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Confusing Maths A-Level Mechanics Forces and Newton Laws Question

There is this question I'm rlly stuck on if anyone is good at mechanics or physics can u pls help this poor soul.

The question is below:
a passenger train unit has five cars. the two end cars and middle one are powered and the other two are trailers. each powered car has a mass of 120 tonnes and each trailer 80 tonnes. the resistance to the motion of each car other than the leading one is 1 kN. the tractive effort of each powered car is 10kN. The train is traveling at a constant speed of 250 km h^-1 on a level straight track.
(i) What is the resistance to the leading car?
(ii) Find the tension or compression in each coupling
if the front power car is shut off owing to a fault, the train can only maintain a lower steady speed.
(iii) assuming that the resistances fall proportionately to speed but the tractive efforts are unchanged what is that steady speed?
The leading power car is still in failure.
(iv) the rear power car's tractive effort is reduced so that the coupling by which it is attached to the car in front is to be in tension rather than compression (when traveling at a steady speed). What is the maximum value of the tractive effort for this to be achieved?

Ive tried to do it but I rlly be getting odd answers

Reply 1

ThomH97

Which are you stuck on, and what have you done so far?

Reply 2

Condused_life

Original post by ThomH97

Which are you stuck on, and what have you done so far?

tbh I was stuck on the whole thing but I had a few ideas:

for i) I thought that there were 3 cars and 2 trailers so the resistance force on the cars either than the leading one was 1kN and that the total forward force applied was 30kN and since it is constant speed a=0
but I was told otherwise that due to the first sentence all of the objects are cars and they all have a resistance force of 1kN except for the leading one.

ii) What I did was did it win couplings but was kind of unsure if the tensions would cancel

for iii) since the total forward force applied is 30kN the resistant force is not equal to the constant speed so I did the forward force - the total resistance force (which I think I did incorrectly!) :frown:

and for iv) I feel as if it was totally dependant on the previous questions answers so I left it out for the meantime and will attempt it once i do the previous questions

Also thank you for replying I rlly do appreciate it :smile:

and sorry for replying late!

Reply 3

ThomH97

Original post by Condused_life

This is correct. Can you see why from the quetion?

ii) What I did was did it win couplings but was kind of unsure if the tensions would cancel

For each car, they're going to have up to 4 (relevant, horizontal) forces acting on them, and the resultant is zero due to the constant speed you've correctly identified previously. The four forces are the front and rear couplings, any driving force, and the resistive force.

I'll leave iii and iv for now until you get the first two.

Reply 4

Condused_life

Original post by ThomH97

This is correct. Can you see why from the quetion?

For each car, they're going to have up to 4 (relevant, horizontal) forces acting on them, and the resultant is zero due to the constant speed you've correctly identified previously. The four forces are the front and rear couplings, any driving force, and the resistive force.

I'll leave iii and iv for now until you get the first two.

Thank you for explaining!☺️ I understand the first bit but the second bit I attempted it again with the information you gave but still a bit unsure.

I’ve attached my working and I apologise in advance as it’s very blurry.

EDDA7E43-6916-49B2-ABE9-4A82C0C4F0A8.jpg87.1KB

1BF2569E-AFD2-44CB-BC7B-56D8043F8725.jpg113.5KB

F8D3328A-0FE0-4FE7-8622-9A393C446B89.jpg112.5KB

D7FB56B0-117B-420A-871B-C2AB97AC898C.jpg103.1KB

756E8770-E133-4E09-A9D1-84A8696A504A.jpg109.8KB

Reply 5

ThomH97

Original post by Condused_life

I'm not entirely sure what you've done, but it looks like you're combining the resistive force on that particular car with the tension in the coupling. And possibly you're adding up all the other resistive forces as well as putting them through the coupling force.

Each car has its own resistive force which you'll need to keep separate from the couplings' forces. The tractive and resisitive forces acting on the other cars will be 'within' the coupling forces at the front and back of each car.

To focus on the front car first, what are the 3 (as it doesn't have a front coupling) forces acting on it? Then do the same for the second car, bearing in mind its front tension is equal to the first car's rear tension.