Can You Solve These Spring and Block Problems?

[johnny872005]

ok guys, I'm a bit stuck on this one. Please look at the images below (this is for problems 11 12 and 13 (they go together)

http://img70.imageshack.us/img70/9058/a17yi.jpg
http://img70.imageshack.us/img70/2062/a20mf.jpg

thanks!

 

[assyrian_77]

What have you done so far? Where exactly are you stuck? You need to show some of your work on the problem.

 

[johnny872005]

well for that one I didnt really know where to start. I've done all the other problems before it mostly.
I would thing for that one, i'd need to use the x distance and velocity formulas, same as the ones that are used in projectile motion

 

[Hootenanny]

(11) Initially you will have an acceleration of $a = \frac{F}{m}$ where the force is from the compressed spring (can be calc. from the spring constant). You can then use kinematic equations to calc. the velocity just before it hits the rough surface. When traveling over the rough surface you will than have a force of $F = \mu R$, which can be converted into an acceleration using $a = \frac{F}{m}$. Then you use kinematic equations again to find its final velocity on the track. Post your working and we'll help.

 

[johnny872005]

i'm still confused on how to calculate the velocity of the blcok after the spring pushes it off.
Also can you please explain how to calculate speeds involivng friction? Truthfully, I didnt quite get it in cdlass, and I wasnt paying attention 100%, i wasnt focused that day.

 

[Hootenanny]

What force will be acting on the block after the spring is released?

 

[johnny872005]

are u talking about 13kN/m?
What I have shown you is all that the question gave me

 

[Hootenanny]

You must know some formulae/ basic principles. We are here to help you, not show you. You need to start working through it on your own.

 

[Hootenanny]

I'll start you off, the first equation you need is $F = -kx$.

 

[assyrian_77]

are u talking about 13kN/m?
What I have shown you is all that the question gave me

13 kN/m is not a force! The unit of force is N (Newton).

 

[assyrian_77]

Hootenannys original post gave really good clues. Divide the problem in three sections. First get the accelaration and final velocity for section one (just before the rough surface). Second, use that to get the acceleration (deceleration actually) and final velocity for section two (the rough surface). Third, use that to get the final velocity for section three (after the rough surface, right before leaving the track.

 

[Dmitri]

11. Elastic energy, when the spring is compresed, should be equal to kinetic energy, when the block is moving
with constant velocity before it hits the surface with friction. To find final velocity subtract energy lost due to friction from kinetik energy before the
rough surface.
12, 13. Are just projectile problems.