# Homework Help: Tension between two objects moving at constant velocity

1. Sep 20, 2015

### moenste

1. The problem statement, all variables and given/known data
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.

(a) 1.6 kN, (b) zero

2. Relevant equations
F = ma
Weight = mg

3. The attempt at a solution
(a) Ftotal = (100+80)*103 * 0.02 = 3600 N
Fengine = 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.

2. Sep 20, 2015

### Staff: Mentor

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.

3. Sep 22, 2015

### moenste

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?

4. Sep 22, 2015

### Staff: Mentor

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