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

Normal scales does not measure your mass directly but the force created by your weight in the gravity of earth and then use that force value to calculate your weight.

However it can not differentiante between gravitional force and any other forces. If you move downwards, you apply an additional force on the scale and then it thinks that your weight has changed.

If you push your hand on the scale you can make it read a bunch of different numbers based on how hard you push. This doesn’t mean you’re getting heavier or lighter when your hand pushes more or less, it means the scale measures something different from weight.

The scale estimates your weight based on how much force is applied and the acceleration due to gravity at sea level. If you go to the top of a tall mountain you’ll measure less heavy than at sea level on the same scale.

As for why the scale momentarily measures higher, well the kinetic energy being dissipated into the scale is measured as increased force. You’re not heavier for a split second when you crouch down, you just exert more force than when motionless.

This is a good illustration of the difference between *mass* and *weight.*

Mass is more or less constant, although the mass of a human body fluctuates due to food and water intake and urine and fecal output.

Weight is your mass relative to earth’s gravity and the vagaries of kinetic energy acting on your body. When you jump on the scale your kinetic energy temporarily and artificially creates a seeming increase in your weight.

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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