Consider the crane as shown in the Fig(1) to which a weight 'W' is hung and a string is attached to it as shown in Fig(1) . The normal reaction force which acts normally or more specifically perpendicularly to the point of contact and the gravitational force due to the weight acts upon CF part of crane and the tension of the string acts upon the AD part of the crane in the direction away from the point of suspension of the string and along the string at point D as show shown in the free body diagram of the crane in (a) part of Fig (2) . There are also many internal forces between the joints of the crane which can be considered as external forces when dealt dismembering the crane as shown in the (b ) and (c ) parts of Fig (2) .
Dealing in detail with all the forces , Consider (a) part of Fig(2) , the tension in the string GD acts along GD (the direction along the string) and away from D (point of suspension) and the gravitational force vertically downwards of the weight hung at F . The normal reaction force at A in vertical and horizontal directions .
consider part (b ) of Fig (2) , the tension force at D same as before and normal reaction in vertical (Ax) and horizontal (Ay ) directions same as before and additional normal forces at C due to CE part of the crane in vertical and horizontal direction and the normal force at B in the perpendicular direction to the part BE .
Consider the part (c ) of Fig (2), the reaction pairs of the forces acting in the previous case act here on the parts considered in this figure because according to Newton's third law every force acts in action and reaction pairs which for a force on a body , a force of equal magnitude acts in the opposite direction of the previous force .
In this way the forces act upon each parts of a joint or conducted bodies internally which are not considered and which are the main things to provide support to other parts in order to maintain equilibrium as a whole system together .
Keeping these points in mind we define three things namely Trusses , Frames and Machines differentiating each other .
Trusses:These points are fully constrained usually stationary structures designed to support loads and consist exclusively of straight members connected at joints located at the ends of each member . Members of Truss are acted upon by two equal and opposite forces directed along the member and are so two force members .
Frames:These are also fully constrained usually stationary structures designed to support loads . Frames always contain at least one multiforce member ( which are acted upon by three or more forces generally not directed along the member) like the crane as discussed above .
Machines:There are structures containing moving parts designed to transmit and modify forces and always contain at least one multiforce member like the case of Frames .
Note:The forces which are said to be internal may become external forces depending upon the case like when the parts of the system are considered individually as discussed above .
