What is the net force on the fish in this fishing scenario?

[reliquator]

I'm on the 3rd unit of my physics unit and we're applying Newton's 3 Laws of motion.

There's this problem:

"A fisherman yanks a fish out of the water with an acceleration of 4.5 m/s^2 using very light fishing line that has a "test" value of 100 N. The fisherman unfortunately loses the fish as the line snaps. What can you say about the mass of the fish?"

Okay, I thought that using the F = MA formula would work, with 100=4.5 * M, which means that the mass of the fish was greater than 22 and 2/9 KG.

WRONG. The answer guide I'm working from says the mass of the fish > 7.0 kg. Did I use the wrong formula?

Thanks ahead of time guys.

 

[Doc Al]

Of course F = ma works. But you've got to consider all the forces acting on the fish, not just the tension in the line. (You forgot gravity.)

 

[reliquator]

Of course F = ma works. But you've got to consider all the forces acting on the fish, not just the tension in the line. (You forgot gravity.)

Okay, I've got it. 100 = (9.8 + 4.5) * M

M = 6.99 Kg, so the fish has to be heavier than 6.99 Kg. But what I don't get is, why would you add 4.5 (acceleration of the line) with accel due to gravity? Thanks to all in advance.

Edit: Question #2

"A 0.145-kg baseball traveling 30.0 m/s strikes the catcher's mitt, which, in bringing the ball to rest, recoils backward 11.0 cm. What was the average force applied by the ball on the glove"

Okay, I don't need an answer to this problem but would like to know how to approach it. I'm inferring that you need to find the FORCE applied to the baseball. Its given a mass but no acceleration. Would acceleration be 0, or 9.8?

Edit2: Okay, so I think I figured it out. I have the displacement, vi, vf, and need a. so vf^2 = vi^2 + 2ad, and I get a = -4090.91 m/s^2.

Aight, so I do F= MA, and get force = 593.18 N, am I correct, and if so, should the 593.18 N be negative? Thanks again all.

 

[Doc Al]

Okay, I've got it. 100 = (9.8 + 4.5) * M

M = 6.99 Kg, so the fish has to be heavier than 6.99 Kg. But what I don't get is, why would you add 4.5 (acceleration of the line) with accel due to gravity?

To apply Newton's 2nd law, you must use the net force on the fish. The net force is Tension (up) - mg (the weight, down) = T - mg. Set that equal to ma: T - mg = ma.

Edit: Question #2

...

Aight, so I do F= MA, and get force = 593.18 N, am I correct, and if so, should the 593.18 N be negative?

Looks OK to me. I'm sure the magnitude of the force is all they are looking for. (The sign is arbitrary--it depends on your coordinate system.)