First, answer this question:
A student in a lab holds a brick of weight W in her outstretched horizontal palm and lifts the brick vertically upward at a constant speed. The force of the student’s hand on the brick is:
A. constant in time and equal to zero.
B. constant in time, greater than zero, but less than W.
C. constant in time and equal to W.
D. constant in time and greater than W.
E. decreasing in time but always greater than W.
Now watch this video. Feel free to pause, rewind, and rewatch as needed.
Finally, answer this question again:
A student in a lab holds a brick of weight W in her outstretched horizontal palm and lifts the brick vertically upward at a constant speed. The force of the student’s hand on the brick is:
A. constant in time and equal to zero.
B. constant in time, greater than zero, but less than W.
C. constant in time and equal to W.
D. constant in time and greater than W.
E. decreasing in time but always greater than W.
Believe it or not, the concept needed to reach the correct answer is given in Khan’s video. Highlight below to reveal:
C. constant in time and W. Why? Since the brick moves at a constant velocity, the forces on the brick (you and gravity) must be balanced.
Action-Reaction 
Was that some positive KA feedback? 😉
It is great to see your blog still fresh and consistently updated. You are an excellent critic of KA. One of these days they will smarten up and hire you.
You’ve sold me. Where can I get material for modeling physics (as much as possible)?
Rob, check out my Modeling Instruction page: https://fnoschese.wordpress.com/modeling-instruction/
It has links to curriculum materials and other resources. Try to get to a workshop this summer!
I love how he says that Galileo and Newton are basically saying the same thing. Nahh, Newton didn’t change anything drastic, he only game a minor change in how we understand the world.
Hi my name is Autumn and I am a student in EDM310 at the University of South Alabama. I am majoring in Elementary Education. I was assigned to read your blog and comment on a few posts. I’m kind of lost when it comes to physics but this video is a great way of explaining the answer. I even understood it after watching the video. I like your idea that traditional instruction is no longer effective. I agree with this idea and going about teaching in different ways helps many learn. Thank you for your post.
OMG I can’t even watch this. If I apply enough force, will he stop being unbalanced?
ughh…
I’m not familiar with Modeling, but I teach intro college physics — for better or worse to classes of 200+.
I think a key piece of the mis-match between the problem in the question, and the KA presentation is that what most students picture when the read the problem the whole process of lifting the brick — from starting at rest to ending at rest — in which case there are short periods of acceleration and deceleration. I think for many students the idea that you can ignore that is as confusing as anything about forces.
You’re absolutely right. And you have identified the mis-match because of your experience with teaching students.
One thing I’ve done to deal with this is to have students map out the entire lifting process in distinct stages: At rest, initially increasing speed, cruising speed, slowing down, and at rest again. For each stage, students draw a system schema, a motion diagram, and a free body diagram. We identify balanced/unbalanced forces based on the constant/non-constant velocity motion. Even AFTER, students have trouble because they think F=mv, not F=ma. Students say, “It’s moving up, so I have to pushing up more than its weight. There must be a force in the direction of motion.”
The Khan Academy video just gives a standard textbook exposition which does little to engage the student. No discussion of misconceptions, no attempts to show EXPERIMENTALLY what he’s talking about. The video will do little to change students answer to the MC question after watching the video.
So help me (and presumably Autumn above) out here. The video contains no major gaffes and contains information relevant to the question posed. What’s the critique?
Hi Chris,
See my reply to Rachel (above). Thanks!
There are no issues with traditional instruction. It is the instructors who destroy the system by receiving tenor and then not teaching to their ability. It is also the students who don’t care but not through any fault of their own but because their instructors who see no need to teach at their full potential.
jay53,
Thank you for posting this comment. I want to let you know I’m not posting the other comments you left, because I find the tone to not match the type of dialogue I’m hoping to foster on this blog.
You raise some valid questions and concerns and you would love to talk to you about them. It’s clear that you didn’t have a positive experience my class, and I’d like some feedback as to how I could have helped you so I can address those concerns with my students this year.
Thanks for taking the time to give your input.
Good for you, Frank, I feel this is not a site/blog in which ideas must be bullied. Concerns can arise and arguments may be posed but it must be done in a respectful manner. On a different note, Modeling as a form of teaching appears to be something that is quite effective and trows off the smothering blanket of ways of old. I would like tor try it in my class room next semester, is it expensive to start? and if so are there any ways to do things differently to offset some costs?
Thanks
The real demonstration of the ineffectiveness of the video is the fact that I couldn’t bring myself to sit through all 9:32 of it. (For that matter, I didn’t even start it.) I would never appreciate a “flipped” classroom, because I don’t have the patience for ten minute videos.
Embarrassing question: could you please explain why the force is equal to W? The way I set up my diagram, I can see that gravity is acting on the brick and so the student must overcome this force. How can she accomplish this by applying an equal force on the brick? In other words, if the force of gravity and the force of the hand are the same, wouldn’t the brick remain motionless? I read the other comments above, but it didn’t “click” yet for me (I thought it would be D). Please help me understand.
Not an embarrassing question at all. It is something people continuously struggle with in physics, even when they can recite back Newton’s 1st Law.
Short answer: If an object’s motion is constant velocity, then the forces on the object are balanced. In this case, that means the upward push of the student on the brick must equal the earth’s downward gravitational pull
on the brick.
Long answer: It’s useful to think about this scenario in stages.
STAGE 1: Brick is at rest in the students hand.
MOTION: Constant velocity (v is constantly zero).
FORCES: Balanced
Upward force of student = Downward force of earth
STAGE 2: Brick is set into motion
MOTION: Accelerating (v is increasing from zero to some new speed).
FORCES: Unbalanced
Upward force of student > Downward force of earth
STAGE 3: Brick is now rising with the constant velocity it ended stage 2 with.
MOTION: Constant velocity
FORCES: Balanced
Upward force of student = Downward force of earth
You see, if you had KEPT the forces unbalanced, the brick would continue to accelerate. So, in order to stop the brick accelerating and make it to move at constant velocity, you need to reduce your upward push until it is equal to W again. Or look at this way: Your push tries to make the brick speed up in the upwards direction while the earth’s pull tries to make the brick slow down. Since your push and the earth’s pull are equal, the brick won’t speed up or slow down … it will just keep doing what is was doing (ie, the speed will remain constant.)
I hope this helps. Ideally, I would have you experiment lifting objects with spring scales so you could convince yourself that this must be true.
The reason your question is so problematic for many even after watching the video is not due to any particular teaching style.
But first, some disclosure. I did HS Physics 31 years ago, have not looked at Newton’s Laws since and before looking at the video I believed that there would have to be greater force in the student’s hand to produce the upward velocity in the brick.
After watching the video I realized two things.
One, that the forces must be balanced as per Newton’s Law since the velocity is constant.
Two, that since velocity needs no force to maintain it, so any prior force – whether an upward kick courtesy of a fellow student, an upward nudge from a helpful teacher or the student’s own prior greater force exerted would completely explain the current constant velocity described in the question.
At least for me the video was effective.
But the question is not about me – it’s about all those who struggle with this question.
As you described in follow-up comments your own efforts in a live classroom to directly address this question (remember, the video was not intended to address this question) still leave students confused with the problem I initially had – “It’s moving up, so I have to pushing up more than its weight. There must be a force in the direction of motion.”
That was also space cadet’s question to you above after your blog post and after your follow-up comments. It’s probably why some of your students were sure it must be F=mv rather than F=ma.
So was the video ineffective or is there something difficult about your question that is unrelated to the teaching method?
Your own discussion of the problem gives no hints as to the true source of the problem – as much as you described how people struggle with the problem you did not identify why people struggle with it.
Ironically the video did. While the video did not address your particular problem, it very clearly addressed the source of confusion – which, strangely you did not.
The real puzzler is, as the KA video stresses, that everything we know from everyday experience seems to tell us that in order to maintain velocity we need to keep applying unbalanced force – and that’s where the real disconnect is.
The best and most direct response to the confusion over your problem is that in Physics we are not describing everyday environments – we are describing simple hypothetical environments without air resistance and without friction.
In such environments, unlike in everyday environments, once a force has brought an object to any given velocity – whether it’s a fellow-student kicking the hand holding the ball upward – a teacher pushing the student’s hand upward or the student initially applying greater force – whichever it is – no further unbalanced force needs to be applied in the simple environments we discuss in Physics and that’s why the velocity at the motion phase described in the question needs no extra unbalanced force to maintain it’s velocity – it’s the product of some prior force that unlike in our real World – does not need to be maintained.
You want to teach that in the classroom? No problem. The video mentions that Galileo may have been able to wrap his head around the idea of constant velocity from his observation of the planets. Thanks to mass media today’s students are far more familiar with the idea of motion in space than students in Galileo’s day and that’s a powerful way to address the core of the problem in this question.
So I concede that the video is not as effective in guiding students to correctly answering your specific question as your own teaching is. But ironically, if your posting and comments here are any indication, the KA video addresses the underlying core principle – the counter-intuitiveness of constant velocity and its evidence in Space far more effectively than your teaching – or at least what is knowable about your own teaching from what you have posted here.
That seems to turn your central argument on its head. Your teaching method may get more students answering the question correctly – but the video did a far better job on the underlying principle. Arguably that’s true education.