السابعة المحاضرة

The Laws of Motion

The Laws of Motion

Chapter 5

5.1 The Concept of Force

The body is moving, When it change its position during a limited time.

5.1 The Concept of Force

Types of Motion:

5.1 The Concept of Force

. An object accelerates due to an external force.

.Force is a vector quantity

Definition of equilibrium

object is in equilibrium if: The net force acting on the object is

zero ,

. The object remains at rest.

.or continues to move with constant velocity.

Measuring the Strength of a Force

It is convenient to use the deformation of a spring to measure force.

Newton ‘s laws of motion

FIRST NEWTON’s LAW

SECOND NEWTON’s LAW

THIRD NEWTON’s LAW

5.2 Newton’s First Law

Newton’s first law (law of inertia):

“In the absence of external forces, an object at rest remains at rest and an object in motion continues in motion with a constant velocity (that is, with a constant

speed in a straight line)”.

5.3 Mass

. Mass (m) is that property of an object that specifies how much resistance an object exhibits to changes in its velocity.

. The SI unit of mass is the kilogram (Kg).

. mass is a scalar quantity

5.3 Mass

Mass and weight

The Weight of an object (W):

. W = the gravitational force exerted on the object

. W varies with locationThe Mass (m) :

. Is the same everywhere

5.4 Newton’s Second Law

“The acceleration of an object is directlyproportional to the net force acting on it and

inversely proportional to its mass.”

: is the net forcea : the acceleration.m: mass.

5.4 Newton’s Second Law

SI unit of force:

CGS unit of force:

2s

mKgNewton

2s

cmgDyne

5.4 Newton’s Second Law

5.4 Newton’s Second Law

Example 5.1

Question 2

Question 3

Problem 5.1

Problem 5.3

Problem 5.11

Problem 5.11

5.5 The Gravitational Force and Weight

Gravitational force Fg :The attractive force exerted by the Earth on an object.

.The magnitude of Fg:

.Direction: Toward the center of earth

5.5 The Gravitational Force and Weight

5.5 The Gravitational Force and Weight

5.5 The Gravitational Force and Weight

Problem 5.9

5.6 Newton’s Third Law

“ If two objects interact, the force F12 exerted by object 1 on object 2 is equal in magnitude and opposite in direction to the

force F21 exerted by object 2 on object 1:”

5.6 Newton’s Third Law

“The action force is equal in magnitude to the reaction force and opposite in

direction ”. The action and reaction forces:

. act on different objects.

. and must be of the

same type.

5.6 Newton’s Third Law

5.7 Some Applications of Newton’s Laws

.Tension Force

.Frictional Force: (in opposite of mosion direction)

.Elevator

.The Atwood Machine

.Inclined surface

Example 5.4 A Traffic Light at Rest

Example 5.4 A Traffic Light at Rest

Inclined Surface

One Block Pushes Another

Elevator

Example 5.8 Weighing a Fish in an Elevator

Example 5.8 Weighing a Fish in an Elevator

The Atwood Machine

Acceleration of Two Objects Connected by a Cord

Problem 21

Problem 21

Problem 23

Problem 24