Dynamics linear movement

Femme_physics

1. The problem statement, all variables and given/known data




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.

3. 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?

ehild

What is the question?

ehild

Femme_physics

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

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?

:

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

Yuqing

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

ehild

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

I like Serena

*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

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.

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

Thanks!!!

I like Serena

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!


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. )

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?