SEPyC Expected learning
1. Describe la fuerza como efecto de la interacción entre los objetos y la representa con vectores.
2. Aplica los métodos gráficos del polígono y paralelogramo para la obtención de la fuerza resultante que actúa sobre un objeto, y describe el movimiento producido en situaciones cotidianas.
3. Argumenta la relación del estado de reposo de un objeto con el equilibrio de fuerzas actuantes, con el uso de vectores, en situaciones cotidianas.
01 THE FORCE
Why do objects change their motion?
Why do objects change their shape?
The following illustrations show more of the effect of forces. Are you able to identify them?
1. The tennis player hits the ball away from her.
(A force can change the course (direction) of a moving object)
2. The driver uses the brakes to stop the car.
(A force can halt a moving object)
3. The force of attraction is exerted on the elastic band.
(A force change the shape of an object.)
4. The force of thrust in the same direction as the motion is exerted on the rolling ball. (A force can change the speed of a moving object)
5. The athlete swings the hammer to rotate it around him
(A force can make an object rotate )
A) What is a force? Motion Effects
A) What is a force? Shape Effects
So, what is a force?
The Meaning of Force
A force is a push or pull upon an object resulting from the object's interaction with another object. Whenever there is an interaction between two objects, there is a force upon each of the objects. When the interaction ceases, the two objects no longer experience the force. Forces only exist as a result of an interaction.
*A force is the result of the interaction of one object with another object.
*Force is the external agent that changes the state of rest or uniform motion of a body along a straight line.
*A force IS NOT inside an object.
B)Classifying Forces
For simplicity sake, all forces (interactions) between objects can be placed into two broad categories:
•Contact forces, and
•forces resulting from action-at-a-Distance
CONTACT FORCES(mechanic)
Contact forces are those types of forces that result when the two interacting objects are perceived to be physically contacting each other.
Examples of contact forces include:
1. The vehicle in the illustration has broken down and cannot move by itself. The tow truck has to pull it. This car also cannot move by itself. The man has to push it.
2. When something has to be pulled or pushed, we say that we must exert force on it. You use the force of thrust to push something away from you, or the force of attraction to pull something towards you.
3. Frictional forces is the force between two objects in contact with each other. It causes moving objects to slow down. It is a resistive force.
The friction force is the force exerted by a surface as an object moves across it or makes an effort to move across it
4. Stretching - Tensional forces
The tension force is the force that is transmitted through a string, rope, cable or wire when it is pulled tight by forces acting from opposite ends. The tension force is directed along the length of the wire and pulls equally on the objects on the opposite ends of the wire.
5. Rubbing
6. Air resistance forces
The air resistance is a special type of frictional force that acts upon objects as they travel through the air. The force of air resistance is often observed to oppose the motion of an object.
7. Applied forces
An applied force is a force that is applied to an object by a person or another object. If a person is pushing a desk across the room, then there is an applied force acting upon the object. The applied force is the force exerted on the desk by the person.
8. Compression
9. Normal forces
The normal force is the support force exerted upon an object that is in contact with another stable object. For example, if a book is resting upon a surface, then the surface is exerting an upward force upon the book in order to support the weight of the book. On occasions, a normal force is exerted horizontally between two objects that are in contact with each other. For instance, if a person leans against a wall, the wall pushes horizontally on the person.
10. Spring Force
The spring force is the force exerted by a compressed or stretched spring upon any object that is attached to it. An object that compresses or stretches a spring is always acted upon by a force that restores the object to its rest or equilibrium position.
DISTANCE FORCES
Action-at-a-distance forces are those types of forces that result even when the two interacting objects are not in physical contact with each other, yet are able to exert a push or pull despite their physical separation.
It is even possible to bring objects into motion or to change the direction of the motion without any contact. Forces that affect objects without making contact are identified as forces that operate over distance.
Three kinds of force can have an effect without any contact with objects:
Gravitational force (also known as Weight)
The force of gravity is the force with which the earth, moon, or other massively large object attracts another object towards itself. By definition, this is the weight of the object. All objects upon earth experience a force of gravity that is directed "downward" towards the center of the earth.
For example, the sun and planets exert a gravitational pull on each other despite their large spatial separation. Even when your feet leave the earth and you are no longer in physical contact with the earth, there is a gravitational pull between you and the Earth.
Magnetic force
Magnetic forces exist between magnets and other magnets, and also between magnets and magnetic materials. The force can be attraction or repulsion.
Magnetic forces are action-at-a-distance forces. For example, two magnets can exert a magnetic pull on each other even when separated by a distance of a few centimeters. These specific forces will be discussed in more detail later in Lesson 2 as well as in other lessons.
Electrostatic force
Forces between electrical charges.
Electric forces are action-at-a-distance forces. For example, the protons in the nucleus of an atom and the electrons outside the nucleus experience an electrical pull towards each other despite their small spatial separation.
REMEMBER
A force can change the course (direction) of a moving object.
A force can halt a moving object.
A force change the shape of an object.
A force can change the speed of a moving object.
A force can make an object rotate.
02 NET FORCE
Resultant Force
Any object can be made to move by the application of force.
Many forces can act simultaneously on a body.
For example, several people can jointly move a boulder but a strong person can move the same boulder all by himself.
That is, the force applied by the strong man produces the same effect as that produced by the net force applied by all.
The force applied by the strong man is the resultant force.
03 FORCES EQUILIBRIUM
Balanced forces
If two forces that have the same magnitude & direction are applied to a body in opposite senses, this won’t move because the effects balance each other.
Forces system analysis is important when designing
Bridges
Building
Oil rigs
UNBALANCED FORCES
WHEN TWO FORCES ACT UPON EACH OTHER IN OPPOSITE DIRECTIONS, AND THEY ARE BOTH THE SAME SIZE, WE SAY THEY ARE BALANCED. YOU CAN SEE BALANCED FORCES IN A TUG-OF-WAR GAME WHEN THE ROPE IS NOT MOVING.
AN UNBALANCED FORCE ACTING ON AN OBJECT CHANGES ITS SPEED OR DIRECTION. THE RESULTANT OF UNBALANCED FORCES IS ALWAYS GREATER THAN ZERO.
THE FIGURES SHOW A BLOCK OF WOOD ON A TABLE. WHEN THE BLOCK IS PULLED AT POINT A, IT BEGINS TO MOVE TOWARDS THE LEFT AND IF THE BLOCK IS PULLED AT THE POINT B IT MOVES TOWARDS THE RIGHT.
ALL OF US KNOW THAT WHEN WE STOP PEDALLING A BICYCLE, THE SPEED OF THE BICYCLE DECREASES DUE TO THE ACTION OF FRICTION, WHICH IS THE ONLY FORCE ACTING ON THE BICYCLE. THUS, IF THE FORCES ACTING ON A BODY PRODUCE ANY CHANGE IN STATE OF REST OR MOTION, THEN THE FORCES ARE SAID TO BE UNBALANCED. AN UNBALANCED FORCE ACTING ON AN OBJECT CHANGES ITS SPEED OR DIRECTION. THE RESULTANT OF UNBALANCED FORCES IS ALWAYS GREATER THAN ZERO.
BALANCED FORCES
IF THE BLOCK IS PULLED FROM BOTH SIDES WITH THE SAME EFFORT THE BLOCK REMAINS STATIONARY. THE FORCES ARE EQUAL AND OPPOSITE AND THEREFORE THE BLOCK DOES NOT MOVE. THE RESULTANT OF THE FORCES ACTING ON THE BODY IS ZERO.
IN A GAME OF TUG OF WAR WHEN BOTH THE TEAMS PULL THE ROPE WITH EQUAL AND OPPOSITE FORCES, THEN THE ROPE REMAINS STATIONARY AS THE FORCES ACTING ON IT ARE EQUAL AND OPPOSITE AND THEIR RESULTANT IS ZERO.
SQUEEZE A RUBBER BALL BETWEEN THE PALMS OF YOUR HANDS. WHAT DO YOU OBSERVE? THE SHAPE OF THE RUBBER BALL CHANGES. THE FORCES APPLIED ON THE BALL ARE EQUAL AND OPPOSITE AND THE RESULTANT OF THESE FORCES DOES NOT MOVE THE OBJECT, INSTEAD THE OBJECT GETS DEFORMED AS LONG AS THE FORCE IS APPLIED. THIS IS A TEMPORARY DEFORMATION.
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Do you want to learn more?
http://www.tutorvista.com/content/science/science-i/force-laws-motion/force-laws-motionindex.php
http://www.physicsclassroom.com/
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