Presentation1

presentation on external & internal

1. Presentation On External & internal forces Made by- saquib faizal

2. What is Force? A push or pull Force has a size & direction All motion is created by a force All forces are exerted by one object on another It has both magnitude & direction, therefore it is a vector quantity.

3. How is Force Measured? Measured in Newtons (N) The more Newtons,the greater the force One tool to measured the the force of gravity is the spring scale.

4. Forces are also described as a push or pull on an object. They can be due to phenomena such as gravity, magnetism, or anything that might cause a mass to accelerate. Common Symbol = F SI unit = newton Derivations from = F=ma other quantities

5. Forces is of two types External Forces Internal Forces

6. EXTERNAL FORCES E xternal forces include the applied force, normal force, tension force, friction force, and air resistance force F app F frict F air INTERNAL FORCES Internal forces include the gravity forces, magnetic force, electrical force, and spring force F grav F spring

7. Examples of External & Internal Forces A baseball. The forces holding the particles of baseball together are internal forces; the forces of a bat hitting the ball is an external force on the ball ; so is gravity. A car . The parts of the car exert forces on each other, the pistons push the rods, the axle pushes the wheels , etc. All these forces are internal to the car as a whole. But the friction that the ground exertion the tires is an external force. So I the weight of the car, and the force the ground exerts to hold up t he car.

8. INTERNAL FORCES When the only type of force doing net work upon an object is an internal force (for example, gravitational and spring forces), the total mechanical energy (KE + PE) of that object remains constant. For example, as an object is "forced" from a high elevation to a lower elevation by gravity, some of the potential energy of that object is transformed into kinetic energy. Yet, the sum of the kinetic and potential energies remains constant. When the only forces doing work are internal forces, energy changes forms - from kinetic to potential (or vice versa); yet the total amount of mechanical is conserved. Because internal forces are capable of changing the form of energy without changing the total amount of mechanical energy, they are sometimes referred to as conservative forces.

9. Internal forces act between diff erent parts of the same structure. There are four types of internal forces: tension, compression, torsion, and shear.

10. Figure 4 The main cables in a suspension bridge, have tension forces acting on them Tension When you pull on an elastic band, the force of your fi nger pulling on the elastic band is an external force. This force creates an internal force called tension , which causes all of the particles of the elastic band to pull apart. Tension can act on a variety of objects, for example, a stretched skipping rope, a trampoline, an electrical power line, guitar strings, and the cables of a suspension bridge.

11. Compression An object that is pressed or squeezed experiences compression. Compression is an internal force that presses the particles of an object together . The springs inside a mattress undergo compression when you lie down on the mattress. Compression also occurs when you kick a soccer ball, step on the sole of your shoe, or lay your head on a foam cushion. Compressed objects usually return to their original shape aft er the external force is removed .

12. COMPRESSION Figure : These mattress springs have compression forces acting on them.

13. Torsion It acts in an object when the object is twisted . Torsion is evident when a skater twists in a jump, a washcloth is wrung out, and a doorknob is turned. Torsion can be created when both ends of a structure are twisted. Torsion can also be created when only one end of a structure is twisted while the other end remains stationary . Figure : This figure skater is experiencing torsion force as she twists her body in a spin .

14. Shear Shear forces occur when forces push or pull in opposite directions within an object. Shear forces usually result in an object being bent, torn apart, or cut. A strong wind that is blowing horizontally against a tree anchored to the ground causes shear forces inside the tree. These forces can cause it to bend or break . Scissors use shear force to cut paper in half. Th e blades of the scissors move in opposite directions and create two pushing forces against the paper, which results in the paper being cut . Figure : Shear forces inside the trees may cause them to snap in a strong wind.

15. EXTERNAL FORCES When net work is done upon an object by an external force,the total mechanical energy(KE+PE) of that object is changed. If the wok is positive work, then the object will gain the energy. If the work is negative work, then the object will lose the energy. The gain or loss in energy can be in the form of potential energy , kinetic energy , or both . Under such circumstances, the work that is done will be equal to the change in mechanical energy of the object Because external forces are capable of changing the total mechanical energy of an object, they are sometimes referred to as nonconservative forces

16. The most obvious external force acting on structures is gravity. On Earth, gravity always acts downward. Gravity is a non-contact force. Non-contact forces are those applied to an object by another object not in contact with it. Applied forces, or contact forces, also act on an object from the outside. You apply external forces when you push a swing, pull an elastic, or throw a ball. External forces on buildings include wind, earthquakes, the weight of people on the floors of the building, and the weight of the building itself. A structure is designed so that external forces will not cause it to break or fall over. In Figure : The magnitude and direction of the applied force are the same.

17. Another important external force is the force in the direction opposite to gravity. Think of the forces on you when you are sitting on a stool. You know that the force of gravity on you (your weight) is an external force that pulls you downard . THE END