Angular acceleration, velocity, momentum of a door?

[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.

 

[haruspex]

I got 0.795 s

Good, if a little inaccurate. (But why does everyone insist on decimals? What's wrong with $\pi/4$?

 

[coffeemanja]

Good, if a little inaccurate. (But why does everyone insist on decimals? What's wrong with $\pi/4$?

Wait... It's pi/2. I do not know, may be because we are used to use SI units..

 

[Haveagoodday]

Wait... It's pi/2. I do not know, may be because we are used to use SI units..

did you use this equation Θf=Θi+ωi*t+0.5αt^2 ?

 

[coffeemanja]

did you use this equation Θf=Θi+ωi*t+0.5αt^2 ?

Yes!

 

[Haveagoodday]

Yes!

what are the values you put in?

 

[Haveagoodday]

Have somebody come to a solution for e)?

 

[haruspex]

Have somebody come to a solution for e)?

I assume you are just looking for a yes/no answer. There's a risk your question might be interpreted as a request for a solution to be posted.

 

[SammyS]

Have somebody come to a solution for e)?

I think somebody posted an answer for part (e).

I don't know if that counts as a solution.

 

[StavangerFinest]

Have somebody come to a solution for e)?

Here's a starting point:

F- Fh = M* a ("h" being in a subscript of F, and "a" being "a" of centre of mass)

I could post whole my work, but I believe It's better for you that you figure it out yourself. But this starting point should be more than enough.
Let me know the result you get in e) and thereby f)

 

[jensjensen]

e) Fh=F(1-3d/(2L))
f) 2L/3

 

[jimjames]

How did you get C) 0.795 s or pi/4

 

[jensjensen]

http://imgur.com/1c9yGRy
where d=0.5

 

[haruspex]

How did you get C) 0.795 s or pi/4

As I said, 0.795 is rather inaccurate.

 

[StavangerFinest]

As I said, 0.795 is rather inaccurate.

It is, but where we study they insist on decimal numbers. It is inaccurate, but the main reason why they do it is so we can use our knowledge about significant digits. You'd be surprised over how many people make silly mistakes on significant digits.

 

[haruspex]

It is, but where we study they insist on decimal numbers. It is inaccurate, but the main reason why they do it is so we can use our knowledge about significant digits. You'd be surprised over how many people make silly mistakes on significant digits.

No, it's more inaccurate than it should be for a three digit decimal. Calculate pi/4.

 

[StavangerFinest]

No, it's more inaccurate than it should be for a three digit decimal. Calculate pi/4.

Oh yeah... 0,785s...Didn't notice that they had nine instead...Sorry, my bad

 

[Bambisu]

Read the book from page 308 to 314, you will understand the physics not just for one question but for all related questions.

 

[haruspex]

Read the book from page 308 to 314, you will understand the physics not just for one question but for all related questions.

The thread is two years old. I doubt StavangerFinest is still interested.

 

[ErCeTe]

Heia Anders