NEWTON’SLAWS OFMOTIONI. Law of InertiaII. F=maIII. Action-Reaction
It is push or pull, which eitherchanges or tends to change the stateof rest or of uniform motion of abody.
While most people knowwhat Newtons laws say, manypeople do not know what theymean (or simply do not believewhat they mean).
NEWTON’S LAWS OF MOTIONNEWTON’S LAWS OF MOTION11ststLawLaw – An object at rest will stay at rest,and an object in motion will stay in motionat constant velocity, unless acted upon by anunbalanced force.22ndndLawLaw – Force equals mass timesacceleration.33rdrdLawLaw – For every action there is an equaland opposite reaction.
11STSTLAW OF MOTIONLAW OF MOTION(LAW OF INERTIA)(LAW OF INERTIA)An object at rest will stay at rest,and an object in motion will stay inmotion at constant velocity, unlessacted upon by an unbalanced force.
1STLAWInertia is thetendency of anobject to resistchanges in itsvelocity: whetherin motion ormotionless. These pumpkins will not moveunless acted on by an unbalancedforce.
1STLAWOnce airborne,unless acted on byan unbalancedforce (gravity andair – fluidfriction), it wouldnever stop!
1STLAWUnless acted uponby an unbalancedforce, this golf ballwould sit on thetee forever.
Why then, do we observeevery day objects in motionslowing down and becomingmotionless seemingly without anoutside force?It’s a force we sometimes cannot see –It’s a force we sometimes cannot see –friction.friction.
Objects on earth, unlike thefrictionless space the moontravels through, are under theinfluence of friction.
There are four main types of friction: Sliding friction: ice skating Rolling friction: bowling Fluid friction (air or liquid): air or water resistance Static friction: initial friction when moving an objectWhat is this unbalanced force that acts on an object in motion?What is this unbalanced force that acts on an object in motion?
Slide a book across atable and watch it slide toa rest position. The bookcomes to a rest because ofthe presence of a force -that force being the forceof friction - which bringsthe book to a restposition.
In the absence of a force of friction, the book wouldcontinue in motion with the same speed anddirection - forever! (Or at least to the end of thetable top.)
NEWTONS’S 1STLAW AND YOUDon’t let this be you. Wear seat belts.Because of inertia, objects (including you) resistchanges in their motion. When the car going 80km/hour is stopped by the brick wall, your bodykeeps moving at 80 m/hour.
2NDLAWThe net force of an object isequal to the product of its mass andacceleration, or F=ma.
2NDLAWWhen mass is in kilograms andacceleration is in m/s/s, the unit of forceis in Newtons (N).One Newton is equal to the forcerequired to accelerate one kilogram ofmass at one meter/second/second.
2NDLAW (F = M X A)How much force is needed to accelerate a1400 kilogram car 2 meters per second/persecond?Write the formulaF = m x aFill in given numbers and unitsF = 1400 kg x 2 meters per second/secondSolve for the unknown2800 kg-meters/second/second or 2800 N
If mass remains constant, doubling the acceleration, doublesthe force. If force remains constant, doubling the mass, halvesthe acceleration.
Newton’s 2ndLaw proves that different massesaccelerate to the earth at the same rate, but withdifferent forces.• We know that objectswith different massesaccelerate to the groundat the same rate.• However, because of the2ndLaw we know thatthey don’t hit the groundwith the same force.F = maF = ma98 N = 10 kg x 9.8 m/s/s98 N = 10 kg x 9.8 m/s/sF = maF = ma9.8 N = 1 kg x 9.89.8 N = 1 kg x 9.8m/s/sm/s/s
CHECK YOUR UNDERSTANDING 1. What acceleration will result when a 12 N net force applied to a 3 kgobject?2 N = 3 kg x 4 m/s/s1 2. A net force of 16 N causes a mass to accelerate at a rate of 5 m/s2.Determine the mass.16 N = 3.2 kg x 5 m/s/s 3. How much force is needed to accelerate a 66 kg skier 1 m/sec/sec?66 kg-m/sec/sec or 66 N 4. What is the force on a 1000 kg elevator that is falling freely at 9.8m/sec/sec? 9800 kg-m/sec/sec or 9800 N
3RDLAWFor every action, there is an equal andopposite reaction.
3RDLAWAccording to Newton,whenever objects A andB interact with eachother, they exert forcesupon each other. Whenyou sit in your chair,your body exerts adownward force on thechair and the chairexerts an upward forceon your body.
3RDLAWThere are two forcesresulting from thisinteraction - a force onthe chair and a force onyour body. These twoforces are called actionand reaction forces.
NEWTON’S 3RD LAW IN NATURE Consider the propulsion of afish through the water. A fishuses its fins to push waterbackwards. In turn, the waterreacts by pushing the fishforwards, propelling the fishthrough the water. The size of the force on thewater equals the size of theforce on the fish; the directionof the force on the water(backwards) is opposite thedirection of the force on thefish (forwards).
3RDLAWFlying gracefullythrough the air, birdsdepend on Newton’sthird law of motion.As the birds pushdown on the air withtheir wings, the airpushes their wings upand gives them lift.
Consider the flying motion of birds. A bird flies byuse of its wings. The wings of a bird push airdownwards. In turn, the air reacts by pushing thebird upwards. The size of the force on the air equals the size ofthe force on the bird; the direction of the force onthe air (downwards) is opposite the direction of theforce on the bird (upwards). Action-reaction force pairs make it possible forbirds to fly.
OTHER EXAMPLES OF NEWTON’STHIRD LAW The baseball forces the batto the left (an action); thebat forces the ball to theright (the reaction).
3RDLAW Consider the motion of acar on the way to school.A car is equipped withwheels which spinbackwards. As the wheelsspin backwards, they gripthe road and push theroad backwards.
3RDLAWThe reaction of a rocket is anapplication of the third law ofmotion. Various fuels areburned in the engine,producing hot gases.The hot gases push against theinside tube of the rocket andescape out the bottom of thetube. As the gases movedownward, the rocket moves inthe opposite direction.