Gigantic pumpkins and physics

From NPR...

October 23rd, 2010

Pumpkins of the Atlantic Giant variety can weigh more than 1,800 pounds. For a mechanical engineer with an interest in plus-sized fruit, like Georgia Tech's David Hu, this raises an interesting physics question: How can the pumpkin get so big without breaking?

IRA FLATOW, host:

Time now for Flora Lichtman. Hi, Flora.

FLORA LICHTMAN: Hi, Ira.

FLATOW: Our Video Pick of the Week, and it's a beauty this week.

LICHTMAN: It's a big one.

(Soundbite of laughter)

FLATOW: It's - oh, it is a big one.

LICHTMAN: It's a seasonal specialty here at SCIENCE FRIDAY. We are looking at giant pumpkins.

FLATOW: Now, we've looked at them before.

LICHTMAN: And, yeah, we've looked at them before. But the story never ends, apparently, with giant pumpkins. So these are the massive Atlantic giant breed of pumpkin that grow to, like, 1,800 pounds.

FLATOW: Wow.

LICHTMAN: And the last time we talked about this - and you can see that video, too - we talked to growers about how they get them so big. And they all have these really funny...

FLATOW: Mm-hmm.

LICHTMAN: ...secret cultivation tricks, like putting blankets on them and...

FLATOW: They do treat them very...

LICHTMAN: They treat them very well.

FLATOW: ...lovingly, yes.

LICHTMAN: Like children, is what they told me. And this week, brand-new research under review, it came out, from David Hu, who is a mechanical engineer at Georgia Tech. And...

FLATOW: Let me just remind everybody that this is SCIENCE FRIDAY from NPR. I'm Ira Flatow here with Flora Lichtman, talking about our Video Pick of the Week. I didn't mean to interrupt you. It's just to pay the bills.

LICHTMAN: Oh, no. I appreciate paying the bills.

(Soundbite of laughter)

FLATOW: So you - you've talked with?

LICHTMAN: So David Hu is this mechanical engineer. So I look at a giant pumpkin and I think, hmm, how many pies could that make? Are they delicious? But he's, you know, he's a physicist, a mathematician. He looks at a giant pumpkin and says, how does it not break under its own weight...

FLATOW: Right.

LICHTMAN: ...because it's so massive?

FLATOW: Yeah. These are, like, sumo pumpkins, right?

LICHTMAN: That's actually a perfect description, because they have that sort of very floppy shape...

FLATOW: Right.

LICHTMAN: ...and that's what he looked at. He modeled the shape of these giant pumpkins to understand the physical forces that sort of lead to this kind of, you know, it's sort of like a pancake.

FLATOW: Right.

LICHTMAN: It really flops over. It's not like a sphere. And his finding was, I thought, really interesting. So his prediction is that the reason they get this droopy shape is because the parts that feel the most stress of gravity, those areas of the pumpkin actually grow more, more quickly, than the other parts of the pumpkin. So the reason why it's not a sphere is because it's growing at different rates in different places, and these internal forces are causing it to grow faster in some places. And the reason why it grows faster in these places of high stress is because it's actually physically pulling the cells apart so they can divide more quickly.

FLATOW: Wow. And you actually have some stop action or what would - a video of a pumpkin growing this way. And it's quite interesting to look at.

LICHTMAN: Yeah. So part of what they did was model it so you can see this sort of mathematical model of a pumpkin growing, and they have really neat time-lapse video of one of these Atlantic giants growing. And that's kind of amazing because you see it start from this little thing and then over the course of, you know, a couple of months, it becomes sumo sized.

FLATOW: And it's amazing. And he's found this sort of a ratio in the size or the width of the - how high it is or something like that?

LICHTMAN: Yes, I mean, he's taken a mathematical approach. I asked him how he got interested in giant gourds and he said that he's really - he's always been interested in plus-sized fruits and, you know, this is applying his body of knowledge...

FLATOW: Oh, we could go so many directions with that.

(Soundbite of laughter)

LICHTMAN: Let's not. Let's just go this one direction.

FLATOW: And so he grows these himself or does he just - he hasn't got the talent for growing them, right?

LICHTMAN: Well, I don't know - I have no idea of whether he's tried or not, but that's a good point. He got growers from across the nation to measure their pumpkins for him.

FLATOW: Right.

LICHTMAN: And, you know, there are people who do this all over, and these weigh-ins all over. And not only do they measure and send in the measurements over the course of the growing season, but they also hollowed them out. So one of the coolest things that I think he showed was what the inside of one of these giant pumpkins looks like, if they're sort of cut crosswise.

FLATOW: Right. And you could - and it's very interesting because he took out a tape measure in one direction, and he's really studying the science of how these things grow so big without breaking.

LICHTMAN: Right. That's the question. I mean, you know, as they get bigger, gravity, the force of gravity sort of tugs on - has more to tug on. And the question is how do these - how do the other forces stand up?

FLATOW: And so the pumpkin has figured out how to do this by the way it divides in cells and things like that.

LICHTMAN: Yeah. I mean, that's so - the weight causes it to grow in a certain way and then it has to look that way to survive, basically.

FLATOW: And if you'd like to look that way or see how that way looks...

LICHTMAN: Thank that you don't look that way is more like it.

(Soundbite of laughter)

FLATOW: Go to our website at ScienceFriday.com, where we have a video of these giant - these pumpkins, giant pumpkins. You can see the cross section, how they're constructed, how the pumpkin knows how to grow that way and has figured it out, and a little video of watching it actually grow from the inside. It's quite fascinating.

LICHTMAN: Yeah. And if you have any fruits that you've grown, big or small, send them our way.

FLATOW: And we'll put them on our website. Thank you, Flora.

LICHTMAN: Thanks, Ira.

FLATOW: Flora Lichtman, our multimedia editor.