# Homework Help: Fish Problem

1. Sep 24, 2006

### cd80187

All right, I am doing my physics homework and I just cannot figure this one out. I have been trying for literally over an hour, so I am running out of ideas. Here it is.

The tension at which a fishing line snaps is commonly called the line's “strength.” What minimum strength is needed for a line that is to stop a salmon of weight 76 N in 13 cm if the fish is initially drifting at 3.1 m/s? Assume a constant deceleration.

All right, so clearly this has to do with force, which is going to end up being the mass of the fish times its decleration. But because they are asking for strength, I am guessing it a magnitude and not a vector. So, I have tried things such as using the velocity over the meters taken to stop the fish, to get time and then apply that to get the acceleration, but that was clearly wrong as the time changes due to the deceleration. I have done all kinds of things, I need help to figure this one out. Thank you

2. Sep 24, 2006

### Staff: Mentor

Use your knowledge of kinematics to find the acceleration. (You are given the initial speed and the distance--that's all you need.) Then use Newton's 2nd law to find the force needed to produce that acceleration. (What's the mass of the fish?)

3. Sep 24, 2006

### cd80187

Thank you so much. I hate those problems that are so simple, yet you overlook. I figured out the answer and it was correct. I used the formula 2a(Change in x) = (final velocity) squared minus (initial velocity) squred. but once again, thank you

4. Oct 20, 2006

### mike_24

another question about the fish problem

I was wondering if someone could explain the fish problem in a bit more detail, I can't seem to get it...

5. Oct 20, 2006

### Hootenanny

Staff Emeritus
Welcome to the Forums Mike,

How would you approach the problem yourself? How much do you know about kinematics?

6. Oct 20, 2006

### mike_24

As far as approaching the problem, im not sure, I was doing a few things with F=ma and replaceing the F with W, but i dont seem to be making a whole lot of progress.

7. Oct 20, 2006

### Hootenanny

Staff Emeritus
Okay, do you know any kinematic equations?

8. Oct 20, 2006

### mike_24

Umm what would be an example of some?

9. Oct 20, 2006

### Hootenanny

Staff Emeritus
Last edited by a moderator: Apr 22, 2017
10. Oct 20, 2006

### mike_24

Yes I do know some kinematics

11. Oct 20, 2006

### mike_24

I am familiar with all of those yes

12. Oct 20, 2006

### Hootenanny

Staff Emeritus
Make two list, one with variables you know. The other with variable(s) you wish to find. Which formulae are applicable in your case?

13. Oct 20, 2006

### mike_24

I don't see how the initial speed and the distance can get you the acceleration though.

14. Oct 20, 2006

### Hootenanny

Staff Emeritus
You also know the final velocity.

15. Oct 20, 2006

### mike_24

For this problem the displacement and velocity equation would be needed i think, and Newtons 2nd law F=ma and maybe W=mg

16. Oct 20, 2006

### mike_24

Ohhh, it would be zero

17. Oct 20, 2006

### Hootenanny

Staff Emeritus
Spot on.
Good.
Sounds good.
You may want to rethink this. What can you say about the net vertical force is a fish is floating (or swimming) at the same depth?

18. Oct 20, 2006

### mike_24

it would zero if the fish was at the same depth?

19. Oct 20, 2006

### Hootenanny

Staff Emeritus
Correct, so you don't need to worry about the weight of the fish. However, you do need the weight to calculate the ....

20. Oct 20, 2006

### mike_24

the mass of the fish?

21. Oct 20, 2006

### Hootenanny

Staff Emeritus
Correct. Can you have a go at the question from here?

22. Oct 20, 2006

### mike_24

Yes, i hope so, but one other question, I rearranged the velocity and displacement equation to solve for acceleration and have calculated the mass of the fish, but i get a really large acceleration, is it because i converted the 13cm to m?, isn't that was is always done?, or am I going wrong somewheres else?

23. Oct 20, 2006

### Hootenanny

Staff Emeritus
So, once you re-arrange your equation you should obtain;

$$a = \frac{v_{f}^2}{2\Delta x}$$

Yes, you should always convert your units to S.I. units in this type of problem.

24. Oct 20, 2006

### mike_24

Doing that gives you an acceleration of 36.96m/s^2, just seems like it should be more like 3

25. Oct 20, 2006

### mike_24

Would 36.96 be the correct acceleration?