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StarTalk Radio

Things You Thought You Knew - Force, Heat, & Speed

Published November 18, 2025
View Show Notes

About This Episode

Neil deGrasse Tyson, joined by comedian co-host Chuck, explores three commonly confused physics pairs: force versus pressure, heat versus temperature, and speed versus acceleration. Using everyday examples like gym spotting, walking on ice, kitchen knives, tornado damage, ocean warming, air conditioners, and sports cars, he shows how precise definitions change how we understand real-world phenomena. The conversation emphasizes how these distinctions explain everything from why houses explode in tornadoes to why Teslas feel so fast and why the ocean can store vast amounts of heat.

Topics Covered

Mathematics and physics Science communication Force and pressure Heat and temperature Speed and acceleration Tornadoes Air conditioning and heat pumps Hurricanes

Disclaimer: We provide independent summaries of podcasts and are not affiliated with or endorsed in any way by any podcast or creator. All podcast names and content are the property of their respective owners. The views and opinions expressed within the podcasts belong solely to the original hosts and guests and do not reflect the views or positions of Summapod.

Quick Takeaways

  • Force is a push or pull that can change an object's motion, while pressure is that force divided by the area over which it acts.
  • Sharpened knife blades and snowshoes work by changing area, thereby changing pressure without changing the total force.
  • Temperature is the average kinetic energy of particles, whereas heat is the total energy summed over all particles in a system.
  • The ocean can have far more heat than a much hotter cup of coffee because it contains vastly more molecules, even if its temperature increase is small.
  • Air conditioners and heat pumps work by moving existing heat from one place to another, not by creating cold.
  • What people often call "motion sickness" is more accurately a response to accelerations, not to constant speed.
  • Thrill in fast vehicles comes from acceleration, changes in direction, and "jerk" (changes in acceleration), not from steady high speed.
  • Tornadoes and bombs destroy structures by creating large pressure differences across walls, resulting in huge total forces spread over large areas.

Podcast Notes

Introduction to the "Things You Thought You Knew" episode

Preview of topics: confusing physics pairs

Episode will cover force vs pressure, heat vs temperature, and speed vs acceleration[1:42]
Framed as another installment of the "Things You Thought You Knew" series[1:32]

Show opening

Neil welcomes listeners to StarTalk[1:18]
StarTalk is described as a place where science and pop culture collide[1:55]

Force versus pressure

Everyday and physics meanings of force and pressure

Neil introduces force and pressure as the first topic[2:13]
He distinguishes physics pressure from emotional or social pressure[2:18]
• Example of emotional pressure: being asked "what are we doing here" after four years of dating and repeated family holiday visits
Notes that words like force and pressure are also used in military and cultural contexts (e.g., "force on the battlefield", "space force")[2:33]
Emphasizes that each word has a precise physics definition that differs from everyday usage[2:45]

Definition of force and Newton's second law

Force is described as a push on something that can set it into motion or break it if it's fragile[3:08]
Neil states that forces make things happen by changing something about an object, typically its motion[3:15]
References Isaac Newton's equation: force equals mass times acceleration[3:30]
• Explains that you use F = m·a to predict the acceleration an object gets from a given force
Introduces the idea of net force: opposing forces can cancel so that no acceleration occurs[3:50]
• If someone pushes one way and another person pushes exactly opposite with equal strength, the net force is zero and nothing accelerates
Even with very large opposing forces, if they're balanced, there is no motion; slight imbalance produces motion[4:04]

Gym spotting example to illustrate balanced forces

Neil recalls a previous discussion about why a spotter in the gym does not need to be as strong as the lifter[3:58]
Chuck jokes that big lifters ask tiny spotters for help and he feels inadequate to save them[4:56]
Typical scenario is a bench press, where the weight is above the lifter's neck, making spotting important[5:18]
• Neil contrasts bench press with rowing-type lifts where you can just drop the weight safely
At the failure point of a lift, the lifter's upward force equals the downward weight, so the bar stops moving[5:55]
• In that moment of stalemate, the bar is motionless because forces are balanced, even though the lifter is exerting maximum effort
The spotter then needs only a small additional upward force to break the balance and move the bar to the rack[6:25]
• Neil says the spotter might even use two fingers and still successfully help, as long as the lifter has not started losing the battle and letting the bar fall
If the bar is already descending, the spotter must first counteract that downward acceleration and then add more force to raise it[7:32]

Balanced forces versus acceleration

Neil reiterates that forces can be balanced even when an object is in motion; in that case it moves at constant velocity[7:47]
If there is a net force, the object's speed will continually change, meaning it accelerates[7:32]
Car example: pressing the accelerator while maintaining constant highway speed[8:32]
• If the car stays at, say, 55-60 mph, the engine's driving force is balanced by tire friction and air resistance
• To accelerate and pass another car, you must press the pedal harder to upset that balance and create a net forward force

Definition of pressure and its equation

After joking about dating pressure, Neil turns to physics pressure[9:32]
He says pressure needs force to be what it is, but is not the same thing as force[9:36]
He introduces the equation for pressure: pressure equals force divided by area[10:35]
• Chuck immediately recognizes the equation as making intuitive sense once stated

Walking on ice and snowshoes: pressure and area

Neil asks whether you will fall through ice when walking on a frozen pond and says it is "all about pressure"[10:55]
If your feet have a small contact area, the same body weight force is divided by a small area, creating high pressure[11:15]
• He explains that putting a smaller number in the denominator of a fraction makes the overall value larger, so smaller area yields higher pressure
High pressure increases the chance of punching through the ice and falling in[11:15]
To avoid breaking the ice, you want to spread your weight over the largest possible area, like wearing clown shoes or snowshoes[11:35]
• Snowshoes are described as large, net-like attachments that distribute your weight across a bigger surface so you don't sink deeply into snow
Neil notes that polar bears have very wide paws, which helps prevent them from sinking into snow and ice[12:33]

Knife sharpness as a pressure phenomenon

Cutting with a knife involves applying a force, but the effectiveness depends on the area of the blade edge[13:09]
To maximize cutting effectiveness, you want the smallest possible edge area so the same force creates maximum pressure[13:09]
A dull knife edge is microscopically chewed up, flattened, and thick, spreading the force over a larger area and reducing pressure[13:09]
• With a dull blade, even 10 pounds of force is spread along the length of the imperfect edge, so food gets mangled and you have to push harder
A sharpened blade has an extremely tiny edge area, so even a mild force creates high pressure that cuts food easily[13:54]
Chefs frequently sharpen their knives to increase the pressure on food without having to exert more force[14:01]
• They effectively reduce the contact area to get the desired cutting pressure

Pressure and destructive forces: tornadoes and bombs

Neil says the distinction between force and pressure appears everywhere and gives tornado damage as an example[14:28]
He explains that tornado centers have very low air pressure compared to the inside of a house[14:54]
He considers a pressure difference of about a tenth of a pound per square inch between inside and outside air[14:59]
• Each square inch of wall feels an outward push of that amount due to the higher interior pressure
Scaling up: 10 square inches gives about a pound of total outward force; 100 square inches (10 by 10 inches) gives about 10 pounds[15:37]
Over an entire wall, those small-per-inch forces sum to thousands of pounds of total force pushing outward[16:04]
Typical house walls are designed to hold up the house or withstand people leaning on them, not to resist thousands of pounds of outward force from pressure[16:15]
Video of tornado damage shows houses exploding outward into matchsticks, rather than simply collapsing, illustrating pressure effects[16:21]
Bombs also work via pressure waves: a sudden high-temperature expansion of gas creates a rapidly expanding air front[16:54]
• In guns, expanding gas drives a bullet; in bombs without bullets, the expanding air itself hits walls with high pressure
If there is much higher air pressure on one side of a wall than the other, the wall can blow inward or outward depending on where the explosion is[17:17]
If the entire force of a blast were concentrated at one tiny spot, it would just punch a hole rather than blow out the entire wall[17:34]
Neil jokes that you don't need special tools like a "tornado airbag" to detect a tornado; you can just look at it[18:00]
He concludes the force-versus-pressure segment, calling it "very cool"[18:24]

Heat versus temperature

Common confusion between heat and temperature

Neil says the difference between heat and temperature is a major source of misconception in civilization[21:37]
Chuck admits that for him, heat and temperature have always seemed like the same thing[22:00]

Physicist's definition of temperature

Neil defines temperature as the average kinetic energy of vibrating molecules (or atoms)[22:17]
In any material, all particles are vibrating; the thermometer reads the temperature when that vibration is communicated to it[22:42]
Temperature is a macroscopic property derived from many particles, not a property of a single particle[22:54]
• Neil explicitly states that a single particle "has no temperature"
At a given temperature, some particles move slower and some faster; temperature is the average over this distribution[23:29]
Example: room-temperature water (around 70-75°F) contains molecules moving at a range of speeds[23:54]
Some water molecules at the surface move fast enough to escape into the air, causing evaporation even below boiling point[24:25]
• Neil notes that the fastest-moving molecules are always escaping, so water can evaporate without boiling
Lower-mass molecules move faster on average than higher-mass molecules at the same temperature[24:39]
• He describes the atmosphere as containing oxygen and nitrogen, where oxygen molecules are slightly heavier
If you separated oxygen and nitrogen, the oxygen would be at a lower temperature on average, but mixed together the gas has a single temperature[25:29]

Physicist's definition of heat

Neil defines heat as the sum of the kinetic energies of all the vibrating molecules in a system[26:05]
Chuck clarifies that temperature is an average, while heat is the total of all those individual energies[26:10]
Neil illustrates with a comparison between a hot cup of coffee and the ocean[27:12]
• A 210°F cup of coffee is hotter than the ocean in terms of temperature
• Despite being cooler, the ocean contains much more heat because it has far more molecules whose energies are summed
Because the coffee's total heat content is small, it cannot start a hurricane, whereas the ocean can[27:12]
Chuck jokes that coffee can still affect your morning by spilling in your lap or speeding up your digestion[27:12]

Heat, temperature, and climate change

Neil connects the heat-temperature distinction to climate change discussions[27:41]
Air temperature changes of a couple degrees Celsius are viewed as dangerous thresholds that can trigger other changes[27:53]
Ocean temperature might rise only a quarter or half a degree, which sounds small[28:00]
Neil emphasizes that even a small temperature increase in the vast ocean represents an enormous amount of total energy[28:08]
When creating an energy budget of Earth's climate system, you must account for energy stored in the oceans, not just in the air[28:23]
• Sunlight heats both the atmosphere and the ocean; heat can "hang out" in the ocean as a reservoir
Even if humans reduce carbon footprints and slow atmospheric warming, the ocean can continue to release stored heat into the air[28:43]
Neil says the relationship between heat retained by land, atmosphere, and oceans is currently imbalanced, with the ocean "winning" as a heat reservoir[29:06]

How air conditioners and heat pumps work

Neil asks if Chuck has ever wondered how air conditioners make indoor air cool when it's hot outside[29:27]
Chuck says he just turns it on and was taught not to leave the door open or "cool the whole neighborhood"[29:43]
Neil explains that any room above absolute zero contains heat, regardless of its temperature[30:14]
An air conditioner contains a pump that removes heat from the indoor air and dumps it outside[30:54]
• He notes that the air around the outside unit is hotter because the device is expelling indoor heat there
The same system can be reversed in winter to act as a heat pump[30:46]
A heat pump can take heat from outside cold air (e.g., 40-50°F) and move it indoors to make the room warmer than the outdoors[31:09]
This is possible because there is still heat present at any temperature above absolute zero[31:17]
Neil calls this "clever engineering" and suggests hugging your favorite engineer in appreciation[31:28]
Chuck admits he doubted the segment would be interesting but ends up enjoying it[31:41]
Neil closes with the image of sipping hot coffee while looking at the ocean, reminding listeners the ocean has more heat than the scalding cup[31:56]

Speed versus acceleration (and jerk)

Opening and Top Gun reference

Neil introduces the topic of speed versus acceleration with mock seriousness, telling Chuck "we're going to have to have this talk"[34:16]
He references a memorable scene in Top Gun where pilots say they "feel the need for speed"[34:53]
Neil wants to push back on the idea that what they crave is speed[34:53]

Why speed alone is not what we feel

Neil argues that the pilots' actual speeds are irrelevant to the sensations they enjoy[36:02]
He notes that at their latitude, Earth's rotation carries them east at about 800 mph, far faster than typical vehicles[35:26]
Earth orbits the Sun at about 18 miles per second, vastly faster than aircraft speeds[36:02]
• Neil gives a local example: in one second at that orbital speed, you could travel from New Jersey across Manhattan and Brooklyn and end up in the water
Despite these huge speeds, we do not feel a "need for speed" from them, showing that constant speed does not produce sensation[36:39]
Neil says what people call motion sickness is really "acceleration sickness"[35:54]

Definition of acceleration and its effects

Acceleration is defined as a change in velocity, which can be an increase or decrease in speed[37:38]
In everyday language, negative acceleration is often called deceleration[37:10]
Velocity includes both speed and direction, so changing direction (e.g., banking in a turn) is also an acceleration[37:51]
Neil emphasizes that when you are in a moving object, you only feel something when speed or direction changes, i.e., during acceleration[38:01]
Examples: accelerating forward pushes your body backward; braking hard throws your body forward; turning makes you lean sideways[38:18]
He argues that the Top Gun pilots actually crave the accelerations from barrel rolls and aerobatics, not steady speed[38:37]

Cars, luxury versus sport, and acceleration

Neil mentions that a Lexus was once described as feeling like "sitting on your living room couch" while driving[39:10]
People who "feel the need for speed" are not buying such smooth-riding cars; they want vehicles that accelerate and bank turns vigorously[39:21]
He points out that car specs list both top speed and the time to go from 0 to 50 or 0 to 60 mph[40:06]
• The shorter the time to reach a given speed, the higher the acceleration and the more "head-snapping" the experience
Neil notes that this high acceleration is why people love Teslas and other well-made electric cars[40:37]
• He says Teslas can go from 0 to 60 mph in roughly 3-4 seconds, and that you can clearly feel this rapid acceleration

Introducing jerk: rate of change of acceleration

Neil states that if acceleration is the rate of change of velocity, you can also consider the rate of change of acceleration itself[41:22]
He explains that the rate of change of acceleration is called "jerk" in physics[41:37]
Example: driving toward a brick wall and braking steadily, then suddenly hitting the wall[41:54]
• While braking, you feel a steady forward push into the shoulder strap as speed decreases smoothly
• At the instant of impact, speed goes to zero almost instantly, representing a rapid change in acceleration and producing a sharp jerk
Neil says jerk is what actually causes musculoskeletal damage in accidents[42:37]
He contrasts tolerable constant accelerations (e.g., 1G, 2G) with sudden changes from one G-level to another that cause the body to snap[42:53]
He frames the classic idea that "jumping out of a 20-story window doesn't kill you; it's the ground" as an illustration of abrupt deceleration and jerk[43:05]

Feeling the road: suspension and acceleration sensations

Neil remarks that some sports cars are marketed with the promise that you can "feel the road"[43:36]
If a road were perfectly smooth, you would not feel anything; bumps and irregularities produce accelerations that you feel[43:51]
He contrasts a Lexus, where the suspension and tires smooth out road bumps, with a sports car that has rigid suspension transmitting those accelerations to the driver[43:58]
Sports enthusiasts, consciously or not, seek these accelerations, twists, and banking turns rather than mere high speeds[44:22]
Chuck gives an example of a motorcyclist on the turnpike doing a wheelie at around 80 mph, interpreting that as craving acceleration[44:44]
Neil notes that even from a dead stop, flooring a high-acceleration car causes an initial head snap, another brief high jerk event[45:20]
He cautions that while these examples are illustrative, listeners should not actually test crashing into walls[45:36]

Closing remarks

Chuck jokes that listeners should take Neil's word and not perform dangerous experiments themselves[45:40]
Neil signs off by reminding listeners to "keep looking up"[46:05]

Lessons Learned

Actionable insights and wisdom you can apply to your business, career, and personal life.

1

Precise definitions matter: concepts that seem interchangeable in everyday language, like force and pressure or heat and temperature, can represent very different physical realities with different consequences.

Reflection Questions:

  • • Where in your work or life do you casually use terms that might actually refer to distinct concepts with different implications?
  • • How could slowing down to clarify definitions before making a decision reduce misunderstandings or errors you currently face?
  • • What is one domain this week where you will deliberately check whether you are conflating two related but distinct ideas?
2

Changing how effort is distributed can be more effective than simply applying more effort, just as increasing area to reduce pressure (snowshoes) or decreasing area to increase pressure (sharpened knives) achieves better results without more force.

Reflection Questions:

  • • In what current project are you trying to "push harder" instead of redesigning how the load or responsibility is distributed?
  • • How might redefining the "area" over which your resources, time, or team are spread change the outcome without increasing total effort?
  • • What is one concrete change you could make this week to redistribute effort or focus for a more leveraged result?
3

Small average changes across very large systems can represent enormous total effects, as with a slight rise in ocean temperature holding vast additional heat energy.

Reflection Questions:

  • • Where are you underestimating the impact of a small change because you are not accounting for the scale of the system it affects?
  • • How could thinking in terms of totals (sum of many small parts) rather than just averages change your approach to metrics or goals?
  • • What is one system in your life or business where you should recalculate the total impact of a seemingly minor shift?
4

Our strongest experiences often come from changes and transitions-accelerations and jerks-rather than from steady states, which suggests that managing how quickly things change can be as important as the final state itself.

Reflection Questions:

  • • In which parts of your life are abrupt changes causing unnecessary "jerk" and stress for you or others?
  • • How could you redesign a current change you are going through so that its pace and transitions are smoother and more tolerable?
  • • What is one process you manage where adjusting the rate of change, rather than the end goal, would improve people's experience?
5

Many powerful technologies, like air conditioners and heat pumps, work by intelligently moving existing energy rather than creating it from scratch; similarly, redirecting existing resources can often be more efficient than generating new ones.

Reflection Questions:

  • • Where are you trying to create new resources (time, money, attention) instead of reallocating or repurposing what you already have?
  • • How might identifying and "pumping" underused assets in your environment give you leverage to solve a current problem?
  • • What is one area this week where you can stop adding more and instead reorganize existing resources for better effect?

Episode Summary - Notes by Kai

Things You Thought You Knew - Force, Heat, & Speed
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