Force And Newtons Laws Of Motion


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ppt on Newton's laws of motion, force,law of coservation of mass with examples ,also details about Newton.

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Force And Newtons Laws Of Motion

  1. 1. Force and laws ofmotionMade byuttam kumarclass – 9th ‘a’ 1
  2. 2. force2 A force can be a push or a pull. For example, when you pushopen a door you have to apply a force to the door. You alsohave to apply a force to pull open a drawer. You cannot see a force but often you can see what it does.Forces can change the speed of something, the direction it ismoving in or its shape. For example, an elastic band getslonger if you pull it.
  3. 3. Balance force• When two forces acting on an object are equal in sizebut act in opposite directions, we say that theyare balanced forces. If the forces on an object are balanced (or if there areno forces acting on it) this is what happens:• an object that is not moving stays still• an object that is moving continues to move at thesame speed and in the same direction3
  4. 4. Example• Hanging objects4The forces on thishanging crate areequal in size butact in oppositedirections. Theweight pulls downand the tension inthe rope pulls up.The forces on thishanging crate are
  5. 5. Unbalanced force• When two forces acting on an object arenot equal in size, we say that theyare unbalanced forces.If the forces on an objectare unbalanced this is what happens:• an object that is not moving starts tomove• an object that is moving changes speedor direction 5
  6. 6. example• Resultant forces• The size of the overall force acting on an objectis called the resultant force. If the forces arebalanced, this is zero. In the example above, theresultant force is the difference between the two6
  8. 8. NEWTONSir Isaac Newton PRS MP (25 December1642 – 20 March 1727) was anEnglish physicist and mathematician who iswidely regarded as one of the most influentialscientists of all time and as a key figure inthe scientific revolution. Newton also made seminal contributionsto optics and shares credit with GottfriedLeibniz for the invention of the infinitesimalcalculus.Newtons Principia formulated the laws ofmotion and universal gravitation thatdominated scientists view of the physicaluniverse for the next three centuries. It alsodemonstrated that the motion of objects on theEarth and that of celestial bodies could be8
  9. 9. NewtON’s Laws Of MOtiON1. 1st Law – An object at restwill stay at rest, and anobject in motion will stay inmotion at constantvelocity, unless acted uponby an unbalanced force.2. 2nd Law – Force equalsmass times acceleration.3. 3rd Law – For every action 9
  10. 10. First law ofmotionlaw of inertia10
  11. 11. First lawthe law of inertiaAccording to Newtons first law, anobject in motion continues in motionwith the same speed and in the samedirection unless acted upon by anunbalanced force. It is the naturaltendency of objects to keep on doingwhat theyre doing. All objects resistchanges in their state of motion. Inthe absence of an unbalanced force, 11
  12. 12. AccordingtoNewtonsfirstlaw, themarble onthat bottomrampshould just12
  13. 13. MATHEMATICALLYFIRST LAWThe first law can be stated mathematicallyas:-13
  14. 14. ‹#›
  15. 15. Factors which determinethe Moment of Inertia of abody The mass of the body. Experiments showthat Inertia is directly proportional to themass. The distribution of mass in the body.15
  16. 16. Affect of inertia on changeOf Object ‘s Mass16A real car has a largemass ,so it has alarge mass , so it hasa large inertia, andhence quite difficult toA toy car has asmall mass, so ithas a small inertia,and hence can bemoved easily by
  17. 17. Affect of inertia if objectis at rest Initially, both the coinand card ,are in stateof rest. Now when wehit the card with ourfingers , a force actson the card andchanges its state ofrest to that of motion.The force of flickerhowever, does notacts on the coin and itfalls into the tumbler.17
  18. 18. Affect of inertia if object ismoving This is what happens if passengers do not wearseat belts while travelling in a car and the carstops suddenly due to an accident. The largeforce of inertia on the body of passengers canthrow passengers violently in forward directioncausing serious injuries.18
  19. 19. Second law ofmotion(F= m x a)19
  20. 20. momentumMomentum can be defined as "mass in motion." All objectshave mass; so if an object is moving, then it hasmomentum - it has its mass in motion. The amount ofmomentum that an object has is dependent upon twovariables: how much stuff is moving and how fastthe stuff is moving. Momentum depends upon thevariables mass and velocity. In terms of an equation, themomentum of an object is equal to the mass of the objecttimes the velocity of the object.Momentum = mass • velocityIn physics, the symbol for the quantity momentum is thelower case "p". Thus, the above equation can be rewrittenas 20
  21. 21. • The units for momentum would be massunits times velocity units. The standardmetric unit of momentum is the kg•m/s.While the kg•m/s is the standard metricunit of momentum, there are a variety ofother units that are acceptable (though notconventional) units of momentum.Examples include kg•mi/hr, kg•km/hr, andg•cm/s. In each of these examples, amass unit is multiplied by a velocity unit to21
  22. 22. Momentum In Everyday Life• A karate playeris able to breakso many tiles,because hestrikes with hishand very, veryfast, producinga extremelylargemomentum.22
  23. 23. Second law of motion• According to the second law ofmotion :- The rate of change ofmomentum of a body is directlyproportional to the appliedforce, and takes place in thedirection in which the force acts.23
  24. 24. •So, newtons second lawof motion can beexpressed as :-Force ∝ change in momentum / time taken24
  25. 25. MATHEMATICAL FORMULATION OFSECOND LAW OF MOTIONSuppose an object of mass, m is moving along a straightline with an initial velocity, u. It is uniformly accelerated tovelocity, ν in time, t by the application of a constant force, Fthroughout the time, t. The initial and final momentum ofthe object will be, p1 = mu and p2 = mν respectively.The change in momentumα p2 – p1α mν – muα m (ν – u).The rate of change of momentum α m (ν −u) / tOr, the applied force, F α m (ν −u) / tOr, the applied force, F = km (ν −u) / t (2)= kma (3)25
  26. 26. Here a [ = (ν-u) / t] is the acceleration, whichis the rate of change of velocity. Thequantity, k is a constant of proportionality.The SI units of mass and acceleration are kgand m s-2 respectively. The unit of force is sochosen that the value of the constant, kbecomes one. For this, one unit of force isdefined as the amount that produces anacceleration of 1 m s-2 in an object of 1 kgmass. That is,1 unit of force = k (1 kg) (1 m s-2). 26
  27. 27. Demonstration of secondlaw of motion27Since the acceleration produced is inversely proportional to the mass ofthe object, it is easier to move (or accelerate) a small ball (having smallmass) than a big truck (having large mass ) by the force of our push.
  28. 28. • The SI unit of force is newton whichis denoted by N. A newton is thatforce which when acting on a bodyof mass 1 kg produces anacceleration of 1m/s2 in it. Wehave just seen thatF=maPutting m=1kg and a=1m/s2, Fbecomes 1 newton.So 1 newton = 1kg 1m/s2.28
  29. 29. Application ofsecond law ofmotion29
  30. 30. In a cricket match a fielder moves his arms back while trying to catcha cricket ball because if he tries to stop the fast moving ball suddenlythen the speed decreases to zero in a very short time. Therefore theretardation of the ball will be very large. As a result the fielder has toapply a larger force to stop the ball. Thus, if he tries to stop a fastmoving cricket ball the fielder may get hurt as the ball exerts a greatpressure on the hands but if he tries to stop it gradually by moving hisarms back then the velocity decreases gradually in a longer intervalof time and hence retardation decreases. Thus the force exerted by30
  31. 31. A cushion like surface is made for a „high jumpathlete‟. This reduces the large momentum offalling athlete more gently. Due to this, lessopposing force acts on the athletes body andinjuries are prevented. 31
  32. 32. NewtON’s 2nd Law proves that differentmasses accelerate to the earth at thesame rate, but with different forces.• We know thatobjects withdifferent massesaccelerate to theground at the samerate.• However, becauseof the 2nd Law weknow that theydon‟t hit the ground 32
  33. 33. third lawof motion33
  34. 34. NewtON’s third Law OfmotionAccording to Newton‟s Third Law of Motion:-To Every Action There is anEqual and Opposite Reaction34
  35. 35. Newton‟s third law of motion says :Whenever one body exerts a force onanother body , the second body exerts anequal and opposite force on the first body.The force exerted by the first body is knownas “action” and the force exerted by thesecond body on the first body is known as“reaction”.35
  36. 36. Examples toillustratethird law of motion36
  37. 37. How Do We Walk37• When we walk on ground , then our footpushes the ground backward. The forwardreaction exerted by the ground on our footmakes us move forward.
  38. 38. Recoiling of gun38• When a bullet is fired from a gun, the force sendingthe bullet forward is equal is equal to the forcesending the gun backward. But due to the highmass of the gun, it moves only a little distancebackward and gives a backward jerk or kick to
  39. 39. Flying of jet aeroplanesand rockets39Modern jet aeroplanes and rockets work on the principle ofaction and reaction. In aeroplanes engines exert abackward force on the exhaust gases; the backwardrushing exhaust gases exert a forward force on the planewhich makes it move forward.
  40. 40. The case of a boat and theship40• Diagram shows “action” and“reaction” when a man steps outof a boat.• The men push the waterbackwards with theoars. The backwardgoing water exerts anequal and opposite pushon the boat, whichmakes the boat move
  41. 41. Conservation ofmomentumMomentum is nevercreated ordestroyed.When two(or more) bodies actupon one another, their totalmomentum remains constant(orconserved) provided no external41
  42. 42. A Newtonscradle demonstratesconservation of momentum.42
  43. 43. •According to law ofconservation of masstotal momentum beforecollision=total 43
  44. 44. Application of law ofconservation of momentumA rocket works onthe principle ofconservation ofmomentum44A jet aeroplane also works onthe principle of conservation ofmomentum
  45. 45. 45