The discussion centers around the implications of subtracting potential energy from total energy, specifically in the context of kinetic and potential energy in a system involving a puck sliding down an incline. Participants explore the relationships between these energy forms, the conditions under which they apply, and the effects of external forces.
Participants express differing views on the implications of negative kinetic energy and the conditions under which energy conservation applies. There is no consensus on the interpretation of negative results or the sources of energy loss in the experiments discussed.
Limitations include potential measurement errors, assumptions about frictionless conditions, and the need for consistent reference points in energy calculations. The discussion highlights the complexity of energy transformations in practical scenarios.
This discussion may be useful for students and practitioners in physics and engineering, particularly those interested in energy conservation, experimental methods, and the effects of forces on energy systems.
Kinetic energy can't be negative. A constant sum for gravitational potential energy and kinetic energy only occurs if there are no other forces involved. If some force other than gravity affects an object, then the work done by that force also affects the total energy.Drew777 said:So is it possible to get a negative number as the kinetic energy of the higher elevation?
rcgldr said:Kinetic energy can't be negative. A constant sum for gravitational potential energy and kinetic energy only occurs if there are no other forces involved. If some force other than gravity affects an object, then the work done by that force also affects the total energy.
Drew777 said:I understand that. When I take the the potential energy of the higher point and subtract it from the kinetic energy I get a negative number. What does this mean?
willem2 said:kinetic energy can't be negative.
It means that a thrown object will fall back before it gets to this altitude.
If you throw something straight up, it will start to move down again when the kinetic energy is 0.
Drew777 said:What I have is the potential energy of a low point that I will call PEf and I got the kinetic energy it took for the object to move down the incline(KEf). I added them together to get Ef.
Now I have to take this number Ef and subtract it from the higher point of potential energy, that I will call PE. I ended up with a negative number on all my trials but one. Why would this be?
Drew777 said:The KEf I measured was using 1/2(m)(Vf^2). So the KEf is the kinetic energy from the high point (PEhp) to the lower point(PEf).
In the ideal (frictionless) case, since mechanical energy is conserved, ΔE should be 0 in all cases. (All potential energy lost during the descent is converted into kinetic energy). The fact that you get a negative ΔE means that mechanical energy is lost. This must be a combination of experimental error + the fact that the table is not entirely frictionless. It's probably mostly the latter, a consequence of which is that although total energy (as always) is conserved, mechanical energy (which means kinetic + potential) is not conserved. Some of the mechanical energy is converted into heat by friction. That's where the shortfall comes in: it's the work done by the frictional force.Drew777 said:To get Ef I added PEf + KEf.
Now I need to find Δ E. The equation for Δ E is Ef-PEhp.
Example
PEf + KEf=Ef
2,070,000 ergs + 623,000 ergs=2,690,000 ergs
Ef - PEhp=Δ E
2,690,000 ergs - 2,810,000 ergs= -107,000 ergs
All of my trials came out with a negative number except my last trial. It came out with a +20,000.
I am trying to understand where these numbers are coming from. Is -107,000 just the energy lost from the top to the bottom. If so then where did the positive number come from?
Thanks
Drew