weight ) which is equal to the weight of the fluid/liquid displaced (by the body).
(iii)
The law of flotation states that a floating body displaces its own weight of the fluid in which it floats.
OR
A body floats when the upthrust exerted upon it by the fluid in which it floats equals the weight of the body.
(b)(i)
- correct diagram showing tank, liquid, body and thread.
-
correct forces shown and labelled
T = U – W [T = tension, U = upthrust, W = weight]
U = ρbVL g
= 1.2 x 10-3 x 5 x 10-3 x 10 = 60 N
W = ρb VL g
= 9.0 x 102 x 5 10-3 x 10 = 45 N
T = 60 - 45 = 15 N
(iii) When the thread is cut off, the body will rise to the surface and float. 
