Frame of reference |
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Delivering newspapers, in the rain, from a moving car is an exacting proposition. The tightly rolled paper must be thrown with just the right impetus, at the right elevation, at the right time, before the recipients house is opposite the moving vehicle. The velocity of the car, added to the arching parabolic trajectory will take the paper neatly on to the front porch. The car is moving at a constant speed relative to the inertial frame of the house and the street. The paper-boy sees the paper arch away perpendicular to the car, the householder sees the paper approaching at an angle. Note: to move directly across a river in a stationary frame of reference, one must swim upstream so that the sum of the two vector velocities, swimmer plus river, is at right angles to the bank. |
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Accelerated frames of reference: when a person walks on a spinning platform or watches wind flowing across the spinning Earth, Newton's first law no longer applies in their accelerated frame of reference. |