Newton’s laws of mechanics can also be applied from the point of view of an accelerated observer, although for him force is no longer equal to mass times acceleration. In this framework, things seem to behave strangely, even though they are very everyday phenomena.
The apparent weight in an elevator
When starting an elevator, during the brief moments in which it accelerates, we can feel that we weigh a little more if it goes up or a little less if it goes down. It’s not just an impression: if we carry a scale, we can measure the effect. What determines that sensation is the direction of the acceleration: when the elevator starts upwards, the apparent weight increases; when it starts down, it decreases.
This occurs during starting and braking, but not during the rest of the journey. So the elevator moves at a constant speed and therefore we are inertial observers. When braking, the acceleration changes direction: if the elevator went up, the apparent weight decreases; If it went down, it increased.
We also accelerate when cornering
An acceleration is any change in velocity, both in magnitude and direction. Therefore, a car taking a curve is accelerating, even though the speedometer shows a constant speed.
In that situation, it seems that the “normal” laws of physics do strange things: the occupants feel a “force” pushing them out of the curve. This occurs because the vehicle changes the direction of its speed when turning.
The occupants feel a “force” that pushes them out of the curve.
The rotation of the Earth also has effects
Even the ground we step on is accelerating. The Earth completes one revolution every 24 hours, so the direction of its speed changes throughout the day.
The effects of this acceleration on everyday life are small, but important in fields such as ballistics and oceanography. Leon Foucault took advantage of precisely these small effects in his famous pendulum experiment.which provided direct experimental evidence of the Earth’s rotation.
