Kinematics

The brakes of a train can produce a retardation of 1.7ms^-2.
If the train is travelling at 100km/h and applies its brakes what distance does it travel before stopping? If the driver applies the brakes 15m too late to stop at a station with what spped is the train travelling when it passes through that station?

Reply 1

Chewwy

17

u = 100*1000/(60*60)
v = 0
a = -1.7
s = s

v^2 = u^2 + 2as
s = (v^2 - u^2)/2a

for the second part: v = sqrt(u^2 + 2a(s-15))

Reply 2

generalebriety

16

I always thought kinematics was a chemistry term. :confused:

Also, I recognise this question out of the Heinemann book, I'm sure of it. :p:

Ok, the first part of the question... the train is originally travelling at 100km/h (u = (whatever that is in m/s)), and has a retardation of 1.7ms^-2 (a = -1.7). It stops (v = 0). Can you find an equation linking u, a, v, and the variable you want to find (s or x, depending what you call it)?

Now take the second part of the question. It's still travelling at 100km/h (u = (whatever that is in m/s)), retardation is still 1.7ms^-2 (a = -1.7), and the distance (s, or x) is whatever you found it to be in the previous part of the question, minus 15. Use the same equation linking u, v, a and s, trying to find v.

Reply 3

generalebriety

16

chewwy

u = 100*1000/(60*60)
v = 0
a = -1.7
s = s

v^2 = u^2 + 2as
s = (v^2 - u^2)/2a

for the second part: v = sqrt(u^2 + 2a(s-15))

Bloody hell. I know what I'm doing and even I would have trouble following that. :p:

Reply 4

ad absurdum

2

generalebriety

I always thought kinematics was a chemistry term. :confused:

Also, I recognise this question out of the Heinemann book, I'm sure of it. :p:

I've only came across kinematics in physics. Last year we were using these textbooks for the higher physics course that probably predated Newton himself, but they referred to stuff involving the equations of motion and stuff as kinematics, and defined kinematics as "the study of the movement of objects without consideration of the forces involved in causing the movements of the objects", or something like that.

OP

thanks for the help

OP

New question:
A, B and C are three points on a straight road such that AB = 80m and BC = 60m. A car travelling with uniform acceleration passes A, B and C at times t = 0, t = 4s and t = 6s respectively. Modelling the car as a particle find its acceleration and its velocity at A

Reply 7

generalebriety

16

birmingham182

New question:
A, B and C are three points on a straight road such that AB = 80m and BC = 60m. A car travelling with uniform acceleration passes A, B and C at times t = 0, t = 4s and t = 6s respectively. Modelling the car as a particle find its acceleration and its velocity at A

Call the initial velocity U.
Along the line AB: u = U, t = 4, s = 80.
Along AC: u = U, t = 6, s = 140.

Which equation involves u, t, s and a? Form two equations (which will end up involving two unknowns, U and a) and solve them simultaneously.