# Homework Help: Free body diagram from whole system-direction of forces in free-body d

1. Aug 20, 2013

### marellasunny

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

A car and a trailer are connected by a joint. The car and trailer system are on a inclined place at a gradient of 8 degrees. The rolling friction coefficient along with other vehicle parameters are given.We are asked to draw the free body diagrams of the car and trailer separately and thereby find the forces at the joint.

2. Relevant equations
Splitting the car-trailer system into 2 separate free body diagrams.

3. The attempt at a solution
I am finding difficulty as to how I should position the forces in the free-body diagram.I have made a logical guess in the solution attached as I assume there is 'dynamic equilibrium'(just as is taken in the D'alembert principle). Am I right in the way I have shown the direction of the joint forces? Can it be done in the exact reverse sense and still be correct?

In the whole system diagram,it does make sense for the joint forces to be so as the joint sags downwards and is pulled forward.

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2. Aug 20, 2013

### PhanthomJay

In the whole system diagram, do not show the joint forces because they are internal to the system. Only show the external forces like traction friction force forward on the driving wheels, rolling friction on the trailer wheel, and weights. When you draw FBDs of the trailer and car, then include the joint forces which are drawn correctly. Do not include fictitious DAlembert forces in the FBD, however,it may be useful to do so by inserting the fictitious inertial force -ma at the cg and summing forces and torques equal to 0, but if so doing, label your diagram as "FBD with ficticious forces".