Weight Versus Mass: Why You Don't Really Want to Lose Weight

Tyson opens by telling Nice that he has another explainer for him, and the two trade a few playful lines about whether it will be awesome and whether it will inspire awe. Tyson says that inspiring awe is his goal every time, and the exchange ends with laughter before the topic is introduced.

Tyson asks Nice whether he has ever really wanted to lose weight. Nice says yes and keeps the exchange light, saying he still wants to lose weight. Tyson responds that what people want to lose is not weight but mass.

Tyson presents mass and weight as related but fundamentally different quantities, which sets up the rest of the explanation.

Tyson says that he can make Nice lose weight simply by sending him to the Moon, because a person weighs one-sixth of their Earth weight there. He says a person who weighs 30-something pounds on the Moon would need to be placed somewhere else to weigh 160 pounds, and that there would surely be a cosmic object where a person would weigh 160 pounds.

Tyson concludes that weight and mass, while related, are fundamentally different.

Tyson defines mass as the sum of all the material substance of the body, meaning all of its molecules. He adds that mass technically counts the energy of the molecules, but that this energy is very small relative to the mass of the molecules, so it is usually not considered. Tyson explains that the energy in a body has a mass equivalent, and that all molecules have energy because they are moving.

Nice agrees that the explanation makes sense, and Tyson concludes that this is what mass is.

Tyson says that if a person eats less, so that metabolism exceeds intake, and goes to the gym, that person is losing mass. He states that after losing mass, a person has fewer molecules in the body than at the beginning.

Tyson says that if a person only cares about weight, climbing to the top of a mountain lowers it, because the person is farther from the center of the Earth and weighs a little less there. He states that if weight is all one cares about, there are ways of losing weight without changing a thing about the body.

Nice jokes that he calls that lying about his weight and says he does it all the time.

Tyson describes a science-fiction rotating space station that is spun up so that the rotation gives occupants more than 1 G. At 2 G, he says, a person would weigh twice as much as on Earth.

When Tyson asks how much a person weighs if put in orbit around Earth, Nice says nothing, and Tyson agrees that weight is not really the issue there. Nice says people want to be weightless, and Tyson agrees.

Tyson says that in orbit a particularly heavy person's heart does not feel the stress of that body weight. He adds that a person might have cholesterol-filled arteries, but that the muscles and heart do not register how fat the person is.

Nice responds that Tyson is saying people probably do not want to be overweight in space. Tyson says that a person might live longer if overweight in space than if overweight on Earth.

Tyson says the problem with being overweight in space is that there is not much room up there, and that there are no fat astronauts. Nice suggests they stick with overweight, and Tyson jokes that he is going with fat like the president. The exchange ends with laughter and a joke about all of them being fat.

Tyson says that on Earth people do not often distinguish mass from weight and think of them as the same thing. He explains that weight is measured in pounds, which is not a measure of mass, and that there is an imperial measure of mass, the slug, though people never use it.

Tyson says the slug is the official unit of mass in the imperial system and that a slug weighs about 32.2 pounds on the surface of the Earth. He notes that scientists have fundamentally abandoned the slug. He adds that Europe's metric unit of weight is the newton.

Tyson says that a kilogram weighs about 9.8 newtons. He notes that people in Europe do not use newtons in everyday life and instead use grams and kilograms. Technically, Tyson says, these units measure mass, so in common conversation in Europe mass and weight are not distinguished from each other.

Tyson gives the example of a weight stated as 2 kg, 1 g or 6 kg and says that this is actually a measure of mass.

Tyson explains that a scale with a spring measures how much a person's weight squeezes it. He describes putting a person on a spring scale in an elevator and cutting the cable, so that the scale and person fall together. The scale shows zero because the person is no longer squeezing it.

Tyson says a spring scale measures weight, while a balance measures mass. He describes placing a standard gram on one side and adding grams to the other until the two sides balance. This is why drugs are measured in grams, and Tyson notes that the balance would still work on the Moon.

Tyson says that a balance would read the same amount on the Moon as on Earth, so the difference between measuring mass and measuring weight does not matter for the balance. He says this is the difference between measuring the mass of something and measuring its weight.

He adds that if interplanetary commerce ever happens, the distinction will matter, because luggage would be checked by mass rather than weight. The exchange closes with a joke about dogs sniffing luggage and the kilogram dogs.

Tyson says that Earth's mass can be found by adding up everything in it, and he gives the figure of about 6 times 10 to the 24th power, which he says is six septillion kilograms. Earth's weight, by contrast, is the gravitational attraction between Earth and something else.

Nice says he would guess his weight at 175 to 180 pounds, then says he weighs 192 pounds and that it is all where it is not supposed to be. Tyson says that Nice's weight of 190 pounds on Earth is how much Earth weighs on him. Tyson adds that if a person floats in space with a scale under the feet and Earth is placed on it, that reading is how much Earth weighs on the person.

Nice says that since the president has been in office, the Earth weighs on him a lot more, and then says that is all he can say. Tyson says that this is emotional weight and was not what the explainer was about, and the two joke about the president.

Nice says that mass is quick and clean, and then compares Catholic mass, which he says is in and out, with services at a Black church, which he says run long. Tyson closes the episode and signs off.