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hi i want to ask why we feel loss and gain of weight when we`re in a lift?

We judge our weight by the forces exerted on us by the surfaces surrounding us called "normal" forces. When a lift is accelerating upward the floor of the elevator has to do two things to you: it must support your weight PLUS it must accelerate your body upward. That means that the force of the floor on your feet as you accelerate upward must be greater than your weight alone.

For example, the normal acceleration of gravity is approximately 10 meters/second^2. If your normal perceived weight is 200 pounds and if you are also accelerating upward at 2 meters/second^2 the force of the floor of the elevator on your feet will have to supply as additional 40 pounds of force to provide the upward acceleration. Therefore, the upward force of the floor on your feet will need to be 240 pounds: 200 pounds to support your weight plus 40 pounds to accelerate your body upward.

As the elevator accelerates to rest the acceleration might be -2 meters/second^2. Now the floor will need to supply 40 fewer pounds of force to your feet. Therefore, as the elevator accelerates to rest the floor of the elevator will only have to supply a force of 200 - 40 = 160 pounds of force to your feet. Therefore, you will "feel" lighter than your normal weight.

Physics

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I am teaching or have taught AP physics B and C [calculus based mechanics & electricity and magnetism] as well as Lab Physics for college bound students. I have a BS in Physics from the University of Pittsburgh and a Master of Arts in Teaching from same. I have been teaching physics for 34 years. I am constantly updating my skills and have a particular interest in modern physics topics.