This post originally appeared on Randi.org
If you have a friend who likes to lift weights, it’s time to make a bet. Bet them that you can use your supernatural abilities to change the gravity around your body, making yourself impossible to lift. And make sure it is a good bet, because you are going to win this one.
When Penn and Teller hosted Penn & Teller Tell a Lie on the Discovery Channel, they featured a woman who claimed exactly that: that she could make herself impossible to lift, even for Muscle Beach body builders. The demonstration was pretty convincing. None of the macho men could lift her once she activated her “powers.” She had all the would-be lifters stumped. She fooled everyone except a physics professor.
So before you bet the farm with your willing friend, let’s look at the physics behind this little trick. At least you won’t be engendering some kind of belief in the supernatural once you fleece your lifter.
The Moment of Deception
The key to making yourself impossible to lift is to subtly control the other person’s movements, as great magicians always do. Get a friend willing to lift you and position yourself as the woman in the Tell a Lie episode did:
She steps in very close to the lifter’s body and lets him proceed (if you watch the video above closely, you can even see her push off a bit on the lift). A woman of 112 pounds, she is lifted easily by these body builders who bench three times that weight.
Now for the trick. Ask your friend to lift you again, but this time puts your hands on their arms so that you can position yourself further away from them, like the woman does:
In this position (at least a foot back from the previous lift), she is impossible to lift. Four or five bodybuilders fail to do so.
How does she do it? Penn and Teller get the physics right, but leave some stuff out. What changes from the first lift to the second is that the woman moves her weight further away from the lifter. Penn and Teller say that she is harder to lift when her “center of gravity changes,” but that’s not the whole story.
What we are really dealing with here are moments. As I explained the last time I wrote about moments, we don’t always realize how much a force can change depending on its distance. A moment is a rotational force that increases or decreases with distance. For example, you can easily hold up a gallon of milk, but try holding up a gallon of milk for even a few seconds with a fully extended arm. The further the milk is from your shoulder, the more rotational force you have to generate in your shoulder to lift the jug. This is because a moment is force multiplied by distance.
So there are two reasons why the bodybuilders couldn’t lift the woman. First, as she moves her body further away during the second attempt, she is effectively tripling her weight. For example, if she is six inches away from the lifter’s shoulders in the first attempt and a foot and a half away in the second, the rotational force needed to lift her up goes from 56 foot-pounds to 168 foot-pounds.
168 foot-pounds doesn’t sound like that much for a body builder to generate, but that is where the physiological reason she is “impossible” to lift comes in. In the first attempt, the lifter’s arms are bent enough that he can use his biceps to aid him. In the second attempt, his arms are extended so that all the rotational force has to come from the smaller muscles in the shoulder instead. The combination of increased forces and smaller muscles is what makes the woman stick to the ground, nothing supernatural.
The “impossible to lift” trick is a simple one if you can perform it effectively. Remember that the lifter must be unaware that you are positioning yourself further away from him or her on the second go. The woman in the video is even perhaps taking advantage of the fact that when she moves her body away, the lifters don’t want to pull her closer or position themselves better for fear of looking “weak” or of how tugging on the woman on camera would look. Trickery is all part of the game.
If you do decide to pull this little trick on your friends, at least debrief them afterwards and explain the physics involved. They will be out of a few bucks, but at least they will have learned something.