36_Physics ProblemsTechnical Physics

36_Physics ProblemsTechnical Physics - Chapter 2 P2.38 37...

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Chapter 2 37 P2.38 Take the original point to be when Sue notices the van. Choose the origin of the x -axis at Sue’s car. For her we have x is = 0, v is = 30 0 . ms , a s = 200 . m s 2 so her position is given by xt x vt a t t t si s i ss ()= + + = + 1 2 30 0 1 2 22 .. ms 2 a f ch . For the van, x iv = 155 m, v iv = 500 . m s , a v = 0 and a t t vi v i vv + = + + 1 2 155 5 00 0 2 s a f . To test for a collision, we look for an instant t c when both are at the same place: 30 0 155 5 00 02 5 0 1 5 5 2 2 tt t cc c −= + =− + From the quadratic formula t c = ±( ) () = 25 0 25 0 4 155 2 13 6 2 . s or 11 4 s. The smaller value is the collision time. (The larger value tells when the van would pull ahead again
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