# Conservation of Energy: Spring pushing a block up an incline

CrazyCatMom

## Homework Statement

"Consider a 250 gram block on a 10 degree frictionless incline and in contact with a spring of constant 1.2 N/cm. If the block is launched from rest by the spring with an initial compression of 6cm, how fast is the block moving at the point of release from the spring? How fast is the block moving at 20cm away from the release? How far up the incline will the block slide before stopping?

KE = 1/2mv^2
PE = mgh

## The Attempt at a Solution

First I converted everything to kg and m to make the answer easier. Then I took the equation for KE and rearranged it to be v^2= 2k/m, but the answer I got wasn't correct. I feel like I am almost there, or at least maybe on the right track, but I'm not sure. I've been working on this for a while and could really use the help. Thanks in advance!

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haruspex
Homework Helper
Gold Member
rearranged it to be v^2= 2k/m
Do you mean you divided k by the mass? What dimensions or units would that yield?
Also, you do not seem to have considered the incline.

CrazyCatMom
I think I'm just kind of grasping at straws and hoping something will come of it. I really don't know how to approach the problem.

haruspex
Homework Helper
Gold Member
I think I'm just kind of grasping at straws and hoping something will come of it. I really don't know how to approach the problem.
Please post the details of the attempt you described.

Chandra Prayaga
Don't look for a formula for v. First, draw a diagram. In the diagram, show all the four different positions mentioned in the statement of the problem. At each position, write down the total mechanical energy. Then you will yourself see how to solve the problem.

ehild
Homework Helper

## Homework Statement

"Consider a 250 gram block on a 10 degree frictionless incline and in contact with a spring of constant 1.2 N/cm. If the block is launched from rest by the spring with an initial compression of 6cm, how fast is the block moving at the point of release from the spring? How fast is the block moving at 20cm away from the release? How far up the incline will the block slide before stopping?

## Homework Equations

KE = 1/2mv^2
PE = mgh
You ignore the spring energy.

## The Attempt at a Solution

First I converted everything to kg and m to make the answer easier. Then I took the equation for KE and rearranged it to be v^2= 2k/m, but the answer I got wasn't correct.
The initial spring energy is converted to kinetic energy and potential energy. What is the initial energy of the spring if it is compressed by 6 cm?

CrazyCatMom
Please post the details of the attempt you described.
I took the equation KE = (1/2)mv^2
Since I know I need to find the velocity, I divided both sides by (1/2)m and got the formula v^2=2k/m
I know the velocity is going to be in cm/s, so those were the units I was aiming towards.

CrazyCatMom
The initial spring energy is converted to kinetic energy and potential energy. What is the initial energy of the spring if it is compressed by 6 cm?[/QUOTE]

If, I use the W=fd formula, would it mean I use 1.2 N/cm for the Force and then 6cm for the Distance?
So, W=(1.2)(6)
7.2 J?

haruspex
Homework Helper
Gold Member
got the formula v^2=2k/m
Where k is what? According to your description of what you did, that k would be the KE, but it would be more usual to use k for the spring constant.
And, as I wrote, you are neglecting the slope. As Chandra Prayaga instructed, consider all the mechanical energy when the system is initially at rest, and again when the spring is fully decompressed.

Chandra Prayaga
As ehild pointed out, you are completely overlooking the spring potential energy in your attempts. You just used the first formula in front of you without looking at all the aspects. You also seem to be unclear about what the symbols mean. I sincerely suggest that you draw a careful diagram. You have three different forms of mechanical energy in your problem. The kinetic energy, the spring potential energy, and the gravitational potential energy. In the equations that you wrote, I don't see an expression for the spring potential energy. The sum of all three forms of energy must remain constant, in the absence of friction. Really that is the clue. There is not a single formula that you can manipulate, plug numerical values in, and get the answer.

haruspex
Homework Helper
Gold Member
you are completely overlooking the spring potential energy
Not if that is what CCM means by "k".

CrazyCatMom
Not if that is what CCM means by "k".
In all honesty, I don't know what it is. I thought it was just a place holder for the compression of the spring.

haruspex
Homework Helper
Gold Member
In all honesty, I don't know what it is. I thought it was just a place holder for the compression of the spring.
So where did you get this k from? It does not appear in the relevant equations you listed.

Chandra Prayaga
If that is so, then CCM is overlooking the gravitational potential energy. In either case, the best way to learn how to solve the problem is by drawing a careful diagram, labeling each position and writing down the energy terms (all three of them) at each position. The solution will then fall out very simply.
CrazyCatMom, in your last post, you were mentioning W = f.d. You cannot use that formula in the case of a spring force. Also, the value of k = 1.2 N/cm is not a force. It is the spring constant. In the case of a spring, the simple formula of W = f.d will not work because the spring force is not a constant. You should look up the expression for the work done by a spring.

CrazyCatMom
If that is so, then CCM is overlooking the gravitational potential energy. In either case, the best way to learn how to solve the problem is by drawing a careful diagram, labeling each position and writing down the energy terms (all three of them) at each position. The solution will then fall out very simply.
CrazyCatMom, in your last post, you were mentioning W = f.d. You cannot use that formula in the case of a spring force. Also, the value of k = 1.2 N/cm is not a force. It is the spring constant. In the case of a spring, the simple formula of W = f.d will not work because the spring force is not a constant. You should look up the expression for the work done by a spring.
I drew out a free body diagram, so I know I have to use cos(10) somewhere in the problem, but I'm not sure where.
I guess this just isn't clicking for me.

haruspex