Chapter 3 Force

CHAPTER 3

Forces

Learning outcomes

 the students will be able to:

·         1.Define the forces, Newton’s Law of motion, forces in equilibrium and moment of forces

·         2.Explain forces, Newton’s Law of motion and moment of forces

·         3.Differentiate between mass and weight

·         4..Solve problems involving resultant forces using resolution method and moment of forces.

“ Science without religion is Lame, Religion without science is Blind ” 

                                               

*Albert Einstein*

TYPE OF FORCES	DESCRIPTION OF FORCE
Applied force	A force that is applied to an object by a person or another object. For example if a person is pushing a desk across the room, then the applied force is the force exerted on the desk by the person.
Gravity Force (weight)	The force with which the earth, moon or other massively large object attracts another object towards itself. (weight of the object) Where g = 9.81 N/kg (on earth) And m = mass (kg) The force of gravity on earth is always equal to the weight of the object as found by the equation: Fg, = mg  (N)
Normal Force	Is the support force exerted upon an object that is in contact with other stable objects. 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.
Friction Force	The force is exerted by a surface as an object moves across it or makes an effort to move across it. There are at least two types of friction force - sliding and static friction. Friction results from the two surfaces being pressed together closely, causing intermolecular attractive forces between molecules of different surfaces. As such, friction depends upon the nature of the two surfaces and upon the degree to which they are pressed together. The maximum amount of friction force that a surface can exert upon an object can be calculated using the formula below: Ffrict = µ • Fnorm
Air Resistance Force	It is a special type of frictional force that acts upon objects as they travel through the air. This force will frequently be neglected due to its negligible magnitude (and due to the fact that it is mathematically difficult to predict its value). It is most noticeable for objects that travel at high speeds (e.g., a skydiver or a downhill skier) or for objects with large surface areas.
Tension Force	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.
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. For most springs (specifically, for those that are said to obey "Hooke's Law"), the magnitude of the force is directly proportional to the amount of stretch or compression of the spring.

THE EFFECT OF FORCES

DIFFERENT BETWEEN MASS AND WEIGHT

COMPARISON	MASS (m)	WEIGHT W =mg
Measurement Tools:	Pan balance, a triple-beam balance, lever balance or electronic balance.	Spring balance
Type of Quantity	Scalar quantity	Vector quantity
Effect of gravity	Always constant (any time or places)	Depends on the gravity at that place.
Symbols	‘m’ for mass	‘W’ for weight
Measurement Unit	kg for kilogram	N for newton or (kg.m/s2)
About	The amount of matters presents in a body and is an intrinsic property of the body.	The strongly gravity pulls on the matter.

NEWTON’S LAWS

 Newton’s First Law of Motion

Newton said that any object in motion will stay in motion and that any object at rest will stay at rest unless acted on by an outside force.  This is generally known as the "Law of Inertia" and inertia was originally a theory that Galileo found. Inertia- an object's tendency to resist a change in motion.

In other words, when no force acts on an object, the acceleration of the object is zero. From the first law, we conclude that any isolated object is either at rest or moving with constant velocity. The tendency of an object to resist any attempt to change its velocity is called inertia.

         

Newton’s Second Law

Newton’s second law answers the question of what happens to an object that has one or more forces acting on it.

Newton's First Law deals with an object with no net force. Newton's Second Law talks about an object that has net force. It states that when the net force acting on an object is not zero, the object will accelerate at the direction of the exerted force. In other words, force and acceleration are directly proportional, while mass and acceleration are inversely proportional.

Newton’s Third Law of Motion

If you press against a corner of this textbook with your fingertip, the book pushes back and makes a small dent in your skin. If you push harder, the book does the same and the dent in your skin is a little larger. This simple activity illustrates that forces are interactions between two objects : when your finger pushes on the book, the book pushes back on your finger. The important principle is known as Newton’s third law.

BALANCED AND UNBALANCED FORCES

Balanced forces occur when the net force acting on an object is equal to zero ( F = 0 ). The object is either at rest or moving at constant velocity.

Unbalanced forces occur when there is a net force acting on the object. The net force is known as resultant force.

 RESOLUTION OF FORCES

 Resolution of Force is process to resolve a single force into a set of component forces which are perpendicular to each other. The principle of the resolution of vector is very important in solving problems which involve several forces which act in different directions.

RESULTANT FORCE

The resultant force is defined as a single force that will produce the same effect as the two or more combined that it replaces. There are a variety of ways to determine the magnitude and direction of the resultant when vectors are directed in other than vertical or horizontal directions. The primary methods used are :

1.  Pythagorean Theorem And Trigonometric Method.

           2. Head-To-Tail Method Using A Scaled Vector Diagram

FORCES IN EQUILIBRIUM

Sum of forces can only be calculated when the forces are acting along one straight line such as on Y-axis or X-axis.

Forces acting to the right an upward are considered as positive and forces acting downward and to the left are considered negative forces

MOMENT OF FORCES

The Moment of a force is the turning effect about a pivot point. To develop a moment, the force must act upon the body to attempt to rotate it. A moment is can occur when forces are equal and opposite but not directly in line with each other.

PRINCIPLE OF MOMENT OF FORCE

The principle of moment state that when in equilibrium,

The total sum of the counter clockwise moment is equal to the total sum of clockwise

CENTER OF GRAVITATONAL FOR EQUILIBRIUM

They are two methods that can be used to determine the center of gravity of an object. The methods are the moment of force method  and the moment of resultant weight method.