Say I’m standing on a scale in the middle of the room and I weigh 150 lbs. Why is it that when I crouch down and apply force to ready a jump, the scale jumps to 200+ lbs even though it’s still my body weight?
Does this kinetic energy make me heavier for a moment? Why does the scale read so many extra lbs?
In: Physics
Scales like this don’t measure your body weight. They measure the force the the scale exerts on you. When you are just standing there, gravity pulls down on you with a force that equals your weight and the scale pushes up on you with this same force, and you stay in place and the force the scale exerts on you is just your weight, so the scale reading matches with your weight.
When you push off, you add an additional downward force from the muscles in your legs pushing down on the scale. By Newton’s 3rd law, the scale then pushes back on you with enough additional force to balance both your weight and the force you are exerting on it. So if you weigh 150lbs and you push down with an additional force of 50lbs using the muscles in your legs the scale will register a value of 200lbs, which no longer matches your weight.
Additionally, since the scale is now pushing up on you with a force of 200lbs and gravity is only pulling down on you with a force of 150lb, there is a net upward force on you of 50lbs and you will accelerate upward with an acceleration of g/3, where g is the acceleration due to gravity and has a value of about 9.8m/s^2 or 32 ft/s^2
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