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Got first part can't get second

A train is uniformly retarded from 35 m/s to 21 m/s over a distance of 350m. Calculate
A) The retardation
b) The total time taken under this retardation to come to rest from a speed of 35m/s

(Letts A level mathematics)

i got first part

v = 21
u = 35
s = 350
a = ?

21² = 35² + 700a
a = 441-1225/700

= -1.12 ms-2

The second part i cannot seem to get, i'm using the formula

s = ut + 0.5at²

350 = 35t + (0.5)(1.12)(t²)

350 = 35t + 0.56t²

0 = 0.56t²+35t-350

bang that in the quadratic formula and you get...
some wierd answers
can anyone help?
Reply 1
thewebmaster
A train is uniformly retarded from 35 m/s to 21 m/s over a distance of 350m. Calculate
A) The retardation
b) The total time taken under this retardation to come to rest from a speed of 35m/s

(Letts A level mathematics)

i got first part

v = 21
u = 35
s = 350
a = ?

21² = 35² + 700a
a = 441-1225/700

= -1.12 ms-2

The second part i cannot seem to get, i'm using the formula

s = ut + 0.5at²

350 = 35t + (0.5)(1.12)(t²)

350 = 35t + 0.56t²

0 = 0.56t²+35t-350

bang that in the quadratic formula and you get...
some wierd answers
can anyone help?


S= 350 doesn't apply to this part.

Use v=u+at with u=35, v=0, and your value for a.

Aitch
Reply 2
thewebmaster
A train is uniformly retarded.....


I don't do Mechanics so I can't answer your question but :rofl: at that part