Question 8
(a) (i) Define a torque.
(ii) State three factors that determine a torque.
(b) (i) Define free fall.
(ii) A body is thrown vertically upwards from the top of a tower 40.0m high with a velocity of 10.0 m s-1. Calculate the time taken for the body to reach the ground. [g= 10.0 m s-2]
(c) A cube of wood of side 8.0 cm, floats at the interface between oil and water with 2.0cm of its lower surface below the interface as shown in the diagram below. Given that the relative densities of oil and water are 0.72 and 1.00 respectively,
calculate the mass of the wood.
Observation
Part (a)(i) This was one of the popular choice of the candidates but the recorded performance was average. Most candidates could not state the definition of torque but rather defined work instead.
(ii) Most candidates were only able to state one correct factor i.e “magnitude of the force”.
Part (bi) Most candidates free fall without stating that such a fall is under the influence og gravity alone.
(ii) Candidates were expected to recall the second equation of motion H = ut + 
at2 and make the necessary substitution taking into account the fact that the body is thrown vertically upwards. This definitely will affect the positive signs in the equation above.
Part (c) Candidates were expected to add the upthrust due to water displaced to the upthrust due to oil displaced to get the mass of the wood but rather opted for other method which got them confused along the line.
The expected answer is:
(a) (i) Definition of a torque
The turning effect of force about an axis/pivot
(ii) Factors that determine a torque
- perpendicular distance of the force from the pivot
- direction of the force
- magnitude of the force
(b) (i) Definition of free fall
The vertical motion of a body only under the influence of gravity/its own weight
(ii) Calculation of the time taken by a body thrown vertically upward to reach the ground
H = ut + ½gt2
-40 = 10t - ½ x 10 x t2
(t + 2)(t – 4) = 0
Time to fall to the ground, t = 4 s
(c) Calculation of the mass of wood
Mass of wood = upthrust due to water displaced + upthrust due to oil displaced
= 1000 x 2 x 8 x 8 x 10-6 + 720 x 6 x 8 x 8 x 10-6
= 0.404 kg