# Can someone assist me with forces and energy loss - Phsyics?

Discussion in 'Homework Help' started by ElecNerd, Mar 13, 2011.

1. ### ElecNerd Thread Starter New Member

Jan 2, 2011
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If an object of a mass of 450kg falls 5 metres onto a pile and the impact takes 0.5 seconds and does not rebound, how can I calculate the applied force exerted onto the pile by the object and the loss of energy upon impact.

I understand that potential energy = mgh and have calculated this energy however am struggling with the above and therefore any assistance is greatly appreciated.

Thanks

2. ### steveb Senior Member

Jul 3, 2008
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When the object falls, the potential energy is converted to kinetic energy. This kinetic energy is then fully dissipated into the pile. The assumption you can make is that the pile makes a constant force for the given time of a half second. Now, you need to ask, what value of force applied for a half second will dissipate the energy? Recall that work is force times distance. Also, recall that the force will decelerate the object from initial speed at the top of the pile to zero. Do you know how to calculate initial speed from kinetic energy?

Last edited: Mar 13, 2011
3. ### ElecNerd Thread Starter New Member

Jan 2, 2011
20
0
Hi

I am trying to find the formulas so that I can use them to calculate the answers.

I have calculated the PE however now am struggling to determine the applied force.

4. ### Kermit2 AAC Fanatic!

Feb 5, 2010
3,893
990
When the object comes to rest, it will have expended ALL its potential energy.

You say you calculated the PE, so, ,,,

5. ### steveb Senior Member

Jul 3, 2008
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Do you want me to just give you the formula so that you plug in height and time and get force? How will that help you on the next problem?

I came up with $F={{m\sqrt{2gh}}\over{\tau}}$ where m is mass, h is height, g is gravitational acceleration and tau is force application time. But, I derived this quickly so there might be a mistake there. Unfortunately, you can't check me if you don't understand the concepts.

6. ### ElecNerd Thread Starter New Member

Jan 2, 2011
20
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I understand the concepts however do not have the formulas etc and therefore its easy enough to understand the concept of it however without knowing the correct formulas to use it is hard to be able to correctly calculate the answers.

7. ### steveb Senior Member

Jul 3, 2008
2,433
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It's still not clear why you can't convert concepts to formulas, but let's go step by step and see if you agree with my approach. Keep in mind that one generally needs to make simplifying assumptions at this level.

My first thought is that the pile will do work to absorb the kinetic energy of the mass. The KE will equal the change in potential energy from falling. Hence, I can say that force F times distance x (work) equals the KE of mgh.

Hence, F=mgh/x with the assumption that x is small compared to h and force is constant during the impulse.

Are you OK with this first step? If so, how might you find x?

8. ### ElecNerd Thread Starter New Member

Jan 2, 2011
20
0
Sorry but call me stupid but I am new to this subject hence why I came for assistance. I am sorry that I am not clearly on your level. Thanks for your attempts to help however it looks as though I will have to obtain assistance elsewhere.

9. ### Kermit2 AAC Fanatic!

Feb 5, 2010
3,893
990
The total energy contained in an object is identified with its mass, and energy (like mass), cannot be created or destroyed. When matter (ordinary material particles) is changed into energy (such as energy of motion, or into radiation), the mass of the system does not change through the transformation process. However, there may be mechanistic limits as to how much of the matter in an object may be changed into other types of energy and thus into work, on other systems. Energy, like mass, is a scalar physical quantity. In the International System of Units (SI), energy is measured in joules, but in many fields other units, such as kilowatt-hours and kilocalories, are customary. All of these units translate to units of work, which is always defined in terms of forces and the distances that the forces act through.

A system can transfer energy to another system by simply transferring matter to it (since matter is equivalent to energy, in accordance with its mass). However, when energy is transferred by means other than matter-transfer, the transfer produces changes in the second system, as a result of work done on it. This work manifests itself as the effect of force(s) applied through distances within the target system. For example, a system can emit energy to another by transferring (radiating) electromagnetic energy, but this creates forces upon the particles that absorb the radiation. Similarly, a system may transfer energy to another by physically impacting it, but that case the energy of motion in an object, called kinetic energy, results in forces acting over distances (new energy) to appear in another object that is struck. Transfer of thermal energy by heat occurs by both of these mechanisms: heat can be transferred by electromagnetic radiation, or by physical contact in which direct particle-particle impacts transfer kinetic energy.

Energy may be stored in systems without being present as matter, or as kinetic or electromagnetic energy. Stored energy is created whenever a particle has been moved through a field it interacts with (requiring a force to do so), but the energy to accomplish this is stored as a new position of the particles in the fielda configuration that must be "held" or fixed by a different type of force (otherwise, the new configuration would resolve itself by the field pushing or pulling the particle back toward its previous position). This type of energy "stored" by force-fields and particles that have been forced into a new physical configuration in the field by doing work on them by another system, is referred to as potential energy. A simple example of potential energy is the work needed to lift an object in a gravity field, up to a support.

FROM WIKIPEDIA ENTRY ON ENERGY

The problem we are having is the objective of this forum!

We want others to LEARN, not serve as an 'internet answer portal' for people simply too lazy to do some mental/manual work.

There are other sites someone interested in such "SERVICES" can peruse.

I, for one,(can't speak for anyone else here, but I gather from the past conversations I've struggled through its a common theme) would bend over backwards, and strain all my brain cells for any individual interested in actually working for his own betterment, and who only seeks guidance on this difficult path.

Everywhere I look, I see lazy people,
Kermit

Jul 7, 2009
1,585
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There are two aspects to this problem. First, you can use Newton's second law to calculate the force on the falling mass that causes it to stop. Your textbook should have taught you this; if not, look up the concept of impulse and realize that, in general, you have an integration to perform to derive the force used to stop the mass (but people usually assume a constant force if no further information is given). Linear momentum is conserved even when mechanical energy is not, so it's a very general notion. This then brings up a question: where does this conserved momentum of the falling mass go (it can't have disappeared)? If you understand the principles like you say you do, then this has a simple answer.

The second aspect is the energy involved. This is an inelastic collision and energy of motion is not conserved in inelastic collisions (because the forces involved are not conservative). But you can still calculate the energy lost by the mass by calculating the work done on it by the applied force. Thus, once you calculate the force used to stop the mass, a simple integration gets you the change in energy (i.e., the work done by the applied force on the mass).

These are some of the key principles that you need to learn in your class -- and when you learn them, the equations you appear to want to plug and chug with go along with them. But if you don't learn the principles, the equations are mostly useless baggage because you won't know when and how to apply them.

Thus, it is obvious to me that in fact you do not understand the principles -- if you did, the problem can be solved in a few simple lines of math. It still appears that people want a royal road to geometry.

And before you think I am trying to insult you (I'm not), you need to realize that everyone struggles with these concepts. You learn them by doing problems, scratching your head, talking with your teacher, and slowly gaining the realization that the geniuses who thought this stuff up over time really did know what they were talking about. The stuff works when you understand it -- and you also need to learn where the boundaries of applicability are. But I do have one certainty -- you don't learn this stuff by just plugging numbers into equations.

11. ### steveb Senior Member

Jul 3, 2008
2,433
469
Ok no problem. I would never call you stupid and don't think that at all. The particular problem you are looking at is an impulse force type, which is not easy for someone starting mechanics problems. I suspect you need to solve other problems first before you are ready for this one.

Last edited: Mar 13, 2011