Considerations for the FBDs of a parachutist traveling at terminal velocity

[greg_rack]

Homework Statement

I'll attach the statement below since it consists of a free-body diagram

Relevant Equations

$F=ma$
$W=mg$

There are a few things I'm not getting about this exercise and related diagram:
-what does "force from parachutist" consists of?
-if the terminal velocity is reached, then $a=0$ which means $F_{tot}=0$, so shouldn't simply the sum of all forces pointing upwards be equal to the sum of all pointing downwards?

 

[berkeman]

Homework Statement:: I'll attach the statement below since it consists of a free-body diagram
Relevant Equations:: $F=ma$
$W=mg$

View attachment 271329
There are a few things I'm not getting about this exercise and related diagram:
-what does "force from parachutist" consists of?
-if the terminal velocity is reached, then $a=0$ which means $F_{tot}=0$, so shouldn't simply the sum of all forces pointing upwards be equal to the sum of all pointing downwards?

The force from parachutist is just their weight and the weight of all of their gear.

And you are correct, at terminal velocity, there is no acceleration of the parachutist and parachute -- they both fall at the constant terminal velocity. So the sum of all forces is zero in that state.

 

[haruspex]

The force from parachutist is just their weight and the weight of all of their gear.

No, it is the force the parachutist and clothing etc. exerts on the parachute.

shouldn't simply the sum of all forces pointing upwards be equal to the sum of all pointing downwards?

Yes... are you saying that, according to the exercise, it is not?
But what are you selecting as the answer to the question?

 

[kuruman]

-what does "force from parachutist" consists of?

Call it the total tension in the ropes that tether the parachutist to the parachute. Note that the upper part of the ropes is in the FBD of the parachute and the lower part of the ropes is in the FBD of the parachutist. Does that help?

 

[greg_rack]

Does that help?

That definitely helped, now I’m understanding the situation much better!
But, shouldn’t either $P=N$ and $M+R=L+Q$?

 

[LCSphysicist]

That definitely helped, now I’m understanding the situation much better!
But, shouldn’t either $P=N$ and $M+R=L+Q$?

$M+R=L+Q$ is " more "second law than third law too me, even if it is true, the question want 3rd Law's implications

 

[greg_rack]

$M+R=L+Q$ is " more "second law than third law too me, even if it is true, the question want 3rd Law's implications

God, I’ve totally missed it... what a fool!
I was thinking in terms of the second law :)
Well, the equation is $P=N$ then, right?

 

[haruspex]

God, I’ve totally missed it... what a fool!
I was thinking in terms of the second law :)
Well, the equation is $P=N$ then, right?

Yes.