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

[joseernesto]

Homework Statement

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).

The Attempt at a Solution

I tried to use the equation D= (Vi+Vf)*t/2 but I am missing the distance

Homework Statement

Homework Equations

D=(Vi+Vf)T/2

The Attempt at a Solution

I tried using the equation D=(Vi+Vf)T/2 but i don't have the dictance(D)

 

[Doc Al]

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

 

[joseernesto]

i don't see how that is going to give me the answer for Vf

 

[Doc Al]

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

 

[joseernesto]

can you give the answer i really need this I am desperate!

 

[joseernesto]

I understand that E=1/2 mv^2 +mgh, but i don't see how that gives you the velocity at half of the high when they don't mention mechanical energy

 

[Doc Al]

I understand that E=1/2 mv^2 +mgh,

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.

but i don't see how that gives you the velocity at half of the high when they don't mention mechanical energy

They expect you to know it without their mentioning it!

 

[joseernesto]

man, i don't see where this **** is going, I don't understand how KE will help me find Vf if there arent values for the variables

 

[Doc Al]

I don't understand how KE will help me find Vf if there arent values for the variables

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.

 

[joseernesto]

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

 

[joseernesto]

hey i couldn't 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

 

[Doc Al]

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