About KE and PE of a stone falling down a tube filled with oil

[Iwanttolearnphysics]

Homework Statement

A stone is falling at a constant velocity down a tube filled with oil. Which of the following statements about the energy changes of the stone during its motion are correct?

I. The gain in kinetic energy is less than the loss in gravitational potential energy.
II. The sum of kinetic and gravitational potential energy of the stone is constant.
III. The work done by the force of gravity has the same magnitude as the work done by Friction.

Relevant Equations

PE = mgh
KE = 1/2mv^2

This question does not have numbers, so I'm stumped. Here's my thinking.

• (I), the gain in KE is less than the loss in GPE is correct according to the key, but I think I don't understand this conceptually. Can you ask me questions to make me think about this a bit more? I can't even form questions to ask myself. I don't get this one at all.
• (II), According to the key, this is wrong the sum of KE and GPE is not constant. But why is that? Isn't there a formula that goes, ME = KE + PE. If that's true, then when KE changes, PE will also change, but they will still both add up to ME. Then shouldn't be ME constant?
• (III), the work done by the force of gravity has the same magnitude as the work done by friction. This is correct because they're opposing forces in this case and they'll be equal in magnitude but in opposite directions.

Here's a picture of the question:

Thank you!

 

[kuruman]

What is the gain in kinetic energy when a mass is moving at constant velocity? As the mass falls, does it gain or lose potential energy? Start from there.

 

[Iwanttolearnphysics]

What is the gain in kinetic energy when a mass is moving at constant velocity? As the mass falls, does it gain or lose potential energy? Start from there.

Oh. I misunderstood the question. Okay, if its velocity is constant, that means that KE will always be the same. As the mass falls, the potential energy will decrease because of h decreasing.

Hmm. Is this something that happens in real life? Can things fall down with constant velocity?

Thank you!

 

[kuruman]

Hmm. Is this something that happens in real life? Can things fall down with constant velocity?

Parachutes? Snowflakes? (no wind)

 

[PeroK]

I've even seen things fall upwards, like balloons or leaves on a windy day.

 

[Iwanttolearnphysics]

Parachutes? Snowflakes? (no wind)

Thank you! :D

 

[Iwanttolearnphysics]

I've even seen things fall upwards, like balloons or leaves on a windy day.

Thank you! I hope you don't mind if I ask another question. Is it possible for the stone in this question to fall at a constant velocity? I mean it's affected by gravity so it should be different each time right?

 

[kuruman]

Thank you! I hope you don't mind if I ask another question. Is it possible for the stone in this question to fall at a constant velocity? I mean it's affected by gravity so it should be different each time right?

Yes, it is possible for the stone to fall at constant velocity just like a parachute. Air is a viscous fluid except that its viscosity is lower than oil. A mass falling through a viscous fluid will eventually reach terminal velocity. Read about drag here. When terminal velocity is reached, the drag force (which is velocity-dependent) is equal to the force of gravity and the net force on the mass is zero. This means that, after that point, the mass will keep on moving at terminal velocity until another force stops it.

What is "it" that "should be different at each time"? A given mass falling through a given fluid will reach the same terminal velocity at all times.

 

[PeroK]

Thank you! I hope you don't mind if I ask another question. Is it possible for the stone in this question to fall at a constant velocity? I mean it's affected by gravity so it should be different each time right?

Why not? Newton's laws allow forces to be balanced. An object floating on the surface of water is moving at constant velocity, with buoyancy balancing gravity. Albeit that velocity is zero. In fact, the same applies to you sitting in a chair. In that case, gravity is also balanced by the normal force.

Moreover, have you ever heard the term "terminal velocity"? A falling object will reach a maximum speed where air resistance balances gravity. For a parachutist this speed is less than for a falling person. But, nevertheless, even without a parachute you reach a maximum speed, where you are effectively no longer accelerating.

 

[Iwanttolearnphysics]

Yes, it is possible for the stone to fall at constant velocity just like a parachute. Air is a viscous fluid except that its viscosity is lower than oil. A mass falling through a viscous fluid will eventually reach terminal velocity. Read about drag here. When terminal velocity is reached, the drag force (which is velocity-dependent) is equal to the force of gravity and the net force on the mass is zero. This means that, after that point, the mass will keep on moving at terminal velocity until another force stops it.

What is "it" that "should be different at each time"? A given mass falling through a given fluid will reach the same terminal velocity at all times.

Sorry for the late reply. I was really busy. Anyway, I think I misunderstood this question. For some reason, even though it says "filled with oil", I imagined something completely different! I should read the question again when I don't understand it. But it makes complete sense to me now. Thanks for the clarification!

 

[Iwanttolearnphysics]

Why not? Newton's laws allow forces to be balanced. An object floating on the surface of water is moving at constant velocity, with buoyancy balancing gravity. Albeit that velocity is zero. In fact, the same applies to you sitting in a chair. In that case, gravity is also balanced by the normal force.

Moreover, have you ever heard the term "terminal velocity"? A falling object will reach a maximum speed where air resistance balances gravity. For a parachutist this speed is less than for a falling person. But, nevertheless, even without a parachute you reach a maximum speed, where you are effectively no longer accelerating.

I missed the "filled with oil" part. I was trying to solve this question during my lunch break and I'm not sure anymore if it was a good idea studying during my break, so I didn't understand what this was even asking. English is not my native language, so there are times I get a bit confused. I'm trying not to do that anymore (not the "not getting confused" part, that's completely out of my control. I mean I'm trying not to ask without reading the question over and over first), which is why I haven't been posting. I try to read the question more than 3x and if it still doesn't make sense, then that's the time I think I need a bit of help. But yeah, the question makes perfect sense now. And yes, I have heard of terminal velocity. Thank you!