This week the crew is joined once again by ‘Physics Commando’ Barry Panas who challenges us to think about the labels we use to describe what we feel when it comes to force and motion. Pat gives us a food themed game of ‘Name That’. Adam checks the numbers on whether it always rains on Canada Day.
Download direct: mp3 file
Perception Of Force and Motion
Barry’s YouTube Video on Force and Motion
Name That: Food Edition
List Of The Largest Fast Food Chains
Fourth Of July: How Many Hot Dogs?
Huffington Post: Quinine in Tonic Water
Live Science: Bugs In Red Food Dye
Huffington Post: Food Compared To A Krispy Kreme Donut
Does It Always Rain On Canada Day?
Historical Data – Climate – Environment and Climate Change Canada
I just finished listening to the segment on Perception of Force, and I must respectfully disagree with the guest.
When I start accelerating in a car, the car starts moving before I do. This is why I perceive myself to be pushed back first before being pushed forward. It comes down to frame of reference. Frame of reference was mentioned several times in the segment, but each time it was mentioned, it was given short shrift.
In every example provided, within which the passenger felt the force in the “wrong” direction, the passenger was inside a moving frame of reference, and the “wrong” direction was only “wrong” with respect to a stationary frame of reference outside of the moving one. In every example provided, within which the passenger felt the force in the “correct” direction, the frame of reference was the same as the observer’s frame of reference.
Since we process vision faster than proprioception, we sense our position with respect to the things around us first–and our brain builds a simple model–and only afterwards do we receive information about the forces acting upon us. This is why we FEEL we’re moving backward even when we KNOW we’re moving forward.
Further, it seems to me that the guest had already decided on what he thought people SHOULD feel, not what they DO feel. Rather than take the data as reported and figure out why it didn’t match his perception, he insisted that the data were wrong and that his preconceived notion was correct.
“The data don’t match my philosophy. Therefore, the data must be wrong.” That is the opposite of skepticism. That is not how science is done.
Hi Jennifer. You started with “When I start accelerating in a car, the car starts moving before I do. This is why I perceive myself to be pushed back” … I have no problem with that statement, provided that this “perception” is based on how things visually appear. I did try to avoid bringing visuals and frames of reference into my segment because everything I said would be equally true in any inertial frame of reference, and I was trying to exclusively focus on how things “felt” and not how they “looked”.
Imagine that you are simply standing on the ground and I am standing a short distance behind you holding the back of a seat facing you. Consider that I then run forwards so that a short time later it hits you in your back and pushes you forwards. In that situation, would you say that you felt like you were pushed backwards before being pushed forwards? I’m guessing not. And yet you seem to be claiming that you do “feel” this way when the car basically does this same thing. I don’t think that is a “feeling” but rather confusion arising from being inside of a car that is accelerating, in which there are visual cues that are consistent with you all of a sudden moving backwards.
What if we remove all visual cues by blindfolding you. If you are in the car accelerating forwards, which way do you feel like you are being pushed then? And should the answer be different if you are not blindfolded if we are discussing how forces feel?
As for your statement about my having decided on what I think people SHOULD feel, I would like to again state that I don’t disagree with how anything actually feels, but rather I think we have our labels backwards. I claim that if you are being pushed forwards, then whatever that feels like should be labeled the “feeling of being pushed forwards” and this label should be consistently used regardless of the presence or absence of how things look.
I claim that we have our labels backwards. It would be like saying that “sugar tastes salty” and “salt taste sugary” …. whatever the taste of salt is, it should be named “salty”. I don’t agree with saying that you “feel like you are being pushed backwards” when you are clearly being pushed forwards. And then saying that you feel like you are being pushed forwards when you are in fact pushed backwards. To me, if you are pushed forwards, then whatever that feels like should be labelled the feeling of being pushed forwards.
Finally, in response to your closing comments, I don’t think we disagree on any of the data or facts … just the labels.
The biggest issue I have with the “Perception of Force” segment is that I will never claim that gravity is pushing me up. I feel gravity pulling me down even though the physics tell me differently. Perhaps we should say the car is pulling us into the seat. I must say I loved the segment. It made be think a lot of the perception of force.
Hi Jeffery – I’m really glad to hear that the segment made you think. I pretty much see that as my job: to make people think. Let’s see if I can make you think about this even more, because the very thing that you “will never claim” is exactly what I do claim(!) Standing still on the ground actually IS the feeling of being pushed upwards. It’s not easy to see how this can be, so hopefully you will have an open mind here (but not so open that your brain falls out)…
Start with an idealized environment with no air resistance, and a uniform gravitational field. Imagine that you are falling in this environment. There is only one force acting on you, and that one force is the force of gravity. This is the purest way that gravity can act on you as it is literally the only force you are experiencing. Now what would that “feel” like?
The answer is that you would feel no forces at all. If you are blindfolded you would be unable to tell up from down, or even vertical from horizontal. In fact, this is (approximately) the same sensation that astronauts have when in orbit (i.e. they are in what we call “free fall”).
There is more than one way to explain why you wouldn’t feel the force of gravity. If we stay with a Newtonian model of gravity, then you won’t feel any force from gravity on your body because it pulls every part (and indeed particle) of your body with a force proportional to its mass, and so everything accelerates at the same rate. In free fall, your head falls at “the acceleration due to gravity” (about 9.8 m/s/s near the earth’s surface), and so do your feet. If you are in free fall in a uniform gravitational field, then your head and feet (along with everything in between) fall together and so you would not feel any compression or tension within your body due to gravity. Although I don’t like the expression, many would say that you would “feel weightless”. I don’t like that expression because, that actually is the “feeling of weight” … which is no feeling at all. Again, I am assuming a uniform gravitational field.
Now let’s try a different scenario. Imagine an idealized environment in which there is absolutely no gravity. There is, however, a solid surface underneath your feet which you are standing on. That surface (perhaps with the help of rockets) is accelerating “up” (i.e. in the direction from your feet towards your head) at 9.8 m/s/s. To be very clear, there would be only one force acting on your body, and that one force would be the force that is from that surface unambiguously pushing you upwards. What would that feel like? Well, just stand on the surface of the earth and you will have your answer. In other words, while standing on the ground you DO feel like you are being pushed upwards (because you are … by the ground). Unlike the force of gravity, this force only acts on your feet (i.e. the part in contact with it) … so it definitely can be felt.
I won’t get into it too much here (largely because my own understanding is incomplete) but if we were to switch to an Einsteinian general relativity model for gravity, there is, in effect, no such thing as a real “force” of gravity. In this model, while standing on the ground (on earth) you would have only one force on you – the ground pushing you up – and because of it, you would be accelerating in 4-dimensional space-time. Again, I would say that you are being pushed up, and you feel like you are being pushed up.
Barry, you have totally lost me.
I throw a ball into the air, and the ball falls to the ground. Being asked to intuit the Earth moving up to ball seems a little silly. The expectation that I tell people the earth was pushed into the ball is beyond the pale.
For example, I see a sunset. I tell people the Sun is setting. I know the Earth rotating so the horizon line is moving above the Sun, but the frame of reference is too big of a factor for common parlance.
In the same way, objects fall to Earth. The frame of reference is too big of a factor.
Perception of Force also: Here’s the thing I’m struggling with; the analogy of pushing someone from behind doesn’t seem to fit with how other objects would behave in the same circumstance… If you push a ball from behind it will roll away from you, but if you sit behind the ball in vehicle and that vehicle begins to accelerate the ball will roll toward you. To follow Barry’s argument shouldn’t it also roll away, just like if you’d pushed it?
Hi Mark – great question! I’ll try to answer it; let me know if this helps or makes things worse!
To best understand this, don’t think of a small ball, but rather a large, massive ball (like a bowling ball). There are a number of possible scenarios here, so let’s start with this one: you are in the back seat of the car, and the ball is on the front seat, strapped in tight with a seat belt (as are you). When the car accelerates forwards, the ball’s seat pushes the ball forwards, and it accelerates forwards. The same thing happens to you in the back seat, so you and the ball accelerate together: the ball does not get any closer to you, nor does it get any farther from you.
Now let’s put you in the front seat and the ball on the floor just in front of you. Assuming the floor is horizontal, the ball just sits there until the car accelerates forwards. You are correct in pointing out that the ball will “roll toward you” in the sense that the distance between you and the ball will get smaller. It does indeed “look like” something pushed that ball towards you. Realize, however, that your body is accelerating forwards quite nicely thanks to your seat back pushing you forwards. The ball does not have this, and so it does NOT accelerate forwards as well (there would be some friction pushing it forwards, but not enough, and so it does not “keep up” with the car, and so it “rolls towards you” though I think it would be better to say that you accelerated into the ball.
What if we instead put the ball on the road a short distance in front of the car. If the car then accelerates forwards as before, you would again see the distance between you and the ball get smaller. In fact, to your eyes, it would look like the ball came towards the car and hit it. What force acted on the ball to make it move like this? None of course. The ball just sat there while you accelerated towards it (thanks to forces on you and your car). The ball on your dashboard is more nearly in that situation: it “rolls toward you” because you accelerated towards it. In other words, don’t blame the ball … blame the car (and you in the car).
One final consideration: What if you hold the ball in your hands? Now when the car accelerates forwards, you would indeed push the ball forwards with your hands. In accord with Newton’s 3rd Law, the ball would push your hands backwards, but there would be a much larger force from your seat back pushing you forwards. In this case, both you and the ball are being pushed forwards (i.e. net force is forwards). You both accelerate together, so the distance between you and the ball would again be constant. To your eyes, it would look like nothing at all is happening to the ball, but you would indeed feel it pushing you backwards, as you push it forwards. Before you jump to the conclusion “aha … there is something pushing the ball backwards” let me point out that the only horizontal force on the ball would be you pushing it forwards. The ball is pushing you backwards, just as your body is pushing your seat backwards. Don’t confuse “being pushed” with “pushing”.
Huh, you know what? You’ve won me over with the holding-the-ball-in-your-hands comparison. I think I’m starting to get it now, even though my brain’s still doing some counter-intuitive pushback. 🙂
I am going to have to replace my bathroom scales. It has been reading the force of the earth pushing up on my body. I wanted ones that would measure the force of my body pushing down on the scales.
It still feels like the weight of my body is pushing down on the scales.
In a frame of reference, we should describe our feelings as if we were the point of reference, despite the feelings being different depending on that frame.
Yes, the sensation is the same. It’s the relative perception of “forward” and “backward” that changes. Standing on the ground, your frame of reference is the ground. In the car, your frame of reference is NOT the ground; it is the inside of the car.
Well, to be strictly accurate, it’s both… You can have multiple frames of reference simultaneously. But the ground outside is secondary. The inside of the car dominates your immediate surroundings. You perceive all the things inside the car (your coffee cup, loose change, sunglasses, phone, etc.) as stationary objects relative to the car. And here’s the important part: your body is also one of those things! If you accelerate, decelerate, or turn too fast, and these objects slide around, your immediate perception is that THEY are sliding around within the car, not that the car is sliding out from under them. Again, that also goes for your body! The seat you’re sitting in is a stationary object as far as your immediate, visceral perceptions are concerned. So if you feel the seat pressing against your back, your brain MUST interpret that as your body pushing backwards into it.
Now suppose you are in a moving car, but traveling in reverse, and you hit the brakes. The sensation of the seat pushing against your back is exactly the same as in the other scenarios. By Barry’s logic, since the sensation is the same, we should describe it with the same words. Even though neither you nor the car are ever moving forward relative to any sensible frame of reference, Barry would presumably insist that you are actually being pushed forward! I hope the absurdity of this is clear.
Of course, one can intellectually envision a frame of reference relative to which the car and driver could be described as moving forward, i.e. the place where the car *would be* if it weren’t decelerating. But that is much too abstract to be the frame of reference where you *perceive* yourself to be. We’re talking about how something FEELS, so perception is all that matters. It makes no sense (no pun intended) to pedantically insist otherwise.