This is a suggested solution for GCE O Level Pure Physics, Year 2018, Paper 2, Section A, Question 2 (a) &(b). This question is under the topic of Dynamics.

In this question, the following data was provided:

  • The tanker starts from rest and travels in a straight line.
  • Mass of tanker: 1.5 x 106 kg
  • Force acting forward on tanker due to propellers is a constant at 2.5 x 105 N
  • Acceleration of tanker is not constant
  • Drag force on tanker at one instance: 1.2 x 105 N.

Candidates are required to calculate the acceleration on the tanker when the drag force is 1.2 x 105 N and to explain in terms of the forces acting, why the acceleration is not constant.

Suggested Solution

(a)The resultant force on the tanker =2.5 x 105 N-1.2 x 105N =1.3 x 105N

However, the formula of resultant force is:

Resultant Force, F = Mass x acceleration

       hence, the acceleration, a   = 1.3 x 105N/1.5 x 106 kg

    =0.087 m/s2 ( 2 sig. fig.)


(b) The resultant force on the tanker is a result of the force by the propeller acting forward on the tanker (which is given as 2.5 x 105 N and the drag force. This resultant force, F=ma. The acceleration changes as the resultant force changes since the mass remains constant. Since it is given that the force acting forward on the tanker due to the propellers has a constant value of 2.5 x 105 N, then the acceleration changes is solely dependent on the drag force. The drag force is a frictional force that increases as the velocity of the tanker increases. As the drag force increases, the resultant force decreases and the acceleration also decreases. Hence, the acceleration of the tanker is not constant.