1. Ms.Jocelyn Buenconsejo Physics Teacher
The PHYSICS of Bridges
Ms.Jocelyn Buenconsejo
Physics Teacher

2. Relate PHYSICS to BRIDGES or BRIDGE CONSTRUCTION

3. WHAT PHYSICS GOT TO DO with BRIDGES?

4. How can we relate PHYSICS to BRIDGES or BRIDGE CONSTRUCTION?
A bridge has a deck, and supports.
Supports are what holds the bridge up.

5. These deck and supports are affected by the forces such as:
1. LOAD on the bridge.
(Forces acting on the bridge.)
2. ENVIRONMENTAL FORCES such as:
……. Forces of water, air, and gravity.
(Forces exerted on a support are called reactions.)

6. The Physics of BRIDGES 1. FORCES – the push or the pull
* Weight / Load on Bridges
Weight is the mass of the object
as affected by gravity.
Weight = (mass) x (acceleration
due to gravity)
In Equation: Wt = m x g

7. Forces exerted on a support are called reactions.
Loads are the forces acting on the bridge.
A bridge is held up by the reactions exerted by its supports and the loads are the forces exerted by the weight of the object plus the bridge itself.

8. Beam Bridge Consider the following bridges: The beam bridge
One of the simplest bridges

9. What are the forces acting on a beam bridge?
So what are the forces?
There is the weight of the bridge
The reaction from the supports

10. Forces on a beam bridge
Here the red represents the weight of the bridge and the blue represents the reaction of the supports
Assuming the weight is in the center, then the supports will each have the same reaction
Newton’s Third Law of Motion
The Law of Action (weight of the bridge) and Reaction (supports).

11. Limitations
With all bridges, there is only a certain weight or load that the bridge can support (Bridge Capacity).
This is due to the materials and the way the forces are acted upon the bridge.

12. MORE FORCES on the BRIDGE!
2 more other forces to consider in a bridge.
Compression forces and Tension forces.
Compression is a force that acts to compress or shorten the thing it is acting on
Tension is a force that acts to expand or lengthen the thing it is acting on

13. There is compression at the top of the bridge and there is tension at the bottom of the bridge
The top portion ends up being shorter and the lower portion longer
A stiffer material will resist these forces and thus can support larger loads

14. The ARCH Bridge
The arch bridge is one of the most natural bridges.

15. In a arch bridge, everything is under compression
It is the compression that actually holds the bridge up

16. A Stronger Bridge
Another way to increase the strength of a bridge is to add trusses.
What are trusses?
A truss is a rigid framework designed to support a structure.

17. A truss is essentially a triangular structure built on top of a basic beam bridge.
How does a truss help the bridge?
A truss adds rigidity to the beam, therefore, increasing it’s ability to dissipate the compression and tension forces

18. This is how the forces are spread out.

19. Limitations of a Truss
The truss can easily hold up weights, but there is a limitation.
Truss bridges are very heavy due to the massive amount of material involved in its construction.

20. Suspension Bridge
Another bridge type that is needed for long spans.
A suspension bridge can withstand long spans as well as a fairly decent load.

21. How Suspension Bridge Works?
A suspension bridge uses the tension of cables to hold up a load. The cables are kept under tension with the use of anchorages that are held firmly to the Earth.

22. Suspension Bridge
The deck is suspended from the cables and the compression forces from the weight of the deck are transferred the towers.
Because the towers are firmly in the Earth, the force gets dissipated into the ground.

23. Suspension Bridge
The supporting cables that are connected to the anchorages experience tension forces. The cables stretch due to the weight of the bridge as well as the load it carries.

24. Limitations
With all cable type bridges, the cables must be kept from corrosion
If the bridge wants to be longer, in most cases the towers must also be higher, this can be dangerous in construction as well during windy conditions
“The bridge is only as good as the cable”
If the cables snap, the bridge fails.

25. 2. Newton’s Laws of Motion
He was an English physicist, mathematician, astronomer, natural philosopher, alchemist, and theologian.
Newton described universal gravitation and the three laws of motion which dominated the scientific view of the physical universe
Sir ISAAC NEWTON

26. The NEWTON’S FIRST LAW OF MOTION (Law of Inertia)
This law states that a body at rest remains at rest, due to balanced forces. There is no motion. There is no net force.

27. This law also states that a body in motion remains in motion at a constant speed, no acceleration, unless acted upon by an outside force.
It is the natural tendency of objects to keep on doing what they're doing.  
INERTIA is the tendency of an object to persist on its state of rest or uniform motion in a straight line.
Situations showing Inertia:
A passenger tends to move forward when the car suddenly stops, and move backward on seat when the car starts and accelerates. The passenger is on the state of Inertia.
A moving car remains in uniform motion and is retarded by the force applied on the brakes.
Why is there a need to use seatbelts on moving vehicles?

28. The NEWTON’S THIRD LAW of MOTION
This law states that when an object exerts a force on a second object, the second object always exerts a force of equal magnitude and in opposite direction.

29. If there is less or greater load;
The NEWTON’S SECOND LAW OF MOTION (Law of Acceleration)
This law states that the greater the mass, the greater the acceleration; the lesser the force the lesser the acceleration.
Bridges can hold or might fall if and only if there will be less or more than its capacity to withstand the load (forces).