# Finding Vf when you dont have distance and only height without numerical value

1. Aug 1, 2010

### joseernesto

1. The problem statement, all variables and given/known data
13. A box of mass m is released from rest at point A, the top of a long, frictionless slide. Point A is at height H above the level of points B and C. Although the slide is frictionless, the horizontal surface from point B to point C is not. The coefficient of kinetic friction between the box and this surface is µk, and the horizontal distance between points B and C is x.
a. Find the speed of the box when its height above point B is ½H.
b. Find the speed of the box when it reaches point B.
c. Determine the value of µk so that the box comes to rest at point C.
d. Now assume that points B and C were not on the same horizontal level. In particular, assume that the surface from B to C had a uniform and upward slope so that point C were still at a horizontal distance of x from B but now at a vertical height y above B. Answer the question posed in part (c).

3. The attempt at a solution I tried to use the equation D= (Vi+Vf)*t/2 but im missing the distance
1. The problem statement, all variables and given/known data

2. Relevant equations
D=(Vi+Vf)T/2

3. The attempt at a solution I tried using the equation D=(Vi+Vf)T/2 but i dont have the dictance(D)

2. Aug 1, 2010

### Staff: Mentor

Hint: Use energy conservation. Express your answer in terms of the given quantities, like H.

3. Aug 1, 2010

### joseernesto

i dont see how that is going to give me the answer for Vf

4. Aug 1, 2010

### Staff: Mentor

Write expressions for the total mechanical energy at the initial and final points. Compare.

5. Aug 1, 2010

### joseernesto

can you give the answer i really need this im desperate!!!!

6. Aug 1, 2010

### joseernesto

I understand that E=1/2 mv^2 +mgh, but i dont see how that gives you the velocity at half of the high when they dont mention mechanical energy

7. Aug 1, 2010

### Staff: Mentor

That's all you need.

What's the initial KE? The initial PE?

The final KE? The final PE?

Set up an equation and solve for the final KE and then the final velocity.

They expect you to know it without their mentioning it!

8. Aug 1, 2010

### joseernesto

man, i dont see where this **** is going, I dont understand how KE will help me find Vf if there arent values for the variables

9. Aug 1, 2010

### Staff: Mentor

You are not going to get a numerical answer; your answer will be in terms of H. You'll solve for the answer by setting up an equation and doing a bit of algebra.

Stop avoiding the problem and TRY IT! Answer my questions.

10. Aug 1, 2010

### joseernesto

bad bro... jaja ill give it a shot and ill probably ask for more help later...ajaja

11. Aug 1, 2010

### joseernesto

hey i couldnt figure out the problem can u please help me a little bit more, this is the only question that i have left and i want to finish

12. Aug 1, 2010

### Staff: Mentor

Answer the questions I asked in post #7. Step by step.