Newton’s LawsNewton’s Laws

And forces, vectors, gravity, And forces, vectors, gravity, momentum, impulse and momentum, impulse and

momentsmoments

Newton’s First LawNewton’s First Law

Objects at rest remain at restObjects at rest remain at restObjects in motion remain in Objects in motion remain in

motionmotion

UNTIL YOU APPLY A FORCEUNTIL YOU APPLY A FORCE

Newton’s Second LawNewton’s Second Law

F = maF = ma

What forces are important in What forces are important in sports?sports?

Newton’s Third LawNewton’s Third Law

For every action there is an equal For every action there is an equal and opposite reaction.and opposite reaction.

But do they act on the same body?But do they act on the same body?

VectorVector

Force is a vector - ?Force is a vector - ? It has a magnitude and a direction.It has a magnitude and a direction.This allows forces to be cancelled This allows forces to be cancelled

out.out.Force brings about accelerationForce brings about accelerationWhich law relates to that?Which law relates to that?Newton’s 2Newton’s 2ndnd Law LawF=maF=ma

Non-parallel forcesNon-parallel forces

Can add vectors in different Can add vectors in different directions:directions:

cc22 = a = a22 + b + b22 ; angle x = tan ; angle x = tan-1-1 a/b a/b

a

b

c

x

Resultant force

VectorsVectors

Remember: the resultant force is not Remember: the resultant force is not a new forcea new force

It is the sum of the 2 componentsIt is the sum of the 2 componentsUse either the resultant or Use either the resultant or

component in any calculation, not component in any calculation, not both!both!

NewtonNewton

What is a Newton?What is a Newton?Unit Unit 1N = force that accelerates 1kg mass by 1N = force that accelerates 1kg mass by

1ms1ms-2-2

weightweight

What’s the weight of 1 litre of water?What’s the weight of 1 litre of water?1kg is the mass1kg is the massThe weight = mass x gThe weight = mass x gg is the gravitational field strengthg is the gravitational field strengthg = 10 N kgg = 10 N kg-1-1

or 9.81 N kgor 9.81 N kg-1-1 (more accurately) (more accurately)Weight of 1 litre of water is 10N or Weight of 1 litre of water is 10N or

9.81N9.81N

QuizQuiz

What is my acceleration due to weight?What is my acceleration due to weight?9.81 ms9.81 ms-2-2

What is your acceleration due to weight?What is your acceleration due to weight?9.81 ms9.81 ms-2-2

What about the marker?What about the marker?9.81 ms9.81 ms-2-2

Acceleration due to weight is always gAcceleration due to weight is always g

MassMass

Amount of matterAmount of matterMeasure of inertiaMeasure of inertia Inertia is the resistance to accelerationInertia is the resistance to accelerationMass attracts gravityMass attracts gravityGravitational force is directly proportional Gravitational force is directly proportional

to massto massNote: Gravitational force is inversely Note: Gravitational force is inversely

proportional to distanceproportional to distance22 between 2 between 2 pointspoints

QuizQuiz

As one moves away from the centre of As one moves away from the centre of the earth, his/her weight decreases.the earth, his/her weight decreases.

At 3000m, g = 9.8009 N kgAt 3000m, g = 9.8009 N kg-1-1 What if we throw shot put at 3000m?What if we throw shot put at 3000m?What if we run marathon at 3000m?What if we run marathon at 3000m?At 3000m, oxygen pressure is reduced At 3000m, oxygen pressure is reduced

by more than 10%by more than 10%Always consider other factors!Always consider other factors!

ImpulseImpulse

Momentum = mass x velocityMomentum = mass x velocitySince F = ma and a = Since F = ma and a = ∆∆v / t , F = m v / t , F = m

∆∆v / tv / tTherefore F t = m Therefore F t = m ∆v, or∆v, or Impulse = change of momentumImpulse = change of momentumThe larger the impulse, the greater is The larger the impulse, the greater is

the change in velocity.the change in velocity.Application?Application?

ImpulseImpulse

Hence, to increase velocity, we either Hence, to increase velocity, we either increase F or t (time of contact).increase F or t (time of contact).

Remember, Remember, F = m F = m ∆∆v / tv / tTo reduce impact, encourage soft To reduce impact, encourage soft

landinglandingUse cushioning, bend joints, etcUse cushioning, bend joints, etc

Conservation of energyConservation of energy

Energy is always conservedEnergy is always conservedRegardless of whether you can observeRegardless of whether you can observeMechanical energy is not always Mechanical energy is not always

conservedconservedMechanical energy is only conserved in a Mechanical energy is only conserved in a

closed systemclosed system Initial (total kinetic + potential energy) = Initial (total kinetic + potential energy) =

final (total kinetic + potential energy)final (total kinetic + potential energy)

Conservation of momentumConservation of momentum

Momentum is conserved when there Momentum is conserved when there is no external forcesis no external forces

E.g. snooker, assuming no frictionE.g. snooker, assuming no frictionApplication?Application?Momentum can be transferred in a Momentum can be transferred in a

collisioncollisionApplication?Application?

Conservation of MomentumConservation of Momentum

If m1 = m2 , then v1 = v2

http://www.phys.washington.edu/~young

MomentsMoments

Turning effect of a forceTurning effect of a forceMoment = force x perpendicular Moment = force x perpendicular

distancedistanceAlso called torqueAlso called torque