Determine the difference between force and net external force.?

Answer 1

Net external force is the sum of all the forces on a system or other entity. It is the force causing the momentum of a mass to change with time per Newton's second law.

Answer 2

With means with which to describe forces, we can now look at the types of forces which can exist in a structural system. Forces or Moments which are applied directly to a structure are described as being external. Reactive Forces are generated at locations where two or more bodies meet (connections and supports) and are a direct result of the applied external forces. (Newton's 3rd Law: For every action there is an equal and opposite reaction) Forces or Moments which develop within a structure in response to external forces are described as internal.


Assume that a beam is supported at its ends. A load is applied to the top of the beam that is acting downward. This load could be a person standing on the beam. The load creates reactions that push up at the two points of support. The line of action of the load (person) on the beam also passes through a hook that is attached to the underside of the beam. Now, if the person standing on top of the beam would climb down and hold on to the hook exactly below the point where they were previously located so that the lines of action were exactly the same, the reactions at the ends of the beams would not change. This is because the load of the person is still acting along the same line of action. As long as a load is applied at any point along the line of action the external reactions will not change.

ONLY the EXTERNAL forces (reactions) remain unchanged. Some of the internal resisting forces within the elements of the structure change as the load is applied at different points along its line of action. This illustrates one important difference between INTERNAL and EXTERNAL forces

Answer 3

Newton said that an object can feel force only by 2 ways.If it is placed directly in contact to other object or if it comes in the field of other object.
suppose a block is placed on table and on it one other block is placed then uppermost block has 3 forces
1) GRAVITATION BY EARTH (a field force)
2) CONTACT FORCE BY BLOCK BELOW IT ( A contact force )
3) gravitational force by table (it is a field force again but very-very small that we can safely neglect it.)

so FORCE is the influence by other objects which are either directly 'touching' our object under consideration ,or in the
the influence by the fields created by other objects when our object comes under the field of that object ( as we are in the influence of the gravitational field created by the earth )

And NET FORCE is the vector sum of all the forces on the object .as in above example the net force on block will be VECTOR SUM OF
"GRAVITATIONAL FORCE BY EARTH + CONTACT FORCE BY BLOCK BELOW It + gravitational force by table".

AND IN THE NEWTONS SECOND LAW'S EQUATION it is the 'net force ' which is written equal to the mass X acceleration of the body.
never equate any one of the forces on the block singly to the mass X acceleration.