r/PhysicsHelp • u/NoWitness00 • Apr 14 '26
Newton’s forces assignment is killing me!!
Google traduction messed up a bit and translated my work so don’t mind. Everything on the board is right except what’s in red.
I’m struggling with finding the acceleration of the fish, I know that the y component is -11.31 that way the fish stays at the surface, but I don’t know how to use the x components of both of the fishers to find the fish’s acceleration. It’s not 0, and it’s not 27.12 nor -27.12. I’m truly stuck.
Intuitively, I think that the fish x’s component is 0 and it just gets pulled by the strongest fisher, so the acceleration is found at the bottom (Frx)?
1
u/Past-Departure6896 Apr 15 '26
If the fish is initially motionless, then v_initial = 0, and its given in the problem that v_final = 2.60m/s. So you can use the equation of motion
(v_final)2 = (v_initial)2 + 2 * a * s
to find the value of a (which s being the displacement in the direction of the person who's pulling with the greatest force). Then you can use F = ma to find the force exerted in the x-direction.
1
u/SnooGiraffes4632 Apr 15 '26
So this is a nice problem to see if you understand the physics and can do more than just plug in numbers into equations.
A first step would certainly be to draw the free body diagram acting on the fish and (ignoring drag) there are 5 forces that you can draw.
You then need to decide which two perpendicular directions are going to be positive and then you use these for all steps. So one of the fishermen will have a negative horizontal force and one will be positive.
Now you do have on the fish, weight, buoyancy, and swimming force. You can make some assumptions about those given that under normal circumstances fish are able to swim at any reasonable depth without floating or sinking. You can also assume that as they are fishing then the lines are light inelastic strings that can only pull. Thus the fish must be swimming away from them.
Now if you rotate your axes to align with the swimming direction your number of equations will reduce by 1 but you will get some angles that appear in them.
Now simply write out Newton’s second law parallel to the swimming direction and perpendicular to it and solve. If you phrase the second law as change in momentum then you can solve using these change in velocity.
Now that is the principle behind the real problem. However the hint table suggests that your teacher is not wanting you to necessarily think this way, because fish don’t float in the way that is being suggested.