Calculating Acceleration on a Slide with Kinetic Friction

[hakojackie]

At a playground, a child slides down a slide that makes a 42° angle with the horizontal direction. The coefficient of kinetic friction for the child sliding on the slide is 0.20. What is the magnitude of her acceleration during her sliding? Ok so I tried to use the equation ax=-Uk*g to find the acceleration and that did not work any pointers on how to start this I drew a picture but that didnt help? I think the problem is missing some information.

 

[Cyrus]

Step 1, draw a picture and include all the forces.

Step 2, write a force balance with all the equations you think you might need

step 3, solve the equations

step 4, see if the answer is reasonable.

 

[hakojackie]

I think I understand what you mean by a force balance. (find each force and break it into its components) but with the only numbers being 42 degrees and .2 i am not sure how to find the components. I would use sin and cos but I don't know what to use as the magnitudes.

 

[Cyrus]

I think you need to draw a picture still.

So you don't have to wait to get approval, upload the picture to something like www.imageshack.cc[/URL] and then post the link.

 

[hakojackie]

I do have a picture I am using the ground as my x-axis the normal force is perpindicular to the slide the wieght of the child is perpendicular to the ground and the friction is the opposite direction in which the child is sliding down the slide. I don't know how to make a picture like that

 

[Cyrus]

Ok, that sounds about right.

Now did you label the forces?

(As for making the picture, I was talking about something like MS-paint, but don't worry about it)

 

[hakojackie]

yes I labeled them with the name of the force but I have no values.

 

[Cyrus]

So you should have labeled:

-W
-N
-[itex]F_f[/tex]
-[itex] \theta [/tex]

Right?

 

[hakojackie]

Yes that is what I have

 

[Cyrus]

Ok, now what is the angle between W and the slide?

 

[hakojackie]

42 degrees?

 

[Cyrus]

http://img96.imageshack.us/my.php?image=angih1.png

Try again, does this help?

What is the value of angles b and c? I asked for the angle b in my first question.

 

[hakojackie]

48 is what is left from the right triangle. My teacher did an example similar to this in class and she said the angles were the same.

 

[Cyrus]

Maybe you misheard your teacher, but angle b is 48, and c is 42.

Angle 42 (c) is the one that is the same as the ramp, not angle 48 (b).

You need to learn how to come up with these angles. Dont rely on your teacher telling you what the angles are.

 

[hakojackie]

ok I know how to do that. What i need help with is how to do the problem I need a mass or some other number to put into my table so I can use sin and cos.

 

[Cyrus]

Don't fret, we are getting there.

I want to make sure you know this stuff becuase it is critical.

Now, we need to find out the normal force. Any Ideas?

 

[Cyrus]

Hint: Review what sin and cos mean in terms of a triangle.

 

[hakojackie]

nevermind I give up my homework is due.

 

[Cyrus]

What!? Why did you wait until the last minute to do it?