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Why do people feel heavier when they tense up their muscles?
Week of June 15th Question
The first weekly Stump the CPE question has been selected, and luckily I won't be stumped this week!
I have actually pondered this question before. We all as children become quickly aware that if someone is trying
to pick you up if you tense up you can make yourself much harder to move! Obviously, by tensing your muscles you
can't actually be gaining any weight that would violate physical laws! So then why do people suddenly feel heavier!
It comes down to several simple concepts of mechanics, impulse, momentum, and body rigidness. When a person does
not have their muscles tensed their body is far less rigid. Therefore, when someone tries to lift the relaxed
person, the force exerted is able to get the person moving incrementally. The body "stretches" as it is lifted
and therefore, the force required to start moving the person is less. The relaxed body therefore allows the person
lifting them to apply less force immediately and slowly increase that force over time (impulse). The time of the lift
is therefore increased until the force and energy applied is enough to lift the person.
If the person is tensed up they become much more rigid. Their body will not "stretch" like when they are relaxed and
therefore the immediate force applied to change the motion or start motion in the person being lifted is much higher.
Why is the immediate force higher? Instead of having milliseconds to apply the force, the rigid body requires all
the force be applied at once.
Mathematically speaking, the longer the time allowed to apply
the force the less force required. Fdt = mdv
The less rigid an object the more time available to start motion in that object. As time increases the instantaneous
force can be less and still provide the same manipulation of an object.
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