Question

Please help with the attached question.

Answer 1

the net force will be applied forward, calculate the net force.

Answer 2

I caluclated 200N. But the marking scheme says it's 2000N. >_<

Answer 3

yeah 200N will bet he net force that is pushing the boat forward.
But 2000N will be the total force applied by propeller to the boat.

Answer 4

How come? :O

Answer 5

Quote: "How come? :O"

The problem says "Newton's 3rd law", and that's the only reason.

You do not need to calculate the net force to find the answer.
On the contrary, finding the answer to this question using Newton's 3rd law enables you to work out the net force if you need it later.

Answer 6

Oh. Okay! Thank you @Vincent-Lyon.Fr !