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R. Barrett and P. P. Delsanto
6.3 Walking on a Train
We all feel that we can distinguish when we are in motion from when we
are at rest. However, the work of Galileo and Newton showed that making
such a decision is actually impossible, except when our velocity changes. At
the moment, sitting in front of a computer screen, and reaching for a cup
of delicious Ethiopian Harar coffee, I do not have any feeling of motion.
However, I live on the surface of a planet that is rotating at a speed of roughly
460 m per second (at the equator). Also the earth is revolving around the
sun at about 30 km per second, and the whole solar system is plummeting
towards the constellation of Leo at 371 km per second. Would anyone call
this “being at rest”?
The reason that I have no perception of these enormous speeds is explained
by Newton’s first law of motion:every body continues in a state of rest or uniform
motion unless acted upon by an external force. I, and my coffee cup, and my
computer, continue on our journey towards Leo, maintaining the same relative separation from each other unless some force acts upon one of us, e.g.
if I inadvertently knock the cup and spill the coffee. Suppose I move to the
study I have set up in my private jet aircraft. 2 Once again, after the plane has
taken off and reached cruising altitude, I will have no sensation of motion,
unless we encounter turbulence, which is an external force likely to send my
coffee mug sliding across the desk.
Can we all, as young children, remember sitting in a train at a railway
station, waiting impatiently for our journey to begin. Looking out of a
window, we notice that at last we have begun to edge forwards and are finally
underway. We sit back in our seat, turn towards our parents to tell them, and
are astonished to see that the station platform on the other side of the train
remains absolutely still. What we had observed was another train pulling into
the platform on the track next to us and have mistaken its movement for our
own.
Technically we can summarize the above considerations by stating that any
system moving at constant velocity (which we call an inertial frame of reference) is equivalent to any other, in the sense that any experiment performed
in one gives the same result if it is performed in another. Consequently, it
is impossible to decide which frame of reference is moving, and with what
velocity. Of course, if the speed of the train is not constant (i.e. it is accelerating or braking) we feel the effect of the acceleration or deceleration, but not
2 We are all allowed our cosy private dreams.
R. Barrett and P. P. Delsanto
6.3 Walking on a Train
We all feel that we can distinguish when we are in motion from when we
are at rest. However, the work of Galileo and Newton showed that making
such a decision is actually impossible, except when our velocity changes. At
the moment, sitting in front of a computer screen, and reaching for a cup
of delicious Ethiopian Harar coffee, I do not have any feeling of motion.
However, I live on the surface of a planet that is rotating at a speed of roughly
460 m per second (at the equator). Also the earth is revolving around the
sun at about 30 km per second, and the whole solar system is plummeting
towards the constellation of Leo at 371 km per second. Would anyone call
this “being at rest”?
The reason that I have no perception of these enormous speeds is explained
by Newton’s first law of motion:every body continues in a state of rest or uniform
motion unless acted upon by an external force. I, and my coffee cup, and my
computer, continue on our journey towards Leo, maintaining the same relative separation from each other unless some force acts upon one of us, e.g.
if I inadvertently knock the cup and spill the coffee. Suppose I move to the
study I have set up in my private jet aircraft. 2 Once again, after the plane has
taken off and reached cruising altitude, I will have no sensation of motion,
unless we encounter turbulence, which is an external force likely to send my
coffee mug sliding across the desk.
Can we all, as young children, remember sitting in a train at a railway
station, waiting impatiently for our journey to begin. Looking out of a
window, we notice that at last we have begun to edge forwards and are finally
underway. We sit back in our seat, turn towards our parents to tell them, and
are astonished to see that the station platform on the other side of the train
remains absolutely still. What we had observed was another train pulling into
the platform on the track next to us and have mistaken its movement for our
own.
Technically we can summarize the above considerations by stating that any
system moving at constant velocity (which we call an inertial frame of reference) is equivalent to any other, in the sense that any experiment performed
in one gives the same result if it is performed in another. Consequently, it
is impossible to decide which frame of reference is moving, and with what
velocity. Of course, if the speed of the train is not constant (i.e. it is accelerating or braking) we feel the effect of the acceleration or deceleration, but not
2 We are all allowed our cosy private dreams.