{"id":55,"date":"2025-02-27T12:58:03","date_gmt":"2025-02-27T12:58:03","guid":{"rendered":"https:\/\/sebstack.com\/?page_id=55"},"modified":"2025-02-27T12:58:03","modified_gmt":"2025-02-27T12:58:03","slug":"science-you-can-feel","status":"publish","type":"page","link":"https:\/\/sebstack.com\/index.php\/science-you-can-feel\/","title":{"rendered":"Science you can feel"},"content":{"rendered":"\n




When we accelerate in a plane for takeoff, we feel ourselves pressed into the seat\u2014this is Newton\u2019s Second Law in practice. As linear momentum changes, we experience the opposite reaction. A similar sensation occurs just after landing, though in reverse\u2014a sudden deceleration as the plane slows down.
Similarly, on a childhood swing, the same reaction plays out. At the highest point of its journey, just before changing direction, the swing momentarily pauses. Its velocity drops to zero, and just before that, we feel a jolt\u2014the sensation of momentum fading.
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The young woman in the picture above would no doubt come to understand the nature of changing momentum better than most. The forces she uncovered later in life were also found to carry momentum, just as objects do in everyday life\u2014only on an atomic scale. Marie Sk\u0142odowska Curie and her work would be of momentous importance.<\/a>

During her time and before, electrical pioneers spoke of electromagnetic induction as something akin to a change in momentum<\/a>\u2014with the so-called Lenz\u2019s Law perceived as an action-reaction phenomenon. This is not a view that has lasted to the present day, an interesting topic in its own right.
Let\u2019s start with what we can see and feel. Imagine a wooden chair that appears to be made of wood, yet is, in fact, constructed from an unexpectedly heavy material. Recall that sense of surprise\u2014your mind anticipates lifting something of a certain weight, only to realize in the moment that it is much heavier. That instant, where expectation clashes with reality, is a direct encounter with mass, momentum, and inertia.

On the other side of the imaginable, a tightrope walker may carry a long but relatively light stick to maintain balance. We can all imagine the peculiar sensation of its “reluctance” to motion, offering stability\u2014something that tangibly resists movement and yet has no appreciable weight.

And again, turning the problem around in our minds, we return to Newton, who spoke of “quantity of motion”\u2014what we now call momentum. Experimentally, we may ask: Can weight<\/strong> alone be a reliable measure of “quantity<\/strong>“? Imagine a wooden-looking chair, deceptively made of lead, aboard a spaceship. In the absence of gravity, it would feel like nothing to lift. And yet, if you gave it momentum\u2014a quantity of motion\u2014you would definitely feel the force when it collided with your hand.
This tells us something important: mass alone does not determine an object\u2019s effect\u2014motion does. In Newton\u2019s idea of quantity of motion, two elements come together: mass and speed. Their product\u2014their multiplication\u2014is what we now call momentum, and it was this very principle that Newton took as his starting point for understanding motion.
Now, let\u2019s explore this idea in the context of rotation. When an object moves in a straight line, it carries linear momentum, but in a curve, things change. A centripetal<\/strong> force pulls the object inward to keep it on its circular path, while an opposing centrifugal <\/strong>tendency resists this, trying to push it outward.
This interplay between forces is strikingly evident when liquids of different densities are spun rapidly. If a tube holds two fluids liquids, that would not mix well\u2014one heavier, one lighter\u2014and is set into motion, the denser liquid shifts outward, while the lighter liquid remains nearer the axis. For a given speed, the heavier liquid possesses greater momentum (or quantity of motion), causing it to be flung further outward. A similar effect occurs with solid objects in liquid\u2014a metallic ball moves outward, while a wooden ball, paradoxically, shifts inward, its lower mass resulting in a weaker outward force.
This reveals something fundamental about motion: momentum is not just about mass\u2014it is about how mass interacts with force and motion itself. Just as a wooden chair that feels weightless in space can still hit hard if thrown, the denser liquid or heavier ball moves outward more forcefully in rotation, showing that momentum influences not only the impact of motion but also how objects rearrange themselves when movement is curved.
Whether it\u2019s a swing slowing at its peak, the unexpected heaviness of an object when lifted, or the way substances behave in a spinning system, all of these phenomena trace back to the same principle. Motion is not just a property of objects\u2014it is a relationship between mass, velocity, and force.
Having read a great many old physics\u00a0books,\u00a0I feel at liberty to introduce an older term into our musings–one that carries a certain elegance in language and may help us restore a way of speaking that once made ideas more\u00a0accessible.\u00a0In modern literature, inertia is often described as resistance<\/strong> to motion, but perhaps we could reclaim an older\u00a0expression,\u00a0one used by the brilliant yet often overlooked British mathematician Oliver\u00a0Heaviside. Just as we intend\u2014throughout this article and hopefully in a future book\u2014to recover the clarity of older scientific language, we will use the term “reluctance”<\/strong> in place of resistance to motion<\/strong>, which fundamentally means inertia<\/strong>.
By doing so, we emphasize not just a passive property of mass, but an active unwillingness<\/strong> to accept changes in motion, a concept that resonates with both mechanical and electromagnetic principles<\/strong>. In this sense, reluctance (or inertia) and momentum stand on opposite sides of the equation<\/strong>, defining motion and its resistance in equal measure.
In this vein, we can reframe our earlier\u00a0thoughts.\u00a0Momentum describes how much motion an object carries, while reluctance (inertia) tells us how unwilling an object is to change its state of\u00a0motion.\u00a0These two ideas stand on opposite sides of an equation, balancing one\u00a0another.
Think of momentum as the quality of motion, the property that allows movement to persist and be\u00a0transferred.\u00a0Think of momentum as the quality of motion<\/strong>, the property that allows movement to persist<\/strong> and be transferred<\/strong>. But if momentum is what carries motion forward, then reluctance<\/strong>, in the sense Oliver Lodge described, is the tendency to overshoot<\/strong>\u2014<\/a>to continue beyond the intended point, resisting any attempt to bring motion to a halt.
Lodge\u2019s idea reframes inertia not as mere resistance to movement, but as an insistence on continuing<\/strong>, even when external forces try to restrain or redirect it. A moving body does not simply stop when a force ceases\u2014it carries on, often beyond<\/strong> the expected limit, overshooting the target before finally settling into equilibrium. This is something we feel<\/strong> every day.
A train that comes to a stop still causes passengers to lean forward<\/strong>, their bodies overshooting<\/strong> the halt because their motion is not immediately canceled. A swing, rather than stopping at its highest point, momentarily defies gravity<\/strong>, stretching just beyond before reversing direction. Even in electrical circuits, current does not instantly cease when a switch is turned off\u2014it lags, overshooting before stabilizing<\/strong>.
This brings us back to the duality of motion<\/strong>: momentum sustains movement, while reluctance resists change\u2014but that resistance does not simply end motion, it causes it to push beyond, to linger, to overshoot.<\/strong> Lodge\u2019s perspective reminds us that inertia is not just a passive property but an active persistence<\/strong>, a reluctance not just to start moving, but also to stop.

Reluctance (inertia), is therefore not so much motion itself but rather the unwillingness of an object to accept changes in motion–it expresses how much effort is needed to force a\u00a0change.\u00a0In a straight line, reluctance is tied to mass, but in rotation, it behaves\u00a0differently.\u00a0When something spins, its reluctance to change is shaped not just by how much mass it has, but where that mass is\u00a0positioned in respect to the center of rotation.\u00a0A wheel with its weight concentrated at the rim behaves very differently from one with its weight near the center–both in how easily it spins up and in how strongly it resists being\u00a0stopped.
This balance between momentum and our reluctance (inertia) can be felt when making a sharp turn in a car or an\u00a0airplane.\u00a0You feel yourself pressed sideways, not because a force is throwing you outward, but because your body\u2019s reluctance resists the change in\u00a0direction.\u00a0The vehicle turns, but your body wants to keep going straight, and only the seatbelt or the cabin wall provides the force to pull you into the new\u00a0motion.
Now consider what happens inside an airplane as it comes to a\u00a0stop.\u00a0Small objects–a pen, a bottle–may slide forward along the\u00a0floor.\u00a0The plane slows down, but these objects, lacking friction to anchor them, keep\u00a0moving.\u00a0Heavier objects, on the other hand, show greater reluctance to sudden shifts in motion, making them harder to\u00a0stop.
And so, the two sides of the equation emerge again: momentum sustains motion, while reluctance resists its\u00a0change.\u00a0This same principle governs the behavior of liquids with different densities when spun\u00a0rapidly.\u00a0If a tube holds two immiscible liquids–one heavier, one lighter–and is set into motion, the denser liquid shifts outward, while the lighter liquid remains closer to the\u00a0axis.\u00a0The heavier liquid carries more momentum but also exhibits greater reluctance–so when a force tries to alter its motion, it does not bend easily; instead, it moves\u00a0outward.\u00a0A similar effect occurs with solid objects in liquid–a metallic ball moves outward, while a wooden ball shifts inward, its lower mass giving it less reluctance to being\u00a0moved.
Momentum is not just about motion–it is about how motion interacts with\u00a0reluctance.\u00a0Whether it\u2019s a swing slowing at its peak, the unexpected weight of an object when lifted, or the way substances arrange themselves in a spinning\u00a0system,\u00a0all of these phenomena trace back to the same equation, where momentum and reluctance stand opposite one another, defining the nature of motion\u00a0itself.
To give the reader a sense of the mathematical relationships at play without diving into formal equations, we can use simple proportional thinking and intuitive\u00a0balance.\u00a0Let\u2019s construct a makeshift math–one that doesn\u2019t rely on symbols but still lets us grasp the structure of these\u00a0ideas.
1. The Balance Between Momentum and\u00a0Reluctance
Let\u2019s say you\u2019re trying to push a heavy door\u00a0open.\u00a0The amount of effort you need depends on two\u00a0things:
How heavy the door is (its reluctance to\u00a0motion)
How fast you want it to start moving (its change in\u00a0momentum)
A heavier door (more reluctance) requires a stronger push to get moving at the same speed as a lighter\u00a0one.\u00a0On the other hand, a faster push (more momentum) means you can make even a heavy door move more\u00a0easily.
So we see a trade-off: if reluctance is high, you need more momentum to create the same\u00a0effect.\u00a0If reluctance is low, less momentum is needed to cause\u00a0motion.
Now imagine this in a rotational sense–think of a merry-go-round at a\u00a0playground.\u00a0If all the kids sit near the center, it\u2019s easy to start spinning\u00a0it.\u00a0But if they all move to the edge, suddenly, it feels much harder to get it\u00a0turning.\u00a0Why? Because reluctance in rotation depends on how mass is\u00a0distributed.
If we imagine this as an equation without writing one, it might look\u00a0like:
momentum change required = reluctance \u00d7 how fast we change\u00a0motion
This means if reluctance is doubled, it takes twice the effort (momentum change) to make the same shift in\u00a0motion.
2. Why Objects Behave Differently in a Spinning\u00a0System
Now, let\u2019s revisit the spinning liquids\u00a0example.\u00a0Imagine we have two types of marbles–heavy metal ones and light wooden ones–inside a bowl, and we start spinning the bowl\u00a0quickly.
The heavier marbles (more reluctance) don\u2019t want to change their motion as easily, so instead of bending with the rotation, they push outward as if trying to maintain their straight-line\u00a0motion.
The lighter marbles (less reluctance) follow along more easily, staying closer to the\u00a0center.
So, if we think of outward motion in the spinning system, we can\u00a0say:
How far an object moves outward is linked to its reluctance and how much motion it originally\u00a0had.
This is why, in a washing machine, heavy clothes get pressed against the drum more forcefully than light ones–their greater reluctance makes them less flexible in following the circular\u00a0motion,\u00a0so they get flung\u00a0outward.
3. How This Connects to Newton\u2019s Third\u00a0Law
Every time we push something, it pushes back on us with the same force–this is Newton\u2019s Third\u00a0Law.
If you push a wall, the wall pushes back, stopping you from going through\u00a0it.
If you push a ball on ice, it moves away because nothing is pushing back as\u00a0strongly.
Now think of the airplane braking example from\u00a0earlier:
The heavy objects on the floor resist the stopping motion more because of their greater reluctance, so they don\u2019t slow down as\u00a0quickly.
The lighter objects get stopped more easily because they have less reluctance, so they slide more\u00a0readily.
This also explains why inertia dampers in spacecraft or vehicles use heavy spinning wheels–their reluctance makes them resist changes in motion, helping stabilize\u00a0movement.
4. The Big Picture: Momentum vs. Reluctance is the Backbone of\u00a0Motion
So, in every\u00a0case:
Momentum is what carries motion\u00a0forward.
Reluctance is what resists changes in\u00a0motion.
The greater the reluctance, the harder it is to change\u00a0momentum.
The greater the momentum, the harder it is to stop or change\u00a0direction.
This trade-off is what governs every motion we see, from a swing at the playground to the arrangement of substances in a rotating system, to even the motion of galaxies on a cosmic\u00a0scale.
A Simple Thought Experiment for the\u00a0Reader
To check if you truly understand these ideas, imagine\u00a0this:
You are spinning in an office\u00a0chair.
Your arms are stretched out\u00a0wide.
Now, you pull your arms\u00a0in.
What\u00a0happens?
If you feel yourself speeding up, you\u2019ve just experienced how reluctance in rotation works–pulling mass closer to the axis reduces reluctance, so your motion responds\u00a0faster.
If you let your arms back out, you slow down because reluctance increases, demanding more effort to change\u00a0speed.
That\u2019s the invisible equation of motion at work–whether in a child\u2019s game or the motion of\u00a0planets.




<\/p>\n","protected":false},"excerpt":{"rendered":"

When we accelerate in a plane for takeoff, we feel ourselves pressed into the seat\u2014this is Newton\u2019s Second Law in practice. As linear momentum changes, we experience the opposite reaction. A similar sensation occurs just after landing, though in reverse\u2014a sudden deceleration as the plane slows down.Similarly, on a childhood swing, the same reaction plays […]<\/p>\n","protected":false},"author":1,"featured_media":0,"parent":0,"menu_order":0,"comment_status":"closed","ping_status":"closed","template":"","meta":{"footnotes":""},"class_list":["post-55","page","type-page","status-publish","hentry"],"_links":{"self":[{"href":"https:\/\/sebstack.com\/index.php\/wp-json\/wp\/v2\/pages\/55","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/sebstack.com\/index.php\/wp-json\/wp\/v2\/pages"}],"about":[{"href":"https:\/\/sebstack.com\/index.php\/wp-json\/wp\/v2\/types\/page"}],"author":[{"embeddable":true,"href":"https:\/\/sebstack.com\/index.php\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/sebstack.com\/index.php\/wp-json\/wp\/v2\/comments?post=55"}],"version-history":[{"count":1,"href":"https:\/\/sebstack.com\/index.php\/wp-json\/wp\/v2\/pages\/55\/revisions"}],"predecessor-version":[{"id":56,"href":"https:\/\/sebstack.com\/index.php\/wp-json\/wp\/v2\/pages\/55\/revisions\/56"}],"wp:attachment":[{"href":"https:\/\/sebstack.com\/index.php\/wp-json\/wp\/v2\/media?parent=55"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}