This is a suggested answer to the GCE O Level Pure Physics examination, year 2018, Paper 1, multiple choice question 4. A similar question occurs in year 2015, Paper 1, question 6 (N2015/P1/Q6)

The question stated the propella of a boat pushes the water backwards with a force of 2000N. The boat moves through the water against a resistance of 1800N. The student is required to compute the forward force on the propeller due to the water.

Suggested Solution

Answer is C. That is, 2000N.


According to Newton’s third law, there is an equal and opposite reaction. Hence, if the propeller pushes the water backwards by 2000N, the forward force on the propella due to the water will be 2000N.