Calculating Force and Motion on an Inclined Plane

[Femme_physics]

Homework Statement

http://img20.imageshack.us/img20/4519/objectssss.jpg

A body whose weight is 12 [kN] progresses upwards on an inclined plane, slanted to the horizontal line at angle alpha. Force F, acting on the body, parallel to the inclined plane, remains constant during the entire action. During the body progression at segment AB (whose length is L) its velocity decreases from Vo to V. The kinetic friction between the object and the surface is u.

A) Calculate force F
B) If force F won't act on the object, will the object stay at rest or will it slide down the inclined plane? Presume that the static and kinematic friction coeffecients are equal.

The Attempt at a Solution

Did I get it right so far? I'm not sure where do I input velocity. I do know there is no movement on the y-axis (with the coordinates I've chosen) but only on the x axis. So it made sense to me that sigma Fy = 0 and sigma Fx = ma.

What do you say?

http://img864.imageshack.us/img864/145/90689814.jpg http://img853.imageshack.us/img853/989/66580583.jpg

 

[ehild]

What is the question?

ehild

 

[Femme_physics]

What is the question?

ehild

A) Calculate how stupid is Femme_Physics for not including the question?
B) Calculate how hard Femme_Physics slapped her forehead for that?...


...


The questions really are:

A) Calculate force F
B) If force F won't act on the object, will the object stay at rest or will it slide down the inclined plane? Presume that the static and kinematic friction coeffecients are equal.

 

[ehild]

A) Calculate how stupid is Femme_Physics for not including the question?
B) Calculate how hard Femme_Physics slapped her forehead for that?...

Oh don't do it! It will damage your brain and it would be catastrophic ...

ehild

 

[ehild]

I do not understand the last two lines on your second sheet.

You can calculate the acceleration "a" from L, vo and v, don't you?

ehild

 

[Femme_physics]

Hmm. So "a" is equal delta v divided by L?

I just wrote the sum of all forces = ma.

Is my sum of all forces correct, at least?

Oh don't do it! It will damage your brain and it would be catastrophic ...

::shy:

I'm applying just enough Newtons to jolt not to permanently damage

 

[Yuqing]

Given what you have, consider using
$\Delta d = \frac{v_{2}^{2}-v_{1}^{2}}{2a}$
to find the acceleration. Your force sum looks fine, and that should give the unknown force.

 

[ehild]

Hmm. So "a" is equal delta v divided by L?

Have you slapped your head already? What is the dimension of acceleration, is it the same as that of (delta v)/L?
You have a simple one dimensional motion with constant acceleration.

ehild

 

[Femme_physics]

So I used the wrong formula. Huh.

Alright, well how about this for F? Looks good?

*crosses fingers* please say it's all correct please say it's all correct

http://img231.imageshack.us/img231/7210/acccf.jpg

http://img560.imageshack.us/img560/9983/accc2.jpg

 

[I like Serena]

So I used the wrong formula. Huh.

Alright, well how about this for F? Looks good?

*crosses fingers* please say it's all correct please say it's all correct

*Looking at your fingers*
Yes, your fingers look nice!

And yes, your a and F are correct too!

Btw, I would write L instead of Δd.
And I would write v0 and v instead of v1 and v2.
And actually, I find F = 7133.6 [N], that is, without the rounding errors you made.

 

[ehild]

Good job! F is correct for three significant digits, (7.13 kN) and it is all right.
What about question B?

ehild

 

[Femme_physics]

*Looking at your fingers*
Yes, your fingers look nice!

And yes, your a and F are correct too!

Btw, I would write L instead of Δd.
And I would write v0 and v instead of v1 and v2.
And actually, I find F = 7133.6 [N], that is, without the rounding errors you made.

I beg your pardon good sir! I made no such "rounding errors". I may have overrounded, but I did not erroneously round. To do such would mean that I rounded 1.7 to 1. That is a rounding error. Overrounding is to round 1.7 to 2 instead of leaving it 1.7.

What about question B?

Question B seems like a basic statics question. I'll tackle that knuckle tomorrow^^

Thanks!

 

[I like Serena]

I beg your pardon good sir! I made no such "rounding errors". I may have overrounded, but I did not erroneously round. To do such would mean that I rounded 1.7 to 1. That is a rounding error. Overrounding is to round 1.7 to 2 instead of leaving it 1.7.

I pardon you good madam! I may have been overzealous with rounding. (Forgive me for being a nitpicker. ;))
The proper answer is actually F = 7 kN.
Yes, that is only 1 significant digit! Because really, that is al you have!

Question B seems like a basic statics question. I'll tackle that knuckle tomorrow^^

Thanks!

See you tomorrow then? Because then I will have a bit more time than today and I would like that!

 

[ehild]

FP, check the text of the problem. "During the body progression at segment AB (whose length is L) its velocity decreases from Vo to V." Vo=6 m/s, V=3 m/s. Is the acceleration positive then?
All of us overlooked this mistake.ehild

 

[I like Serena]

(Just woke up. :zzz:)

FP, check the text of the problem. "During the body progression at segment AB (whose length is L) its velocity decreases from Vo to V." Vo=6 m/s, V=3 m/s. Is the acceleration positive then?
All of us overlooked this mistake.

ehild

Yes, you're right.

 

[Femme_physics]

Ahh...so I just switch the 3 and the 6. Let me get to it :) And then question B looks like statics. I'll be cracking it now! Thanks! :)

 

[Femme_physics]

I'm looking for the acceleration formula we were using.. this is it right?

 

[I like Serena]

I'm looking for the acceleration formula we were using.. this is it right?

Yes. :)

 

[Femme_physics]

I get the same result for a just in minus. I see why now, since the object is decelerating, it makes sense.

Statics was a cakewalk ;)

http://img163.imageshack.us/img163/425/yayv.jpg

Thanks, you two! XoX

 

[I like Serena]

I get the same result for a just in minus. I see why now, since the object is decelerating, it makes sense.

Sorry, there is still more to it, since the result for F will be different now...

Statics was a cakewalk ;)

Thanks, you two! XoX

Very good!

 

[Femme_physics]

I know there's more to do but I solved it using a pencil and I'd really hate to start recopying it with black marker. I just changed 0.45 to -0.45, used that as acceleration and got my answer :)

Here...see if you can see anything

http://img815.imageshack.us/img815/3388/ifucansee.jpg

 

[I like Serena]

I know there's more to do but I solved it using a pencil and I'd really hate to start recopying it with black marker. I just changed 0.45 to -0.45, used that as acceleration and got my answer :)

Here...see if you can see anything

*looking*
*looking*
*looking*
*looking*
*looking*
*looking*
*looking*
*looking*
*looking*

Yep! I have the exact same answer!

Aw, this was over all too quickly! Do you have another cookie?

 

[Femme_physics]

*looking*
*looking*
*looking*
*looking*
*looking*
*looking*
*looking*
*looking*
*looking*

Yep! I have the exact same answer!

Aw, this was over all too quickly! Do you have another cookie?

w00t ;)
*chuckles* Yes am posting it right now, :) I just didn't want two dynamics problems to clash! That'd create a hole in the space time continuum, and all.

 

[Femme_physics]

Actually, as I'm writing the problem, I'm coming up with ideas so that's why I still haven't posted a new problem :) It's amazing what just typing the problem or reading it out loud can do.

 

[Quinzio]

Actually, as I'm writing the problem, I'm coming up with ideas so that's why I still haven't posted a new problem :) It's amazing what just typing the problem or reading it out loud can do.

Indeed. That's why usually people work in groups :)

 

[Femme_physics]

An exact similar problem just with different figures, but the same method as before doesn't seem to work

http://img846.imageshack.us/img846/6564/samemethod.jpg

Solution attempt

http://img862.imageshack.us/img862/7376/attempt1.jpg

http://img135.imageshack.us/img135/4429/attempt2.jpg

 

[I like Serena]

Good morning Fp!

Calculate that last expression again?

 

[Femme_physics]

Good morning, ILS!

Am getting same result

 

[ehild]

Why do you multiply the forces?

ehild

 

[Femme_physics]

It's a plus sign!