# Spring and block question

1. Feb 22, 2006

### 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)

thanks!

2. Feb 22, 2006

### assyrian_77

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

3. Feb 22, 2006

### 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

4. Feb 22, 2006

### Hootenanny

Staff Emeritus
(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 travelling 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.

5. Feb 22, 2006

### 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.

6. Feb 22, 2006

### Hootenanny

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

7. Feb 22, 2006

### johnny872005

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

8. Feb 22, 2006

### Hootenanny

Staff Emeritus
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.

9. Feb 22, 2006

### Hootenanny

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

10. Feb 22, 2006

### assyrian_77

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

11. Feb 22, 2006

### 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.

12. Feb 22, 2006

### 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.

Last edited: Feb 22, 2006
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