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Old Dec 12th 2015, 10:45 PM   #1
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Conservation of Energy

Hello, I'm having trouble trying to solve and explain this problem to my friend and was wondering if you guys can provide a solution and a simple explanation, that would be great! Thanks


a. If Spring 2 can only compress by 2 meters, what mass must the bock have, given the setup depicted in diagram 1, to compress spring 2 by exactly that amount? Note: Assume that the block starts from rest

b. What is the block’s maximum velocity and when and where does it occur?
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Old Dec 12th 2015, 10:47 PM   #2
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Also have one more problem relating to work energy theorem....

a. What physical condition must be true for the Work-Energy Theorem to become a statement of Conservation of Energy?

b. Use your answer from part a) to derive a statement of Conservation of Energy from the Work-Energy Theorem.

c. Use your answer from part b) to demonstrate that energy is conserved by passing back and forth between Kinetic Energy and Potential Energy.
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Old Dec 13th 2015, 03:04 PM   #3
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These two posts should perhaps be better posted as two separate threads,
It then makes it easier for us to answer them separately.
The various people who provide answers on this site have different strengths on the various topics and will naturally pick the threads that they feel match their strengths, but will leave other threads to other contributors.

I like to think I can sometimes help to pick out the key points of an awkwardly presented question.
Thus your initial post in this thread is a bit unclear, (you yourself indicate you were having difficulty describing it to your friend).

I think the intention is that spring 1 is initially pulled down (storing energy) then released, transferring energy to the mass and propelling it up until it hits spring 2, compressing spring 2 with its residual energy.

I will leave the maths to someone else (like ChipP perhaps).

Your second post is perhaps suited to ThermoSpokenHere,
Which is why it should have been presented in a separate thread to avoid the replies from these two becoming tangled.
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Old Dec 14th 2015, 09:20 AM   #4
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Originally Posted by MBW View Post
I think the intention is that spring 1 is initially pulled down (storing energy) then released, transferring energy to the mass and propelling it up until it hits spring 2, compressing spring 2 with its residual energy.

I will leave the maths to someone else (like ChipP perhaps).
I think your interpretation of the problem is probably correct - the block gets shot upward by spring 1, collides with spring 2, and compresses spring 2 by precisely 2 meters.

We can use energy principles on this. When you compress spring 1 by 5m it develops potential energy that must be enough to shoot the block upwards and compress spring 2 by 2 meters. Thr ePE induced in spring 1 is:

PE_1 = (1/2) K_1 (5m)^2

which must equal the PE gained in rising up through the Earth's gravitational field a distance of 41 meters plus the initial 5 meters plus the 2 meters, and must also induce PE into spring 2 of (1/2)K_2 (2m)^2. So:


PE_1 = mgh + PE_2, or:

(1/2) K_1 x^2 = mg(5m + 41m + 2m) + (1/2) K_2(2m^2)

Now solve for m.
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Old Dec 15th 2015, 08:19 AM   #5
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Charles: You appear to be under the mistaken impression that the helpers in this forum will do your homework for you and your question is clearly a homework question, either yours or belonging to your friend. We're not allowed to do peoples homework for them. Even if we were I myself wouldn't since it doesn't help anybody to do someone's homework for them. Students can't learn that way.

Originally Posted by Charles View Post
Also have one more problem relating to work energy theorem....

a. What physical condition must be true for the Work-Energy Theorem to become a statement of Conservation of Energy?

b. Use your answer from part a) to derive a statement of Conservation of Energy from the Work-Energy Theorem.

c. Use your answer from part b) to demonstrate that energy is conserved by passing back and forth between Kinetic Energy and Potential Energy.
Charles: When someone states a question like this then it's most likely from a question from a text or some other homework assignment. The use of "Use your answer from..." gives it away. We are forbidden by forum rules to help you unless you show that you've made an attempt to solve it yourself. Please read the rules in the thread entitled Must read before posting a problem : please show your attempt! located at:
Must read before posting a problem : please show your attempt! which says
The Helpers and the staff of the forum have agreed that they won't give a full answer to a problem posted with no attempt (except maybe in some cases). The Helpers are here to help you to understand your weaknesses by going through the asked questions with you and not here to solve your physics problems.
It is imperative to show an attempt to a problem. If you don't know how to start, say it. Helpers will give you a push in the right direction.

Welcome to the world of physics!
Also, please read the rule in the thread entitled Forum Rules located at:
Forum Rules which says in part
(12) Do not post questions that will result in class credit, i.e. homework or take home exams. Whenever possible an Administrator or Moderator will attempt to contact the school or instructor. Additional penalties on the site will be taken as needed.

Last edited by Pmb; Dec 15th 2015 at 08:22 AM.
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