24Chapt. 2One-Dimensional Kinematicspositive, and about which object is chosen as defining the relative reference frame is almostthe whole problem.For the last question, ask yourself doBOTHof the trains have toaccelerate relative to the ground in order to arrange an innocuous collision?b) What is the magnitude of relative acceleration ing’s (i.e., in units of the gravitationalacceleration)? Acceleration expressed ing’s is called theg-force which is misnomer sinceit is an acceleration, not a force.c) Fundamentally unaccelerated frames of reference are called inertial frames.It’s hard tofind an exactly inertial frame actually. But approximate ones are common. The groundis approximately an inertial frame of reference for most, but not all human, purposes. Innon-inertial frames, there are what are called inertial forces (or fictitious forces). Considera frame moving in a straight line and accelerating relative to an inertial frame: e.g., theground.
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