[hzay]

> Tense muscles will exert more force on the things they're attached to like bones and organs, usually in opposing directions, which is what causes damage from otherwise trivial impacts. That's why, all other things being equal, drunk drivers experience fewer injuries in auto accidents - slower reaction times mean they don't react in time to tense up for the crash.

Wow, is this true? I've noticed that when small babies are picked up, there's a difference in their weight, depending upon whether they want to be picked up or not. Like, by doing something with their body, they are able to make themselves lighter or heavier -- perhaps by clinging more (or less) to the person picking them up, and perhaps by tightening (or loosening) their muscles.

I'm a pilates instructor, if you have any sources that discuss the effect of tense muscles & force, I would really appreciate it. Thanks :)

[delfinom]

Babies don't magically change weight. It's just when they are fighting being picked you are probably being forced into a position with less mechanical advantage.

[hzay]

It doesn't seem to make sense logically, if you think of weight as mass * gravity. But hypothetically speaking, if the baby can do this -- https://www.youtube.com/watch?v=f5Wks63Ng0Y -- don't you think the weight will be different to whoever is picking the baby up?

We don't pick up the baby as a single movement. If we did, then yes there can be no magical weight change. But in reality, there's a bit of a choreography involved. Perhaps the baby holds up its arms, you put your arms below its shoulders, you raise it slowly. During this process, surely it can control how much of its weight you perceive?

[curryst]

You're right, just using the wrong terms. The baby's weight is fixed at mass * gravity, it doesn't change. What changes is the force required to accelerate the baby upwards, which is the delta between the force of gravity (the baby's weight) and any upward forces opposing that gravity. So if the baby pushes upward with it's feet to help you, you have to exert less force to lift the baby.

Scientifically speaking, the weight of an object cannot change unless gravity changes or it's mass changes. The forces required to accelerate that object in a particular direction vary wildly based on all kinds of things that exert a force on the object.

Off topic, but that does make me curious how "weight" works in an environment where you're exposed to more than one gravitational field. Like if you're close enough to two black holes to be affected by the gravity from both.