Chapter 3 NEWTON'S

7/29/2019 Chapter 3 NEWTON'S

1/55

PUSAT ASASI SAINS PERTANIANASP0501 Introduction to Mechanics

Force and Motion

The Laws of Motion


2/55


Classical Mechanics

Describes the relationship between the motionof objects in our everyday world and the forcesacting on them

Conditions when Classical Mechanics does not

apply very tiny objects (< atomic sizes)

objects moving near the speed of light

Quantum and relativistic mechanics

Kinetics analyzes the cause of motion force


3/55


Sir Isaac Newton

1642 1727 Formulated basic

concepts and laws ofmechanics

Universal Gravitation

Calculus

Light and optics


4/55


Natural characteristicof an object

Tendency tocontinue in motion

Tendency toremain at rest

inertia momentum


5/55


Inertia

The natural tendency of an object toremain at rest.

Measured by its mass

In order to move the object you need aforce (eg push or pull) acting on theobject.


6/55


Momentum

Common meaning:tendency of a body to continue in motion

Physics definition:the quantity to measure the tendency tooppose change of motion


7/55


Momentum

The natural tendency of an object tomaintain uniform motion (constant velocity).

This tendency is dependent on the mass ofthe object and its velocity

Define momentum,

Momentum = mass velocity,

A vector quantity

In order to change the velocity of the objectyou need a force acting on the object.

vmp


8/55


Force

Effect of force - change of velocity

From rest to move increase/ decrease in value change in direction both

Other effect of force - change of shape


9/55


External and Internal Forces

External forceAny force that results from the interaction

between the object and its environmentexample: push, pull, gravitational force, magnetic force

Internal forces Forces that originate within the object itself

They cannot change the objects velocity

example: vibration of atoms


10/55


Forces on an object

Usually think of a force as a push or pull Vector quantity

May be a contact force or a field force (non-contact)

Contact forces result from physical contact betweentwo objects

Field forces act between disconnected objectsAlso called action at a distance


11/55


The forces can be: contact forces field (non-contact) forces

Forces on an object


12/55


Single or many forces

Net effect is due to the net force (sum of allforces)

Forces on an object


13/55


Newtons First Law

An object will remain at rest or moveswith a constant velocity (that is constantin magnitude and direction) unless actedon by a nonzero net force

The net force (also known as resultant force) isdefined as the vector sum of all the external forcesexerted on the object

Newtons First LawAlso known as Law of Inertia


14/55


Newtons First Law

Nonzero net force

No force act on the object

Sum of the forces act on the object is zero


15/55


Newtons First Law

As long as no net force acting on an object An object at rest remains at rest

An object moving with constant velocity continuesmoving with the same velocity

The object are said to be in equilibrium.

If Fnet = 0 thenv = 0

v = constant

or acceleration a = 0


16/55


Newtons First Law

First condition of equilibriumThe net (external) force acting on the object is zero.

0F

1F

2F

4F

3F


17/55


Newtons First Law

The object are in equilibrium if:

It is at rest ( v = 0) static equilibrium.

It is in uniform motion ( v = constant ) dynamic equilibrium

First condition of equilibrium

The net (external) force acting on the object is zero.

0F


18/55


Newtons First Law

First condition of equilibriumThe net (external) force acting on the object is zero.

0F

yx

yx

yx

yx

FFF

FFF

FFFFFF

444

333

222

111

0xF

0yF

and

then


19/55


Force and momentum

Object can change its velocity only whenthere is a force (eg push or pull) acting onthe object.

A force will change the velocity of anobject.

A force can move an object initially at rest

A force can stop a moving object

A force can make object to move faster orslower.


20/55


Force and momentum

Everyday experience: Force depends on mass

Force depends on velocity

Momentum : p = m v

What is the relationship between forcewith momentum?


21/55


Newtons Second Law

The rate of change of momentum of an object isdirectly proportional to the net force acting on it.

F D tnetDp


22/55


Newtons Second Law

Change of momentum Dp

Change of mass Change of velocity

D t

Dp= m D t

Dv + v DmD t

Rate of Change of momentum

F D t

Dpnet


23/55


Newtons Second Law

Assume: no change in mass.

Change in momentum change in velocity only

Rate of change of momentum

D tDp = m D tDv

= m a

Newtons Second Law

Fnet m a


24/55


Units of Force

SI unit of force is a Newton (N)

The force to change 1 kg of mass to

accelerate at 1 m s-1.

2s

m

kg1N1 1

Newtons Second Law

Fnet = m aThis equation is only true if the mass is constant


25/55


Newtons Second Law

If there is change in mass.

Rate of change of momentum

Newtons Second Law

Fnet = m a

D t

Dp

= m D t

Dv

+v

Dm

D t


26/55


Newtons Third Law

If object 1 and object 2 interact, the forceexerted by object 1 on object 2 is equal inmagnitude but opposite in direction to theforce exerted by object 2 on object 1.

Equivalent to saying a single isolated force

cannot exist

12 21 F F


27/55


Newtons Third Law

F12 may be called theactionforce and F21 thereactionforce

Actually, either force can

be the action or thereaction force

The action and reactionforces

act on different objects


28/55


Newtons Law of motion

Summary: First law

Second law

Third law


29/55


Newtons Law of motion

Application:

First law

Particle in equilibrium

Acceleration, a = 0

Velocity, v = 0 or v = constant

Second law

Particle in motion

Acceleration a 0

Third law

What are

the forcesacting on

the object?


30/55


Forces on an object

1. Gravitational Force

2. Normal reaction force

3. Friction4. Tension of string

5. Other external force ( push/ pull)


31/55


Forces on an object

1. Gravitational Force Mutual force of attraction between any

two objects

Expressed by Newtons Law of UniversalGravitation:

2

21g

rmmGF


32/55


The magnitude of the gravitational forceacting on an object of mass mnear theEarths surface is called the weightWof the object

W = m g is a special case of NewtonsSecond Law gis the acceleration due to gravity

Forces on an object


33/55


The acceleration due to gravity gdependsupon location (altitude above the Earth).

Near the Earth, g taken as constant

g = 9.81 m s-1

Direction of g is vertically downward (directedtowards the centre of the Earth).

Gravitational Force


34/55


Forces on an object

2. Normal Reaction forceNormal Reaction Force alwaysexists when two objects are in

contactIt is the reaction of thesurface of the second objectdue to the force (action) by

the first object. (Newtonsthird law)

12


35/55


Normal (Reaction) Force

Normal Reaction Force always existswhen two objects are in contact It is the reaction of the surface of the

second object due to the force (action) bythe first object. (Newtons third law)The direction of Normal Reaction Force isalways perpendicular to the surface


36/55



R

R


37/55



RR


38/55


Forces on an object

3. Friction forceNormal Reaction Force alwaysexists when two objects are in

contact

This is due to theinteractions between the

object and the surface

1

2


39/55


Force of Friction

When an object is in motion (tends tomove) on a surface or through a viscousmedium, there will be a resistance to themotion

This is due to the interactions between theobject and its environment

This is resistance is called friction Static friction (stationary object)

Kinetic friction (moving object)


40/55


Forces of Friction

This is due to theinteractions betweenthe object and itsenvironment

The force ofstaticfriction is generallygreater than the forceofkinetic friction


41/55


Forces of Friction

Friction is proportional tothe normal force

Ffriction = m R m is the coefficient of friction

The direction of thefrictional force is parallelto surface opposite thedirection of motion


42/55


Forces of Friction

The direction of the frictional force isopposite the direction of motion

Friction is proportional to the normal(reaction) force, parallel to surface

Ffriction = m R The coefficient of friction () depends on

the surfaces in contact

The coefficients of friction are nearlyindependent of the area of contact


43/55


Forces of Friction


44/55


Forces on an object

4. Tension forceTension Force always existswhen the object is

connnected by a rope/ string/cable etc


45/55


Tension Force

The force exerted by a rope, string, wire.

The direction is always towards the centre ofthe rope.


46/55


Tension Force

The force exertedby a rope, string,wire.

The direction isalways towards thecentre of the rope.

If pulley is smooth,tension on bothsides are equal inmagnitude


47/55


Tension Force


48/55


Free Body Diagram

Treat the object as a particle (representedby a point).

Treat each object separately.

Only forces acting directly on the objectare included in the free body diagram

Indicate the direction of each force.

If the free body diagram is incorrect, the

solution will likely be incorrect


49/55


Free Body Diagram


50/55


Free Body Diagram

F B d Di


51/55


Free Body Diagrams

Free Body Diagram


52/55


Free Body Diagram Multiple Objects

Draw free diagram separately for each object

Free Body Diagram


53/55


Free Body Diagram Multiple Objects

Draw free diagram separately for each object

A li ti N t L f ti


54/55


Need to identify the forces acting on the

object Weight of object (gravitational force)

Normal (reaction) force

Frictional force Tension of string/ rope

Other external force

Need to draw a free body diagramA rough sketch to show all the forces on the

object

Application Newtons Law of motion

Object in contact

with a surface

Connected objects

A li ti N t L f ti


55/55

Need to find the NET FORCE (addition of

vectors) Resolve vectors into components.

Parallel component

Perpendicular component

Apply the Newton law of motion equationApply separately for each object component

by component

Solve the equations Simultaneous equation

Application Newtons Law of motion

Chapter 3 NEWTON'S

Documents

Transcript of Chapter 3 NEWTON'S