Homework help!!? physics?

Question

A truck covers 40.0 m in 7.05 s while smoothly slowing down to final speed of 1.90 m/s.

(a) Find the truck's original speed.

m/s

(b) Find its acceleration.

m/s^2

Answer 1

three equation of motion

v  = u+at    ----------------------------------------...

s  = ut+1/2 at^2  -------------------------------------2)

v^2 = u^2+2as  ----------------------------------------...

we have   v = 1.9  m/s    s = 40     t  = 7.05

we see no relation connecting this three quantities

however

u   = v-at                    --------------------------------4) rearrange eq  1

sub val of u in eq 2

   s=  (v-at)t  + 1/2 at^2

   s  = vt  - 1/2at^2  ------------------------------------5)

   40  = 1.9 * 7.05  - 1/2 a *7.05 *7.05

  a = -1.07 m/sec^2  (negative indicates it is slowing down )

v = u+at

1.9  = u +(-1.07)(7.05)

u  = 9.4435 m/sec