A 100 N force applied by the brakes stops a car traveling at an initial velocity of 25 km/ hr. Solve for mass.

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I'm getting a negative mass, so I must be doing something wrong.

$\displaystyle v = V_0 t + 1/2 a t^2$

$\displaystyle

0= (6.94 m/s * 15 seconds) + (.5)(a)(15)^2$

$\displaystyle 104.1 + 125a$

Which simplifies to $\displaystyle -.93 m/s$

I plug that into $\displaystyle F= ma$

so...

$\displaystyle 100= (m)(-.93)$

Which comes out to -107.53