About steel balls of different masses and net force

[Iwanttolearnphysics]

Homework Statement

"Two steel balls, of mass M and 2M fall at constant speeds in a tube filled with oil. Which of the following correctly compares the magnitude of the net force and of the drag (resistance) force on the two balls?"

Relevant Equations

Fnet = ma

I don't understand why the answer is B. Here's my thinking:

• Since it wasn't mentioned that there's any other force aside from the drag force, then Fnet = Drag force
• I know that Fnet = ma, and since they have different masses, Fnet must be different for both balls?
• Acceleration cannot be different because acceleration due to gravity is what's causing them to move downwards.
• What other force am I missing here? Is it wrong to equate Fnet = drag force?

 

[haruspex]

it wasn't mentioned that there's any other force aside from the drag force

Then why are they descending?

Fnet = ma, and since they have different masses, Fnet must be different for both balls?

Unless a=...?

Acceleration cannot be different because acceleration due to gravity is what's causing them to move downwards.

Gravity exerts a force. It does not dictate the acceleration. That depends on the net force.

You are told the balls move at constant speeds (though not necessarily the same speed). What does that tell you about their accelerations?

 

[Iwanttolearnphysics]

Then why are they descending?

Unless a=...?

Gravity exerts a force. It does not dictate the acceleration. That depends on the net force.

You are told the balls move at constant speeds (though not necessarily the same speed). What does that tell you about their accelerations?

Thank you. I think I understand it now. They're descending because of the force of gravity. But I still don't understand why they have the same net force. In other kinematic problems where the balls are interacting, we solve them by adding their masses.

• Fnet = ma
• Fnet = 3a

For both of them, Fnet is 3a if we were to solve it like above. But I don't see why because they're not even interacting.

Thank you for pointing out that gravity exerts a force and that it does not dictate acceleration. I completely forgot about that.

For your last question, I solved it like this (supposing that Fnet is 3a)

Ball M = Suppose that M is 1.
Fnet = Fg-FF
3a = mg-FF
3a = 9.81-Ff
Ff = -3a+9.81

Ball 3M = Suppose that M is 1.
Fnet = Fg-Ff
3a = 3(9.81)-Ff
Ff = -3a+29.3

I still don't completely get it. Please tell me if I made any mistake. Appreciate it.

 

[haruspex]

You still have not considered the implication of the constant speeds. What does that tell you about the accelerations?

 

[Iwanttolearnphysics]

You still have not considered the implication of the constant speeds. What does that tell you about the accelerations?

Constant speed means zero acceleration. I completely get it now. Thank you!

 

[PeroK]

Constant speed means zero acceleration. I completely get it now. Thank you!

I suggest you failed to answer this question correctly not because of a direct lack of understanding of physics, but because you were fooled to some extent by the question setter's trickery.

If the question setter had added a third column for gravitational force, then you would have analysed the question very differently. You even assumed explicitly in your original post that there were no other forces.

Also, by giving you only the options "same" and "different", the question setter managed to fool you again, because constant speed means zero net force.

If the question had asked you to list the forces on each object, you might have done something like this:

M: Net force = 0, Gravity = Mg, Drag = -Mg

2M: Net force = 0, Gravity = 2Mg, Drag = -2Mg

I suggest that if you had not been given the table, you might have analysed and solved this question completely?

The lesson I would learn from this is to broaden the way you think about these questions. If you are given a multiple choice like this, there is a case to be made for ignoring the options until you've analysed the problem independently yourself - so as not be be led astray by the specific information you have been given and the information the question setter chose not to include.

 

[Iwanttolearnphysics]

I suggest you failed to answer this question correctly not because of a direct lack of understanding of physics, but because you were fooled to some extent by the question setter's trickery.

If the question setter had added a third column for gravitational force, then you would have analysed the question very differently. You even assumed explicitly in your original post that there were no other forces.

Also, by giving you only the options "same" and "different", the question setter managed to fool you again, because constant speed means zero net force.

If the question had asked you to list the forces on each object, you might have done something like this:

M: Net force = 0, Gravity = Mg, Drag = -Mg

2M: Net force = 0, Gravity = 2MG, Drag = -2MG

I suggest that if you had not been given the table, you might have analysed and solved this question completely?

The lesson I would learn from this is to broaden the way you think about these questions. If you are given a multiple choice like this, there is a case to be made for ignoring the options until you've analysed the problem independently yourself - so as not be be led astray by the specific information you have been given and the information the question setter chose not to include.

Thanks. That's a good tip. I won't read the choices next time, so that I can work through it on my own. I was constantly doubting myself while answering this question.

 

[PeroK]

Thanks. That's a good tip. I won't read the choices next time, so that I can work through it on my own. I was constantly doubting myself while answering this question.

I certainly got confused by those options until I stopped looking at them and took a fresh look at the physics of the scenario.