# Energy changes moving through fluids

• Jimmy87
In summary, when a ball is dropped into a tube containing oil, the gravitational potential energy (GPE) of the ball is converted into kinetic energy (KE) as it falls. However, when the ball reaches terminal velocity, the KE decreases due to the drag force of the oil. At this point, both the decrease in KE and GPE go into doing work against the drag force. As the ball continues to fall, the GPE is gradually converted into thermal energy due to the friction with the oil. This is because the work done by the drag force and the thermal energy generated by the friction are essentially the same thing. As the ball falls, its potential energy decreases and its velocity decreases, causing it to do work on the fluid and
Jimmy87

## Homework Statement

A ball is dropped from a height 'h' above a tube containing oil. Describe what happens to the GPE when the ball has reached terminal velocity in the oil.

## Homework Equations

KE = 1/2mv^2 GPE = mgh

## The Attempt at a Solution

When the ball is dropped,if we ignore air resistance then GPE will be converted directly into KE. When it strikes the surface of the oil, the ball's velocity will decrease which means the KE will also decrease. There is also still GPE which is not going into KE. So I think that both the decrease in kinetic energy and GPE both go into doing work against the drag force. When the ball reaches terminal velocity the KE will no longer change so there will be no more contribution from the kinetic energy, however GPE is still being lost which will go into work against the drag force. I am confused as I checked the answer for this question in my book and it just says "at terminal velocity GPE of the ball is converted into thermal energy". Surely GPE is being converted into both thermal energy and energy from doing work against the drag force or are these two the same? i.e. is heat lost due to friction with the oil the same as work done against the drag force?

Yes, the latter.

Chet

Chestermiller said:
Yes, the latter.

Chet

So, all the GPE goes into thermal energy as soon as it hits the oil? How is work done against drag and thermal energy mean the same thing? If work done is force x distance doesn't some of the energy go into moving the object therefore it doesn't go into heat, e.g. if you push a block over a certain distance, the energy goes into the motion of the block. Does the work done by gravity change as it falls through the oil?

Jimmy87 said:
So, all the GPE goes into thermal energy as soon as it hits the oil?
No. As the object is slowed down by the oil, it is losing KE, and that is being converted into internal energy of the oil.
How is work done against drag and thermal energy mean the same thing?
As the object moves through the fluid, it is causing the fluid to deform, and this causes viscous forces to develop at the surface of the object to slow it down. Meanwhile, the deformation that the object causes in the fluid combines with the viscous forces to do work on the fluid locally, and, as a result, to increase its internal energy.
If work done is force x distance doesn't some of the energy go into moving the object therefore it doesn't go into heat?
The drag force of the fluid on the object acts in the direction opposite to the motion of the object, and this reduces its kinetic energy (i.e., velocity). At the same time, the fluid that the object is contacting is caused to accelerate by the object, and to deform. So the kinetic energy of the object does work on the fluid, and the deformational work on the fluid causes the mechanical energy to be dissipated within the fluid, and converted into internal energy (heat).
Does the work done by gravity change as it falls through the oil?
The potential energy change resulting from the decrease in elevation of the object within the fluid is converted to internal energy of the fluid also.

Chet

Jimmy87

## What is the definition of "energy changes moving through fluids"?

Energy changes moving through fluids refer to the transfer of energy from one point in a fluid to another. This can occur through various mechanisms such as conduction, convection, and radiation.

## What is the role of fluids in energy changes?

Fluids play a crucial role in energy changes as they act as the medium through which energy is transferred. They can also store and transport energy, making them essential in many industrial and natural processes.

## What factors affect the movement of energy through fluids?

The movement of energy through fluids is affected by several factors, including the properties of the fluid (such as density and viscosity), the temperature gradient, and the presence of external forces (such as gravity or pressure differentials).

## What are some real-world applications of energy changes moving through fluids?

Energy changes moving through fluids have numerous practical applications, including heating and cooling systems, power generation through steam turbines, weather patterns, ocean currents, and even the movement of blood and nutrients in living organisms.

## How is energy conservation related to energy changes moving through fluids?

Energy conservation is closely related to energy changes moving through fluids. The laws of thermodynamics dictate that energy cannot be created or destroyed, only transferred or converted. Therefore, understanding how energy changes in fluids can help us design more efficient systems and processes to conserve energy.

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