Truss vs Arch Equilibrium
Balanced & Unbalanced Forces
Force: A push or pull that change the speed or direction of an object. Two or more forces can also change shape.
An object is in equilibrium when the forces ( & torques) acting on the object are balanced. Using a free body diagram (as shown on the right side of the page) there are four forces acting on an object. The forces are considered to be balanced if the rightward forces are balanced by the leftward forces and the upward forces are balanced by the downward forces. This shows that the net force is zero but this does not mean all the forces are equal . Objects at equilibrium must have an acceleration of 0 m/s/s. But having an acceleration of 0 m/s/s does not mean the object is at rest. When this happens we say the object is in uniform position. This means that the mass of the object is evenly distributed. Static & Dynamic Equilibrium
An object at equilibrium is either static or dynamic .
● Static:­ when all forces acting on an object are balanced (i.e. there is no resultant forces) and the object is not in motion and does not have a velocity. This means the object is static or stationary .
● Dynamic equilibrium is when all the forces acting on an object are balanced but the object it moving. The object is therefore moving with a constant velocity.
The object is in motion moving in the same direction with the same speed. This report consists of the aspects of two types of bridges therefore we will focus on static equilibrium.

Torques

Torques is defined as the turning effect of a force ( sometimes called moment). This simply means that when a force (internal/external) is applied to an object it changes the shape which in this situation causes a twisting effect.
The formula for torques is τ = F × d
We multiply the ‘force’ by the ‘distance to pivot’. Using the concept of torques we can find out the twisting effects that a force has applied to an object.

Newton’s Law We can apply Newton’s law to this concept. The first law;
I. Every object in a state of uniform motion tends to remain in that state of motion unless an external force is applied to it.
The object will remain in its position (static equilibrium) unless there is an external force such as acceleration (thrust) applied (dynamic equilibrium). This can apply to either static or dynamic equilibrium because without a force the object cannot move.
The second law;
The relationship between an object's mass m , its acceleration a, and the applied force
F
is
F = ma
.
Acceleration and force are vectors (as indicated by their symbols being displayed in slant bold font); in this law the direction of the force vector is the same as the direction of the acceleration vector.
The object will move and the acceleration will change if there is a force acting on it. The velocity of the force will enable the object to move in the same direction. This law applies to a dynamic equilibrium. A dynamic equilibrium is in motion and therefore this law states the concept of how the object moves by the mass and acceleration.
The Third Law; For every action there is an equal and opposite reaction. This law is applied to an equilibrium because it explains the concept of how one force will respond to the force in the opposite direction.

Beam Bridge Beam bridges are the simplest structural forms for bridge spans supported by a pier at each end. In its most basic form, a beam bridge consists of a horizontal beam that is supported at each end by piers. The weight of the beam pushes straight down on the piers. This is shown in the diagram below.

Under load, the beam's top surface is pushed down or compressed while the bottom edge is stretched or placed under tension .
The force of compression pushes the sides on top of the bridge inwards. This causes the upper portion of the deck to shorten. The result