What is the tension in the towbar?

[Stroodle]

Hi there, I have a question that I'm not sure to go about solving:

A car of mass 700kg is towing a trailer of mass 500kg, on an incline of 30 degrees to the horizontal.
Friction on the trailer is 400N, friction on the car is 600N. If the engine provides a driving force of 700N, what is the tension in the towbar?

I guess I need to first work out the acceleration of the system, but I think a bit unclear as to what driving force is...
In this case wouldn't the car and trailer no be moving, as the driving force is less than the combined friction on the system?
Thanks for your help.

 

[kuruman]

The problem clearly states that the engine produces a force of 700 N. This would be the same situation as in the more familiar picture of a "disembodied hand" applying the force. Having said that, you are correct in noting that the force of the engine is less than the combined forces of friction. However, also note that the problem says nothing about the direction of this force or the direction in which the car and trailer are moving.

What if the engine force is applied uphill and the car and trailer are moving backwards downhill?
What if the engine force is applied downhill and the car and trailer are also moving downhill?
Is there a difference between these two possibilities as far as the "tension" in the towbar is concerned?