Everyday Q&A: How to Lose Weight

April 6, 2008

“Why do astronauts floating in space feel weightless?” “Why does riding that rollercoaster give me the sensation that my organs are lurching out of my body?” “I feel light and skinny when the elevator starts going down!”

To explain these phenomena, a clearer definition of “weight” is needed. When we stand on a scale, what we are really considering is the normal force of the scale pushing up against our feet. You sense your weight only because there is some contact force pushing up against your body. Standing on the sidewalk, the concrete exerts the normal force on the soles of you feet and you feel weight.

Logically, a change in the strength of this contact force will inevitably lead to a change in your sensation of weight. As Newton famously states in his second law, force equals mass times acceleration; hence, if the acceleration changes, so does the force.

When an elevator starts moving down, our natural acceleration in the upwards direction decreases from the usual acceleration due to gravity, and, with it, so does the normal force and thus your weight. This weight decrease results in the light, bouncy sensation at the start of a downward elevator ride.

Weightlessness occurs in the absence of a contact force. Astronauts experience this feeling in the void of space because they do not encounter any force pushing against them. Similarly, we feel weightless when we fall because there is no normal force. As a rollercoaster rapidly accelerates, the force of the seat against our bodies varies intensely, causing lurching sensations of weight in different directions.

Thus, like many other transient quantities in modern physics, “weight” is also relative, a measure depending on the situation at hand.