Class 9 Force and laws of motion by rohan sir iitj

PhonePe stand poses are cool in nature.Force is an external effort to reach which may make a body at rest.Or stop a movie body or change the speed of moving in body or change the direction of meeting body or change the safe and size of their body need of forced return and seriously time

1. CHAPTER : 8
FORCE AND
LAW’S OF MOTION
BY ROHAN SIR
IIT/JEE/NEET
FOUNDATION

2. Force :
Force can be push or pull in nature .Forceisanexternal
effortwhichmaymoveabodyat restorstopamoving
bodyorchangethespeedofa movingbodyorchange
thedirectionofamovingbodyor changetheshape
andsizeofabody.
Unit of force “Newton”
CGS unit dyne

3. 1. Which of the following is the SI units of force?
a Kgm/s
2
b Kgm/
-s
c Newton-metre
d Newton

4. Q. Which of the following is true about force?
a. Force is invisible
b. Force can move a body
c. It can deform a body
d. All of the above
Answer:(d) all of the above

5. Effectsofforce:-
i)Forcecanmoveabodyatrest.
ii)Forcecanstopamovingbody.
iii)Forcecanchangethespeedofamoving
body.
iv)Forcecanchangethedirectionofamoving
body.
v)Forcecanchangetheshapeandsizeofa
body

6. Forcecanmoveabodyatrest.

7. Forcecanstopamovingbody.

8. Forcecanchangethespeedofamoving
body.

9. Forcecanchangethedirectionofa
movingbody.

10. Forcecanchangetheshapeandsizeofa
body

11. Balancedandunbalancedforces:
i)Balancedforces:-
Iftwoforcesactonabodyinoppositedirectionandif
boththe forcesareequal,thentheresultantforce
actingonthebodyiszero. Suchforcesarecalled
balancedforces.
Balancedforcescannotchangethestateofrestor
motionofabody

12. ii)Unbalancedforces:-
Iftwoforcesactonabodyinoppositedirectionand
ifoneforceis greaterthantheother,thentheresultant
forceisnotequaltozero.Such forcesarecalled
unbalancedforces . Unbalancedforceschangesthe
stateofrestorthemotionofabody.

13. Difference between balanced and unbalanced force.
Balanced force Unbalanced force
1. When two or more forces act on a body
and produce a net force equal to zero
then the forces are called balanced forces.
1. When two or more forces act on a body
and produce a net force not equal to zero
then the forces are called unbalanced
forces.
2. A balanced force does not produce any
change in the state of uniform motion or
the rest of the body.
2. An unbalanced force can produce the
change in the state of uniform motion or
the rest of the body.
3. A balanced force does not cause to
accelerate a body.
3. An unbalanced force can accelerate a
body.

14. Types of forces :
TherearetwomaintypesofforcesTheyare:-
i)Contactforces
ii)Noncontactforces

15. Contactforces:-
Forcesexertedbyoneobjectonanotherwhentheyare
incontactwitheachother.
i)Muscularforce
ii)Forceoffriction
iii)Normal force
iv)Tension force

16. Noncontactforces
Forcesexertedbyoneobjectonanotherwithoutany
contactwitheachother.
i)Magneticforce
ii)Electrostaticforce
iii)Gravitationalforce
iv)Nuclear force

17. Differentiate between contact and non contact force?
Forces
Contact Forces Non-Contact Forces
1.
These forces require a physical touch
or contact between the two objects.
These forces do not require any
physical touch to work on the
object.
2.
Since, in contact force, contact is to be
made between the two objects therefore it
does not work from a distance.
Since, in non-contact force contact
is not required between the two
objects, therefore it can work from
a distance.
3.
Contact force includes muscular force, the
normal force, the tension and the frictional
force.
Non-contact force includes electro-
magnetic force, gravitational force
and electrostatic force.
4.
For Example, muscular force is required to
kick the ball, else the ball would not move.
For example, There is no contact
between a free falling object and
Earth.

18. Forceof friction :
Frictional force is the force generated by two surfaces that contact
and slide against each other.
Forceoffrictionistheforcewhichopposesthemotionofan
object overasurface.

19. Eg:-1. Aballrollingongroundgraduallyslows
downand comestoastopduetoforceof
friction.
2. Ifwestoppedalingabicycle,itgradually
slows downandcomestoastopduetoforce
offriction.

20. Anobjectwithuniformmotionwillcontinuetomove
withuniform motioniftheforcesactingonit(pushing
forceandfrictionalforce)are balanced.
Ifanunbalancedforceactsonthemovingbody,
thenitsspeedor directionofmotionchanges.
Iftheunbalancedforceisremoved,thenitwill
continuetomovewith thespeedithadacquiredtill
then.

21. Galileo'sexperimentofmotionofanobjectonan
inclinedplane:-

22. Step 1
Whenamarblerollsdownaninclinedplane,itsvelocity
increasesandwhenit goesuponthesecondinclined
plane,itsvelocitydecreases.Iftheinclinations ofboth
theplanesareequal,thenthemarblewillreachthe
sameheightwhichit rolleddown.
Iftheinclinationofthesecondplaneisdecreased,it
willtravelmoredistancetoreachtheoriginalheight.

24. Step 2:Iftheinclinationofthesecondplane ismade
horizontal,themarblewilltravelforevertryingto reach
thesameheight. Anunbalancedforceisrequiredto
changethemotionofthemarblebut noforceisneeded
tosustaintheuniformmotionofthemarble.

25. Newton'slawsofmotion:-
Newton'sfirstlawofmotionstatesthat:-
'Anobject remainsinastateofrestorinuniform
motioninastraight lineunlesscompelledtochange
thatstatebyanapplied force.'

26. Inertia:-
Thenaturaltendencyofobjectstoremainina state
ofrestorinuniformmotioniscalledinertia.Thisis
whythefirstlawofmotionisalsoknownasthe lawof
inertia.

27. Examplesofinertia:-
1.Ifastrikerhitsapileofcoinsonacaromboard,thelowestcoin
movesoutandduetoinertiaofrest,theothercoinsfalldown

28. 2. Ifacoinplacedonaplayingcardoveratumbleris
flickedwiththe finger,duetoinertiaofrest,thecoin
fallsdownintothetumbler.

29. 3. Whenwetravelinacarandthedriverappliesthebrakes
suddenly, wetendtofallforwardduetoinertiaofmotion.

30. 4.Whenwearestanding inabusandthebusbeginstomove
suddenly,wetendtofallbackwardbecauseourfeetincontact
withthe floormovesforwardbuttheupperpartofthebody
continuestoremain atrestdueto inertiaofrest.

31. InertiaandMass:-
Abodyatrestcontinuestobeatrestandabodyin
motioncontinuestobeinmotion.Thispropertyofa
body iscalleditsinertia.
Theinertiaofabodyismeasuredbythemagnitude of force
requiredtochangethestateofthebody.Theforce required
tochangethestateofaheavierbodyismore thanthe
forcerequiredtochangethestateofthelighter body.This
isbecausethemassoftheheavierbodyis morethanthe
massofthelighterbody.
So'Themassofabodyisameasureofitsinertia.'

33. Q; The inertia of an object causes the object to
a. decrease its speed
b. Increase its speed
c. resist any change in the state of its motion
d. decelerate due to friction
Answer:(c) resist any change in the state of its motion

34. Q; Which law is also known as the law of inertia?
a. Newton’s first law of motion
b. Newton’s second law of motion
c. Newton’s third law of motion
d. Law of conservation of momentum
Answer:(a) Newton’s first law of motion

35. Momentumofabody:-
Themomentum ofabodyistheproductofitsmassand velocity.
Momentum=massxvelocity
wherepisthemomentum ofabody
misthemassofthebody
visthevelocityofthebody
p=mv

36. Q: If the mass of the body is doubled and its velocity becomes half, then
the linear momentum of the body will
a. become double
b. remain the same
c. become half
d. become four times
Answer:(b) remain the same

37. Q; The linear momentum of an object is 250 g cm/s. If the
velocity of the object is 5 m/s, then the mass of the object is
a. 0.5 g
b. 5 kg
c. 0.5 mg
d. 5 mg
Answer:(a) 0.5 g

38. Q. A goalkeeper in a football game pulls his hands backwards after holding
the ball shot at the goal. This enables the goalkeeper to
a. increase the rate of change of momentum
b. decrease the rate of change of momentum
c. increase the force exerted by the balls on the hands
d. exert larger force on the ball
Answer:(b) decrease the rate of change of momentum

39. Ifabodyisatrestitsvelocityiszeroandsoits
momentum isalsozero.
TheSIunitofmomentum iskilogrammeterpersecondor kgm/sor
kgms·
1
Eg:-Atruckmovingataverylowspeedcankillapersonstandingin
itspathbecauseoftheheavymassofthetruck.
Abulletofsmallmasswhenfiredfromaguncankillaperson because
ofthelargevelocityofthebullet.
Sotheimpactofabodydependsuponitsmassandvelocity

40. Newton'ssecondlawofmotion:-
Newton'ssecondlawofmotionstatesthat:-
'Therateofchangeof momentumofanobjectis
proportionaltotheappliedforceinthe directionof
force.'

41. MathematicalformulationofSecondlawof
motion:-
Ifanobjectofmassmismovingalongastraightlinewith
initial velocityuandisacceleratedtovelocityvintimetby
applyingaforce F,then
Initialmomentump
1=mu
Finalmomentump
2=mv
Changeinmomentum= p
2-p
1
=mv-mu

42. F=ma
TheSIunitofmassiskgandacceleration ism/s
2or
ms·
2sotheunit ofForceiskgms·
2ornewton.It's
symbolisN

43. Q; A passenger in a moving train tosses a coin that falls behind him. It
means that the motion of the train is
a. uniform
b. accelerated
c. retarded
d. along circular tracks
Answer:(b) accelerated

44. Q;WhichofthefollowingisNOTtrueforanobjectmovingalongastraight
pathinanacceleratedmotion?
a. Its velocity always changes
b. Its speed keeps changing
c. It always goes away from the Earth
d. It always goes away from the Earth
•Answer:c. A force is always acting on it

45. Newton's thirdlawofmotion:-
Newton'sthirdlawofmotionstatesthat:-
'Toeveryactionthereis anequalandopposite
reactionandtheyactontwodifferentbodies.'

46. Toprovethatactionandreactionareequal
andopposite:
TaketwospringbalancesAandBconnectedtogether.Fixthe
spring balanceBtoarigidsupport.Whenaforceisappliedby
pullingthefree endofthespringbalanceA,boththespring
balancesshowthesame readings.Thisshowsthattheforce
exertedbythespringbalanceAonB is equalbutoppositein
directiontotheforceexertedbyspringbalanceB onA.The
forceexertedbythespringbalanceAonBisactionandthe
forceexertedbythespringbalanceBonAisreaction.

48. Q. Which of the following is true for the third law of motion?
a. Action-Reaction pair always acts on the same body.
b. They act on different bodies in opposite directions
c. Action-Reaction pairs have the same magnitudes and directions
d. Act on either body at normal to each other
Answer:(b) They act on different bodies in opposite directions.

49. Examplesofactionandreaction:-
Whenabulletis firedfromagun,itexertsaforwardforce(action)
on thebulletandthebulletexertsanequalandoppositeforceon
thegun (reaction)andthegunrecoils.

50. Whenasailorjumpsoutofaboat,heexertsabackwardforce
ofthe boat(action)andtheboatexertsanequalandopposite
forceonthe sailor(reaction)andthesailorjumpsforward.

51. Whenanairfilledballoonisreleased,theforceofthe
aircomingout oftheballoon(action)exertsanequal
andoppositeforceonthe balloon(reaction)andit
movesupward.

52. Whenarocketisfired,theforceoftheburninggases
comingout (action)exertsanequalandoppositeforce
ontherocket(reaction)and itmovesupward

53. Conservationofmomentum:-
TheLawofconservationofmomemtumstatesthat:-
'Thesumofmomentaoftwoobjectsbeforecollisionisequaltothe
sumofmomentaaftercollisionprovidedthereisnounbalancedforces
actingonthem.'
Thismeansthatthetotalmomentumofthetwoobjectsisunchanged
orconservedbycollision.

54. If two balls A and B of masses
Velocities �
�and �
�and �
�>�
�, the two balls will collide with each other .
During collision at time t , ball A exerts a force ??????
��on ball B and B exerts a force
??????
��on ball A . If �
�and �
�are the velocities of balls A and B after collision ,
??????
�and ??????
�are travelling in a straight line with initial

55. A B
�
�
�
�
??????
� ??????
�
A B
??????
��
??????
��
Before collision
Time of collision

56. A B
�
�
�
�
??????
� ??????
�
After collision

57. The moment of ball A before and after collision are ??????
��
�and
??????
��
�
And the moment of ball B before and after collision are ??????
��
�and
??????
��
�
Change in momentum of ball A during collusion = ??????
��
�-??????
��
�
Rate of change of momentum of ball A (??????
��) =
??????
��
�−??????
��
�
�

58. Change in momentum of ball B during collusion = ??????
��
�-??????
��
�
Rate of change of momentum of ball B(??????
��) =
??????���−??????���
�
According to Newton's third law of motion
(??????
��) =(??????
��)
??????
��
�−??????
��
�
�
=
??????
��
�−??????
��
�
�

59. After solving this we get
??????
��
�+ ??????
��
�= ??????
��
�+ ??????
��
�
Moment of two ball before collision equal to after collision of two
ball

60. Change in momentum of ball B during collusion = ??????
��
�-??????
��
�