How do I calculate the drag force on a motorboat accelerating from a dock?

[Robertoalva]

1. A 940 kg motorboat accelerates away from a dock at 2.2 m/s2. Its propeller provides a thrust force of 3.9 kN. What is the drag force exerted by the water on the boat?
2. I know that the force can be get by F= m a
3. My teacher told us to draw a free body diagram to get all the forces acting on the object, but I'm not so sure how to do the water one.

 

[SteamKing]

You only have two forces to deal with, drag and thrust. There are only so many diagrams which can be drawn. Use the fact that drag acts to oppose motion and thrust acts to cause motion.

 

[Robertoalva]

so, my thrust force would be equal to 3.9kN?

 

[cepheid]

so, my thrust force would be equal to 3.9kN?

Yes of course, it says so in the problem statement that you yourself posted.

Have you drawn the free body diagram yet? As others said above, it's simple. There are only two forces that act. There is nothing special or different about the drag force from the water. You just represent it on the diagram as force that opposes the motion. The thrust is in the direction of motion. Now you can apply Newton's Second Law.

 

[Robertoalva]

do I have to get the Force of the boat? because in my homework the weight is marked in red and also the acceleration.

 

[Bostonpancake0]

It's marked in red because you must calculate the net force acting on the boat (F=ma) which is (940*2.2=2068n)

 

[cepheid]

do I have to get the Force of the boat? because in my homework the weight is marked in red and also the acceleration.

Just worry about the horizontal forces. You have your free body diagram with the two forces, thrust and drag. You know Newton's second law, F_net = ma. What is F_net? I.e. what does it mean and how is it determined from the forces in your free body diagram?

 

[Robertoalva]

thanks! it was 1.832kN

now that I look it, it was really easy. I should study more...

anyways thank you very much!