How Should Forces Be Calculated in a Truck-Trailer System?

[MaxKempar]

I have a truck (m1,27000 kg) pulling a trailer (m2, 8500 kg) with a chain holding the two together (T). The overall acceleration is 0.78 m/s^2.
Find f1 force on the truck, f2 force on the trailer and T, tension force.

Here is what I did: since the force comes all from the truck, f1 = (m1+m2)(a) = 35500*0.78 N
T = -f2 = 8500 * 0.78 N

But apparently this is how you do it:

f2 = m2a=T = 27000*.78 N = T
f1 = m1a = F-T
F = f1+f2 = (m1+m2)a = 35500*.78
f1 = 35500*.78 - 27000*.78;

I just don't understand how f2 which is just being pulled can have force greater than f1 which is doing all the work.

What am I doing wrong?

Thank you

 

[MaxKempar]

The main thing that I don't understand is F = f1+f2, how can a powerless trailer have a positive force. Wouldn't F = f1-f2?

 

[Orodruin]

Find f1 force on the truck,

This part is a bit unclear, the force on the truck from where? Is it the total force or the force from the ground on the truck or the force from the chain on the truck?

 

[CWatters]

+1

Find f1 force on the truck, f2 force on the trailer and T, tension force.

Do you have a diagram showing where f1 and f2 are measured?

As far as I can see these could all be the same. The chain is pulling the trailer so the tension _is_ the "f2 force on the trailer". The force f1 on the truck could be the tension in the chain or the friction force with the ground??

 

[CWatters]

But apparently this is how you do it

f2 = m2a=T = 27000*.78 N = T

That's not consistent with the problem statement (m2 is 8500kg not 27000kg). It should be...

f2 = m2a = 8500 * 0.78 = T = 6630N .......(1)

f1 = m1a = F-T

That implies f1 is the force required to accelerate the truck alone without the trailer. That's OK.

F = f1+f2 = (m1+m2)a = 35500*.78

Ok so F is the total force required to bull both truck and trailer. That's OK

f1 = 35500*.78 - 27000*.78;

That doesn't make sense due to the error above at (1). Should be

f1 = 35500*.78 - 8500*.78

but there is a much easier way to calculate f1...

f1 = m1*a = 27000 * 0.78

I don't see why they need to make it so complicated.