7. Using x =3 t − 4 t 2 + t 3 with SI units understood is eﬃcient (and is the approach we will use), but if we wished to make the units explicit we would write x =(3m / s) t − (4 m / s 2 ) t 2 +(1m / s 3 ) t 3 . We will quote our answers to one or two signiFcant Fgures, and not try to follow the signiFcant Fgure rules rigorously. (a) Plugging in t = 1 s yields x =0.W ith t =2sweget x = − 2 m. Similarly, t = 3 s yields x =0and t = 4 s yields x = 12 m. ±or later reference, we also note that the position at t =0is x =0. (b) The position at t
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