Spring-Type problem (Work done by gravity)

[charmy]

Homework Statement

^ Please ignore my writing. The correct variables:
- Xi (initial position) = 10cm (.01m)
- Mass = 1kg
- Uk (kinetic coefficient) = 0.200
- K (Stiffness constant) = 1000 N/m

Question: What is the value of the work done by gravity?

Homework Equations

Force of gravity= Mass x acceleration
Work by gravity = mgh

The Attempt at a Solution

Force by gravity = 1kg x 9.8m/s2 = 9.8N
Work by gravity = 1kg x 9.8m/s2 x h

I'm having problem finding the last variable, h, the height.

Please help. My final is on wednesday. I have other problems too but I'll start off with this one. Thanks for any input!

 

[Birkeland]

Figure out how much energy is lost due to friction on the horizontal part, subtract that from the total energy it started with (which can be used by looking at the elastic potential energy). Once you have the total energy it has when it starts up the ramp it should be simple to find the height.

However, that being said you don't need to. If you can find the energy at the base of the ramp, think about what it's energy will be at the top.

 

[charmy]

Birkeland, when you said find the energy at the base, does that mean I use the KE equation?... I'm confused with all this... T_T

 

[ashishsinghal]

agree with birkeland but in future be a bit patient in explaining answers. Let the poster identify the logic himself

 

[ideasrule]

Birkeland, when you said find the energy at the base, does that mean I use the KE equation?... I'm confused with all this... T_T

He means the energy at the base of the curved part. Use the spring's potential energy along with the energy lost via friction and you'll get this value.

 

[charmy]

Okay, let me try this:

Spring's potential energy: 1/2 x (1000) (.01)2 = 5J (her answer sheet has - 5J, why is that?)

Work done by friction: -.784J

-5J +.784J = - 4.216J (Yay! this is the answer!)

Sorry that I'm a bit slow but the professor has never gone over how to approach this type of problem before...she just decides to add it in the finals and see how we go from there.

 

[Birkeland]

agree with birkeland but in future be a bit patient in explaining answers. Let the poster identify the logic himself

Yeah, when I am physically teaching that is what I do, something about the forum environment makes me want to rush things.

her answer sheet has - 5J, why is that?

Could be several reasons, maybe she defined directions different, ect. However if you use the numbers you got it should work out to the same answer.

(Yay! this is the answer!)

Sorry that I'm a bit slow but the professor has never gone over how to approach this type of problem before...she just decides to add it in the finals and see how we go from there.

Don't worry about it, that's how you learn things. The main thing I found out in college physics is that in the end, all physics is just a way to apply what you know to completely new situations

 

[charmy]

^ College Physics is hard since I have no foundation at all. My high school physics teacher taught us literature instead...and got fired later.

Regarding the -5J answer, if I had use positive 5J, wouldn't my answer be different though since my answer is - 4.216J ?

Thanks for the all the help! ^_^

Another question that she wants to ask us, What's the speed of the block the instant it passed the frictional section?

 

[Birkeland]

Regarding the -5J answer, if I had use positive 5J, wouldn't my answer be different though since my answer is - 4.216J ?

Yeah, but look what you did.

Spring's potential energy: 1/2 x (1000) (.01)2 = 5J (her answer sheet has - 5J, why is that?)

Work done by friction: -.784J

-5J +.784J = - 4.216J (Yay! this is the answer!)

You found work to be -.784J and energy to be 5J, but you changed it to -5J and +.784, all you did was flip the minus. Don't forget in physics all a negative sign means is direction, as long as you are consistent you are fine. My guess is her negative came from making the .01 in the spring equation negative (because the spring is compressed to the left).

Another question that she wants to ask us, What's the speed of the block the instant it passed the frictional section?

Well, ask yourself this, when the block is past all the friction, where you said the block has 4.216J of energy, what type of energy is that?

 

[flyingpig]

May I ask a question about the work done by gravity now that the OP got it? What happened to the Kinetic energy? Is he just solving for the work done by gravity initially?

 

[cepheid]

May I ask a question about the work done by gravity now that the OP got it? What happened to the Kinetic energy? Is he just solving for the work done by gravity initially?

Since energy is conserved, the 5 J that was stored in the spring was converted entirely into kinetic energy upon release. Some of that kinetic energy was lost due to friction, and the rest of it was converted into gravitational potential energy as the block ascended the curve.

EDIT: 3000th post!

 

[charmy]

Birkeland, I see where you're going with it. I have to use the kinetic energy equation! ^_^

Thanks for all the help! I'm going into take my Physics final!

 

[Birkeland]

Birkeland, I see where you're going with it. I have to use the kinetic energy equation! ^_^

Thanks for all the help! I'm going into take my Physics final!

Excellent, hope you do well!