A review of A review of Newton’s Third law Newton’s Third law

and VECTORS!and VECTORS!Oohhhh….ahhhhh….Oohhhh….ahhhhh….

What is Newton’s Third What is Newton’s Third Law?Law?

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

Two forces involved –Two forces involved –

ActionAction

ReactionReaction It doesn’t matter which is It doesn’t matter which is

which.which.

ForcesForces

Neither one exists without the other.Neither one exists without the other. Equal in strength and opposite in Equal in strength and opposite in

directiondirection Forces always exist in _____.Forces always exist in _____.

ooops…I mean pairs.ooops…I mean pairs.

There are countless examples where we There are countless examples where we see this…see this…

pears

Example One – Gravity and Example One – Gravity and TidesTides

When we say gravity – what is When we say gravity – what is pulling on what?pulling on what?

Think of the moon and the earth…Think of the moon and the earth…what is pulling on what?what is pulling on what?

The Earth pulls on the moon…the The Earth pulls on the moon…the moon pulls on the earth.moon pulls on the earth.

The moon can’t pull the earth, The moon can’t pull the earth,

but it can pull on something.but it can pull on something.

Example 2 – Rocket Example 2 – Rocket EnginesEngines

Rocket pushes fuel exhaust out at a Rocket pushes fuel exhaust out at a very fast speed…so what has to very fast speed…so what has to happen?happen?

The exhaust pushes on the rocket.The exhaust pushes on the rocket. Action: Rocket pushes fuelAction: Rocket pushes fuel Reaction: Fuel pushes rocketReaction: Fuel pushes rocket

SystemsSystems Why don’t the forces cancel themselves Why don’t the forces cancel themselves

out to zero? Because there are systems.out to zero? Because there are systems. A force from OUTSIDE a system is A force from OUTSIDE a system is

needed for acceleration.needed for acceleration. A force from INSIDE the system plays no A force from INSIDE the system plays no

role in acceleration.role in acceleration. It is a bit confusing. It is a bit confusing. Think of a car…if you want to push it, Think of a car…if you want to push it,

what do you have to do? Get out and what do you have to do? Get out and push smurf.push smurf.

Systems - ContinuedSystems - Continued

The smurf couldn’t stay IN the car to push The smurf couldn’t stay IN the car to push it, he had to get OUT of the car in order to it, he had to get OUT of the car in order to apply a force and accelerate the system. apply a force and accelerate the system.

Let’s use Pooh as another example. Is Let’s use Pooh as another example. Is there acceleration…there acceleration…

if it’s only the wagon?if it’s only the wagon? if it’s only Pooh?if it’s only Pooh? if it’s both?if it’s both?

Velocity, Force, and Velocity, Force, and AccelerationAcceleration

Velocity, force, and acceleration Velocity, force, and acceleration require two things to be complete:require two things to be complete: Magnitude (size)Magnitude (size) DirectionDirection

These quantities are fittingly called These quantities are fittingly called vectorvector quantities. quantities.

Mass, Volume, and SpeedMass, Volume, and Speed

These are all measurements of These are all measurements of magnitude (size), but not _______.magnitude (size), but not _______.

They are all They are all scalarscalar quantities. quantities.

direction

size

size

direction

+

Vector, oh VectorVector, oh Vector

Pictures are often better than words – Pictures are often better than words – so we use vectors.so we use vectors.

Once again…a vector is a Once again…a vector is a representation of any vector quantity representation of any vector quantity (velocity, force, acceleration) by an (velocity, force, acceleration) by an arrow.arrow. It shows direction (which way)It shows direction (which way) It shows magnitude (how much)It shows magnitude (how much)

Vectors are used when adding and Vectors are used when adding and subtracting velocitiessubtracting velocities

If the vectors are in line it’s easy…If the vectors are in line it’s easy…

5 N 10 N+ = 15 N

5 N-10 N = 5 N

ResultantsResultants If the vectors are not in line, it gets If the vectors are not in line, it gets

slightly tricky. To make things slightly tricky. To make things simple, we use the parallelogram rule.simple, we use the parallelogram rule.

Bring the tails of two vectors together Bring the tails of two vectors together so they form a point.so they form a point.

Draw lines parallel and of equal Draw lines parallel and of equal length to the vectors.length to the vectors.

From the origin, draw a diagonal line From the origin, draw a diagonal line – this is the resultant, or result of the – this is the resultant, or result of the two forces coming together. It’s two forces coming together. It’s motion in two dimensions baby!motion in two dimensions baby!

Example…Example…

Forces: Forces:

Connect the tailsConnect the tails Parallelogram:Parallelogram:

Same size and angleSame size and angle Diagonal = ResultantDiagonal = Resultant

Example 2…Example 2…

Which way will he go?Which way will he go?

Plane Speed

Wind Speed

Plane’s Direction + Speed