force and laws of force and laws of motion motion

made by : m.d zakaria for class: 9TH

date: 4-8-13

ForceForce The cause of an acceleration, or the change The cause of an acceleration, or the change

in an object’s motionin an object’s motion Can cause an object to move, stop, or Can cause an object to move, stop, or

change directionchange direction SI unit for force is the Newton (N)SI unit for force is the Newton (N)

* 1 N = a kg x m / sec1 N = a kg x m / sec22

* Mass = amount of matter presentMass = amount of matter present

* Weight = measure of gravitational attraction Weight = measure of gravitational attraction * ( force due to gravity; 1 lb = 4.45 N)( force due to gravity; 1 lb = 4.45 N)

Balanced forceBalanced force If the resultant of the several forces acting on a If the resultant of the several forces acting on a

body is zero , the forces are said to be body is zero , the forces are said to be ‘balanced forces’. ‘balanced forces’.

Unbalanced forcesUnbalanced forces If the resultant of the several forces acting on a If the resultant of the several forces acting on a

body is not zero , the forces are said to be body is not zero , the forces are said to be ‘unbalanced forces’. ‘unbalanced forces’.

InertiaInertia

It is the inability of a body to change by itself its It is the inability of a body to change by itself its state of rest or of uniform motion in a straight.state of rest or of uniform motion in a straight.

Inertia of restInertia of restIt is the tendencyIt is the tendency

of a body to of a body to

remain in its state remain in its state

of rest.of rest.

Inertia of motionInertia of motion It is the tendency of a body to remain in its It is the tendency of a body to remain in its

state of uniform motion in a straight line.state of uniform motion in a straight line.

Inertia of directionInertia of direction It is the inability of a body to change by itself It is the inability of a body to change by itself

its direction of motion.its direction of motion.

Momentum Momentum It is the quantity of motion possessed by a It is the quantity of motion possessed by a

body and is equal to the mass and velocity of body and is equal to the mass and velocity of the body.the body.

Momentum = mass multiply by velocity or p= Momentum = mass multiply by velocity or p= mvmv

Momentum is a vector quantityMomentum is a vector quantity . Its unit is kg ms-1. Its unit is kg ms-1

Newton laws of motionNewton laws of motion

Now that we have learned how to describe Now that we have learned how to describe motion, how do we cause the motion that we motion, how do we cause the motion that we want?want?

We apply force on an object!We apply force on an object!But what do forces directly affect: But what do forces directly affect:

location? velocity? acceleration? location? velocity? acceleration? jerk?jerk?Newton answered these questions by postulating Newton answered these questions by postulating

threethree laws of motion. laws of motion.

Newton first law of motionNewton first law of motion

Newton’s First Law of MotionNewton’s First Law of Motion: : an object an object

inin motion will, in the absence of forces, tend to motion will, in the absence of forces, tend to remain in motion with neither the speed nor remain in motion with neither the speed nor direction changing.direction changing.

This, at first, does not seem obvious. Most things This, at first, does not seem obvious. Most things on earth tend to slow down and stop. However, on earth tend to slow down and stop. However, when we consider the situation, we see that there when we consider the situation, we see that there are lots of forces tending to slow the objects are lots of forces tending to slow the objects down such as friction and air resistance.down such as friction and air resistance.

Newton first law of motionNewton first law of motion

When we look at the planets and moon, When we look at the planets and moon, however, it is easier to see that they just keep however, it is easier to see that they just keep right on going!right on going!

Also, when we remove or reduce a lot of the Also, when we remove or reduce a lot of the forces on an object, it does tend to keep right forces on an object, it does tend to keep right on going. Consider a ball rolling on a smooth on going. Consider a ball rolling on a smooth floor. We don’t need forces to keep the floor. We don’t need forces to keep the motion going!motion going!

Newton second law of motionNewton second law of motion

If we want to change the motion, we push on it (that is, If we want to change the motion, we push on it (that is, apply forces). apply forces).

Newton states this in his Newton states this in his Second Law of MotionSecond Law of Motion: : The The resultant force (vector sum of the individual forces) resultant force (vector sum of the individual forces) on an object causes the object to accelerate in the on an object causes the object to accelerate in the same direction as the resultant force and in inverse same direction as the resultant force and in inverse proportion to the mass of the objectproportion to the mass of the object::

F = maF = ma

. . m[v-u \ t] or ft m [v-u]m[v-u \ t] or ft m [v-u]

Newton second law of motionNewton second law of motion

Note that this is a Note that this is a vectorvector equation, and should equation, and should really be worked in component form:really be worked in component form:

FFxx = ma = maxx

FFyy = ma = mayy ..

We can now see that Newton’s First Law of We can now see that Newton’s First Law of Motion is really just a special case of his Motion is really just a special case of his Second Law of Motion.Second Law of Motion.

Newton third law of motionNewton third law of motion

There is one further important aspect of motion There is one further important aspect of motion that Newton identified: the distinction that Newton identified: the distinction between forces that act between forces that act onon an object and forces an object and forces that act that act byby the object. This leads to his the object. This leads to his Third Third Law of MotionLaw of Motion: : For every force by a first For every force by a first object on a second object, there is a force by object on a second object, there is a force by the second object on the first object with the the second object on the first object with the same magnitude but in the opposite same magnitude but in the opposite direction.direction.

Newton third law of motionNewton third law of motion

This is sometimes called the law of action and This is sometimes called the law of action and reaction.reaction.

I like to call it: I like to call it: you can’t push yourselfyou can’t push yourself! You ! You can only push on an object and hope that it can only push on an object and hope that it pushes back.pushes back.

Example: when you walk up a stairs, you use Example: when you walk up a stairs, you use your muscles to push down on the stairs and your muscles to push down on the stairs and you trust that the stairs will push back up on you trust that the stairs will push back up on you lifting you up the stairs.you lifting you up the stairs.

Laws of conservation of momentumLaws of conservation of momentum

its states that the total momentum of any system its states that the total momentum of any system of object remains constant in the absence of of object remains constant in the absence of any external forces. According to total any external forces. According to total momentum before collision = total momentum momentum before collision = total momentum after collision after collision

MM11UU11 + M + M22UU22 = M = M11UU11 + M + M22UU22

Friction Friction

Whenever body slides or rolls over the surface Whenever body slides or rolls over the surface of another body , a force comes into action of another body , a force comes into action which acts in the opposite direction of the which acts in the opposite direction of the motion of the body. This opposing force is motion of the body. This opposing force is called ‘friction’.called ‘friction’.

Unit Unit The units of mass are kilogram. This is the third The units of mass are kilogram. This is the third

fundamental unit (along with meters and fundamental unit (along with meters and seconds) in the MKS system of units.seconds) in the MKS system of units.

The units of force in the MKS system are The units of force in the MKS system are Newton , where a force of 1 Nt will give a Newton , where a force of 1 Nt will give a mass of 1 kg an acceleration of 1 m/smass of 1 kg an acceleration of 1 m/s22 . .

Mass Mass The quantity of inertia possessed by an object The quantity of inertia possessed by an object

or the proportion between force and or the proportion between force and acceleration referred to in Newton's Second acceleration referred to in Newton's Second Law of Motion. Law of Motion.

mass: mass of a body is a measure of its inertia*the SI UNIT IS KG

MCQ QUESTIONMCQ QUESTION1.1. When a force is exerted on an object , it can When a force is exerted on an object , it can

change its- change its-

a. state b. position a. state b. position

c. shape d. all the abovec. shape d. all the above

Answer- dAnswer- d

Q2- S.I unit of force is-Q2- S.I unit of force is- a. joule b. kg ms-a. joule b. kg ms-11

c. dyne d. newtonc. dyne d. newton answer- danswer- dQ3-the unit of momentum is-Q3-the unit of momentum is- a. kg ms-1 b. kg ms-1sa. kg ms-1 b. kg ms-1s c. kg ms-2 d. kg ms-2sc. kg ms-2 d. kg ms-2s answer- aanswer- a

Q4- Measurement of force is done on the basis Q4- Measurement of force is done on the basis of-of-

a. Newton 1a. Newton 1stst law of motion law of motion b. Newton 2b. Newton 2ndnd law of motion law of motion c. Newton 3c. Newton 3rdrd law of motion law of motion d. All the aboved. All the above

answer- b answer- b

Q5- the rate of change in momentum is –Q5- the rate of change in momentum is –

a. Energy b. force a. Energy b. force

c. Acceleration d. implusec. Acceleration d. impluse

answer- banswer- b

Q6- the principle of function of jet aeroplane , is Q6- the principle of function of jet aeroplane , is based law –based law –

a.1a.1stst law of motion b. 2 law of motion b. 2ndnd law of motion law of motion

c. 3c. 3rdrd law of motion d. law of gravitation law of motion d. law of gravitation

answer- canswer- c

Q7- when an object get an acceleration – Q7- when an object get an acceleration –

a.a. It fall always falls toward the earthIt fall always falls toward the earth

b.b. It velocity is always increasedIt velocity is always increased

c.c. It speed is always increased It speed is always increased

d.d. A force act on itA force act on it

answer-danswer-d

Q8- which is incorrect statement about action Q8- which is incorrect statement about action and reaction referred to newton 3and reaction referred to newton 3rdrd law of law of motion-motion-

a. They are equal b. they are opposite a. They are equal b. they are opposite

c. They act on the same object c. They act on the same object

d. They act on two different objectd. They act on two different object

answer- canswer- c

Q9- a force applied on an object is equal to – Q9- a force applied on an object is equal to – a. Product of mass of object and velocity of a. Product of mass of object and velocity of objectobject

b. Sum of mass of object and velocity of objectb. Sum of mass of object and velocity of object

c. Product of mass of object and acceleration of c. Product of mass of object and acceleration of object object

d. Sum of mass of object and acceleration d. Sum of mass of object and acceleration

answer-canswer-c

Q10- A scooter generally slips on an oily road Q10- A scooter generally slips on an oily road because-because-

a.a. Friction between tyres and road increaseFriction between tyres and road increase

b.b. Friction between tyres and road decreaseFriction between tyres and road decrease

c.c. Inertia between tyres and road increaseInertia between tyres and road increase

d.d. All the aboveAll the above

e.e. answer- banswer- b