Tension between two objects moving at constant velocity

[moenste]

Homework Statement

A railway engine of mass 100 tonnes is attached to a line of trucks of total mass 80 tonnes. Assuming there is no resistance to motion, find the tension in the coupling between the engine and the leading truck when the train: (a) has an acceleration of 0.020 ms^-2, (b) is moving at constant velocity.

Answers:
(a) 1.6 kN, (b) zero

Homework Equations

F = ma
Weight = mg

The Attempt at a Solution

(a) F_total = (100+80)*10³ * 0.02 = 3600 N
F_engine = 100*10³ * 0.02 = 2000 N
T = 3600 - 2000 = 1600 N

(b) Constant velocity = 0 acceleration. But I don't quite understand why there is 0 tension. Any tips? Thank you.

 

[mfb]

What else would you expect?
Zero acceleration is just a special case for F=ma, if acceleration is zero the force is zero as well.

You can simplify the calculation for (a) by the way, there is no need to consider the engine mass.

 

[moenste]

What else would you expect?
Zero acceleration is just a special case for F=ma, if acceleration is zero the force is zero as well.

You can simplify the calculation for (a) by the way, there is no need to consider the engine mass.

Thank you. So in general, just calculate the F of the attached part (in this case the trucks) and it would be the tension: F = 80*10³ * 0.02 = 1600 N. And if a = 0 then F = 80*10³ * 0 = 0. I got you words correctly?

 

[mfb]

In general, you have to see what pulls what in which way.
In this case: yes, right.