Angular acceleration, velocity, momentum of a door?

[StavangerFinest]

On c.) could the time be 2,53s (also got t= -1,26), used abc formula with α=10 and ω=2? Used the inertia formula that StavangerFinest did: I = (ML^2)/3.

I got t= 0,40s...What did you use as θf and θi?

 

[haruspex]

used abc formula with α=10 and ω=2?

what is the abc formula?
The time to close is less than 2 s but more than StavangerFinest's 0.4s.

 

[StavangerFinest]

what is the abc formula?
The time to close is less than 2 s but more than StavangerFinest's 0.4s.

I did it again and i get 0,79s... Assistants told me I shouldn't have used abc formula but a simple t= Δθ/ Δω...
Can you help me with question e) and thereby f) in this exercise though? I got in f) that

d= 2L/3 (two thirds of L)

pretty insecure abt this one

 

[haruspex]

I did it again and i get 0,79s... Assistants told me I shouldn't have used abc formula but a simple t= Δθ/ Δω...
Can you help me with question e) and thereby f) in this exercise though? I got in f) that

d= 2L/3 (two thirds of L)

pretty insecure abt this one

Right on both.
But I do not agree with your formula in post #33. There should not be any $\omega$ reference. Did you change your formula?

 

[leifen]

I am stuck on e). What I have done:
T = d x F x sin(theta)
T = I x alpha

I x alpha = d x F x sin(theta)

F (d) = (I x alpha) / (d x sin (theta))
F (d) = (1/3 m x d^2 x a) / (d^2 * sin (theta))

But then all my d's disapear... Am I on the right track?

 

[statii]

I got t= 0,40s...What did you use as θf and θi?

I used tetha final to be pi/2 and tetha initial to be 0, and abc formula with alpha=0, got 0,79s then :)

 

[StavangerFinest]

Right on both.
But I do not agree with your formula in post #33. There should not be any $\omega$ reference. Did you change your formula?

Just ignore that one. I started over from this formula:

F- Fh= M* a (a centre of mass)

and I worked my way down from there.

 

[StavangerFinest]

I am stuck on e). What I have done:
T = d x F x sin(theta)
T = I x alpha

I x alpha = d x F x sin(theta)

F (d) = (I x alpha) / (d x sin (theta))
F (d) = (1/3 m x d^2 x a) / (d^2 * sin (theta))

But then all my d's dissapear... Am I on the right track?

d shouldn't disappear, no.

 

[leifen]

d shouldn't disappear, no.

Badly formulated by me. I am stuck, because the d's disapear, that's my problem. But am I using the right method and formulas?

 

[coffeemanja]

That's it.

Sorry, have been busy. I still have 40 in a . Alpha =( ΣF*L/2)/ (1/3*M*(L/2)^2) is my formula. Since it is a uniform rod with an axis through the end and the push is in the middle... I saw answer 10 somewhere... So now I'm puzzled

 

[SammyS]

As haruspex asked: "What is the abc formula?"

As for finding the time for the door to close:

After MIL (mother-in-law) releases the door, its angular velocity is constant. The time interval during which she applies force is fixed at 0.2 seconds (we think so anyway - no confirmation from Haveagoodday )​

Finally: It strikes me that this thread is very weird. We have several respondents, mostly new PF, who jump in at various points, mostly to get help, as if they are also tasked with solving this problem.

Do you folks know each other?Also: It would greatly help if posters would use units with their numerical answers, as well as using the "Reply" feature so it's a bit clearer as to who you're answering or asking.

 

[coffeemanja]

As haruspex asked: "What is the abc formula?"

As for finding the time for the door to close:

After MIL (mother-in-law) releases the door, its angular velocity is constant. The time interval during which she applies force is fixed at 0.2 seconds (we think so anyway - no confirmation from Haveagoodday )​

Finally: It strikes me that this thread is very weird. We have several respondents, mostly new PF, who jump in at various points, mostly to get help, as if they are also tasked with solving this problem.

Do you folks know each other?

I bet we are in the same class. Abc formula is what they call formula for solving square equation. X= -b+/- sqrt b^2...

 

[SammyS]

I bet we are in the same class. Abc formula is what they call formula for solving square equation. X= -b+/- sqrt b^2...

The quadratic formula.

By the way:
Is it true that the time of contact with the door is 0.2 seconds? (or as some write: 0,2 seconds)

 

[coffeemanja]

The quadratic formula.

By the way:
Is it true that the time of contact with the door is 0.2 seconds? (or as some write: 0,2 seconds)

Yes, that is in the statement of the problem:)

 

[SammyS]

Yes, that is in the statement of the problem:)

Well, it looks like a typo to me in the statement of the problem, as was the l=2 rather than l/2 .

An angry mother-in-law slams the door shut, by pushing at the middle of

the door (l=2 from the hinges) with a force of F = 100N, lasting a time

t = 0:.00 s.

 

[coffeemanja]

Well, it looks like a typo to me in the statement of the problem, as was the l=2 rather than l/2 .

 

[Haveagoodday]

On c.) could the time be 2,53s (also got t= -1,26), used abc formula with α=10 and ω=2? Used the inertia formula that StavangerFinest did: I = (ML^2)/3.

in c i get 0.395s

Sorry, have been busy. I still have 40 in a . Alpha =( ΣF*L/2)/ (1/3*M*(L/2)^2) is my formula. Since it is a uniform rod with an axis through the end and the push is in the middle... I saw answer 10 somewhere... So now I'm puzzled

you are using the wrong value of l, you have to put in the total width of the door in the equation, it doesn't matter where she pushes the door, it only matters when calculating the torque.

 

[coffeemanja]

in c i get 0.395s

you are using the wrong value of l, you have to put in the total width of the door in the equation, it doesn't matter where she pushes the door, it only matters when calculating the torque.

Yeah, got that already. Thanks anyway!

 

[haruspex]

in c i get 0.395s

It's about double that. Please post your working.

 

[Haveagoodday]

It's about double that. Please post your working.

yeah, i recalculated and got 0.785s.

 

[haruspex]

yeah, i recalculated and got 0.785s.

Excellent.

 

[coffeemanja]

It's about double that. Please post your working.

Can I post mine?
Θf=Θi+ωi*t+0.5αt^2
90=0+2t+0.5*10t...and the result is far of the one you say is correct...

 

[Haveagoodday]

Can I post mine?
Θf=Θi+ωi*t+0.5αt^2
90=0+2t+0.5*10t...and the result is far of the one you say is correct...

you have to use this equation t= θf-θi/wf-wi
and you also have to use radians instead of degrees.

 

[haruspex]

Can I post mine?
Θf=Θi+ωi*t+0.5αt^2
90=0+2t+0.5*10t...and the result is far of the one you say is correct...

What is the are value you putting for alpha here? What units do you have for omega?

 

[haruspex]

you have to use this equation t= θf-θi/wf-wi

w_f-w_i? Did you mean that?

 

[Haveagoodday]

you have to use this equation t= θf-θi/wf-wi

you

w_f-w_i? Did you mean that?

yes

 

[coffeemanja]

What is the are you putting for alpha here? What units do you have for omega?

That one was closest to theta. 90 degrees.
That is the formula for rotational motion...if alpha is a constant

 

[haruspex]

you

yes

I've never seen that equation, and if you were to apply it you would get infinity. w does not change after the force on the door ceases.

 

[haruspex]

That one was closest to theta. 90 degrees.
That is the formula for rotational motion...if alpha is a constant

You did not answer either question. What number are you plugging in for alpha, and what units is your number for omega expressed in?

 

[coffeemanja]

You did not answer either question. What number are you plugging in for alpha, and what units is your number for omega expressed in?

Alpha is 10 rad/s...agh...I see now! Let me redo here...

 

[Haveagoodday]

I've never seen that equation, and if you were to apply it you would get infinity. w does not change after the force on the door ceases.

but i

That one was closest to theta. 90 degrees.
That is the formula for rotational motion...if alpha is a constant

dont forget that you have to use radians instead of degrees. But still it is apparently wrong to use that equation.

 

[haruspex]

but i

Yes?

 

[Haveagoodday]

I've never seen that equation, and if you were to apply it you would get infinity. w does not change after the force on the door ceases.

well it is kind of like this equation v= dx/dt, and if you rearrange it you get t=dx/dv, and same goes for rotational motion equation w=dθ/dt, at least i think so.

 

[coffeemanja]

I got 0.795 s

 

[haruspex]

well it is kind of like this equation v= dx/dt, and if you rearrange it you get t=dx/dv.

No, you get dt=dx/v. It is certainly not the case that t=dx/dv.