Static equilibrium, forces and toques

[Jose18]

Homework Statement

Hi, I have 30minutes to resolve question b. I have tried all night to resolve it but I'm not so good in physics. Any help is truly appreciated.

Relevant Equations

I know that the sum of rotational forces and Fnet y should be zero but when it comes to isolated F2 the result doesn't make any sense.

 

[BvU]

Hello @Jose18 , !

I have 30minutes to resolve question b. I have tried all night to resolve it

Sounds a bit strange: all night + 30 minutes ?

the result doesn't make any sense

Says who ? Post your work, in accordance with https://www.physicsforums.com/threads/homework-help-guidelines-for-students-and-helpers.686781/

 

[Jose18]

It is a homework due in 30 minutes. I apologize for my impertinence but I've been working all night literally, 10 hours nonstop and I've been watching videos in youtube but there is not a single similar example as this one. I just discovered this site so I'm a bit of a hurry. The homework is really important. I tried to isolated F2 but I think I'm missing something. My equation was:

F2=(F1*sin60*r1 - F3*r3 - F4*r4)/r2*sin45

 

[haruspex]

It is a homework due in 30 minutes. I apologize for my impertinence but I've been working all night literally, 10 hours nonstop and I've been watching videos in youtube but there is not a single similar example as this one. I just discovered this site so I'm a bit of a hurry. The homework is really important. I tried to isolated F2 but I think I'm missing something. My equation was:

F2=(F1*sin60*r1 - F3*r3 - F4*r4)/r2*sin45

Pity you didn’t try this site sooner.

Hard to say whether your equation is right because you haven't defined r1 etc. or stated the axis you are using, or explained how you got it.

Unfortunately, you have chosen the hardest of the three ways you could have solved it. Since all forces and distances are known except F2, looking at the sum of vertical forces or the sum of horizontal forces would also work, and that last is particularly easy here.

 

[Cutter Ketch]

Things to think about:
A) This is not a difficult problem. We can and will help if you are still interested.
B) We “help” which means we don’t do it for you which means there always has to be a conversation where we figure out what you are doing and guide you. For example, what is your reasoning for writing that equation? From where are you measuring those r’s? This doesn’t happen in one post. You have to be willing to have some back and forth.
C) Teachers do not assign problems that they expect to take you 10 hours. (well not until graduate school any way). If you are struggling, if you don’t know what you are doing, if you’ve made an honest effort to understand and it still seems like Greek, get help. Get help early. Get help as soon as you begin to feel like you don’t get it. Get good help. Think of somebody you are sure understands and call them. If you are struggling with a problem that clearly is intended to take 5 minutes, do not sit for 10 hours Googling stuff that clearly isn’t helping. GET HELP! (much much sooner)

Now, if you would still like help, tell us what you tried and we really will help. What was your reasoning in writing that equation, and from where are you measuring those r’s. What values did you use? What did you get for a result? Given the nature of the problem chances are that if it isn’t making sense you have probably just made a sign error or a calculator error.

 

[Jose18]

Things to think about:
A) This is not a difficult problem. We can and will help if you are still interested.
B) We “help” which means we don’t do it for you which means there always has to be a conversation where we figure out what you are doing and guide you. For example, what is your reasoning for writing that equation? From where are you measuring those r’s? This doesn’t happen in one post. You have to be willing to have some back and forth.
C) Teachers do not assign problems that they expect to take you 10 hours. (well not until graduate school any way). If you are struggling, if you don’t know what you are doing, if you’ve made an honest effort to understand and it still seems like Greek, get help. Get help early. Get help as soon as you begin to feel like you don’t get it. Get good help. Think of somebody you are sure understands and call them. If you are struggling with a problem that clearly is intended to take 5 minutes, do not sit for 10 hours Googling stuff that clearly isn’t helping. GET HELP! (much much sooner)

Now, if you would still like help, tell us what you tried and we really will help. What was your reasoning in writing that equation, and from where are you measuring those r’s. What values did you use? What did you get for a result? Given the nature of the problem chances are that if it isn’t making sense you have probably just made a sign error or a calculator error.

Hi,
first of all, thank your for your answer. I'm still interested in resolving this problem. I got some help from a classmate and haruspex. So I think I finally resolved question b).

This is the information I have for the problem.
Below are the questions and my solutions.
I would like to know if what I did is correct.

a. (2) Do both the forces and torques sum to zero when in static equilibrium?
a. No b. Yes
c. Depends on the chosen pivot point

Answer = b = yes

b. (7) What is the value of F2?
F2y = 4.8 + 1 + 2.4 – F1y

= 8.2 – sin60 * 6

= 3.004 N

F2 = F2y/sin45 = 4.24 N


c. (2) What is the net torque about point of 80 cm?Tnet = -(r1*F1*sin60) - r2*F2*sin45 + F3*r3 + F4*r4

= -(0.6*6*sin60) - (0.1*4.24*sin45) + (4.8*0.55 + 2.4*0.2)

= -0.3 Nd. 3) What is the torque exerted by the 6 N force about the point of 10 cm?

T= F1*sin60 * 0.1m = 0.52 Ne. (3) What is the torque exerted by the 2.4 N force about the center of mass?

T = -F4*r4 = 0.24 Nf. (3) Write an expression, for ONLY the counterclockwise torques, τccw for the above system for a chosen pivot point of 0 cm.

T = F* (x-0)

Which x = position where the force is applied and (x-0) = r

 

[Jose18]

Pity you didn’t try this site sooner.

Hard to say whether your equation is right because you haven't defined r1 etc. or stated the axis you are using, or explained how you got it.

Unfortunately, you have chosen the hardest of the three ways you could have solved it. Since all forces and distances are known except F2, looking at the sum of vertical forces or the sum of horizontal forces would also work, and that last is particularly easy here.

Thank you for your help. I think I got the right answer. And you were right the sum of horizontal forces is the easiest way and it saved me 1 step.

 

[haruspex]

In c) you left out the weight of the meter stick.
Note that you are told the system is in equilibrium, so you did not need to calculate anything.

I don't think your answer for f is what is wanted. The expression should include the actual forces and distances.

 

[Jose18]

For c), let's say I want to prove it is in equilibrium. How do I calculate the torque of the meter stick weight.

 

[haruspex]

For c), let's say I want to prove it is in equilibrium. How do I calculate the torque of the meter stick weight.

Why is that hard? You know the weight, where it acts, and how far that is from the given axis.