we've been talking a lot about the
science of how things move you throw a
ball in the air and there are ways to
predict exactly how it will fall but
there's something we've been leaving out
forces and why they make things
accelerates and for that we're going to
turn to a physicist you've probably
heard of Isaac Newton with his three
laws published in 1687 in this book
prints appear Newton outlines his
understanding of motion and a lot of his
ideas were totally new today more than
three hundred years later if you're
trying to describe the effects of forces
on just about any everyday object a box
on the ground a reindeer pulling a
sleigh or an elevator taking you up to
your apartment then you're going to want
to use Newton's laws and yes I'll
explain the reindeer thing later
Newton's first law is all about inertia
which is basically an object tendency to
keep doing what it's doing it's often
stated as an object in motion will
remain in motion then an object at rest
will remain at rest unless acted upon by
a force which is just another way of
saying that to change the way something
moves to give it acceleration you need a
net force so how do we measure inertia
well the most important thing to know is
mass say you have two balls that have
the same size but one is an inflatable
beach ball and the other is a bowling
ball the bowling ball is going to be
harder to move and harder to stop once
it's moving it has more inertia because
it has more mass make sense right more
mass means more stuff with a tendency to
keep doing what it was doing before your
force came along and interrupted it and
this idea connects nicely to Newton's
second law net force is equal to mass
times acceleration or as an equation F
net equals MA it's important to remember
that we're talking about net force here
the amount of force left over once
you've added together all the forces
that might cancel each other out let's
say you have a hockey puck sitting on a
perfectly frictionless ice rink and I
know they're not usually perfectly
frictionless but stick with it if you're
pushing the puck along with a stick
that's a force on it that isn't being
canceled out by anything else
so the puck is experiencing acceleration
but when the puck is just sitting still
even when it's sliding across the ice
after you've pushed it then all the
forces are balanced out that's what's
known as equilibrium an object that's in
equilibrium can still be moving like the
sliding park but it's velocity won't be
changing its when the forces get
unbalanced that you start to see the
exciting stuff happen and probably the
most common case of a net force making
something move is the gravitational
force say throw five kilogram balls
straight up in the air and then you know
get out of the way because that could
really hurt if it hits you but the force
of gravity is pulling down on the ball
which is accelerating downwards at a
rate of 9.81 m/s^2
so the net force is equal to MA but the
only force acting here is gravity this
means that if we could measure the
acceleration of the ball we'd be able to
calculate the force of gravity and we
can measure the acceleration its 9.81
m/s^2
the value we've been calling small G so
the force of gravity on the ball must be
five kilograms which is the mass of the
ball
times small G which comes to 49 point
zero five kilograms times meters per
second squared we use this equation for
gravity so much that is often just
written as FG equals mg that's how you
determine the force of gravity otherwise
known as weight now those units can be a
bit of a mouthful so we just call them
Newtons
that's right we measure weight in
Newtons in honor of Sir Isaac and not in
kilograms kilograms are a measure of
mass but gravity often isn't the only
force acting on the object so when we're
trying to calculate a net force we
usually have to take into account more
than just gravity this is where we get
into one of the forces that tends to
show up a lot which is explained by
Newton's third law you probably know
this law as for every action there's an
equal but opposite reaction which just
means that a few exert a force on an
object it exerts an equal force back on
you and that's what we call the normal
force normal in this instance means
perpendicular and the normal force is
always perpendicular to whatever surface
your object is resting on at least it is
when you're pushing on something big and
macroscopic like a table if you put a
book down on a table the normal force is
pushing and therefore pointing up but if
you put it on a ramp the normal force is
pointing perpendicular to the ramp now
the normal force isn't like most other
forces it's special because it changes
its magnitude so you have a piece of
aluminium force stretched tightly across
the top of a bowl and then you put one
lonely grape on top of it because of
gravity that grape is exerting a little
bit of force on the foil and the normal
force pushes right back with the same
amount for then you add another grape
which doubles the force on the foil in
that case the normal force doubles to
that I'll keep happening until
eventually you add enough grapes that
they break through the foil that's what
happens when the normal force can't
match the force pushing against it but
what does Newton's famous third law
really mean though when I push on this
desk with my finger right now I'm
applying a force to it and it's applying
an equal force right back on my finger
one that I can actually feel but if
that's true and it is then why are we
able to move things how can I pick up
this mug or how can a reindeer pull a
sleigh basically things can move because
there's more going on than just
the action and reaction forces for
example when a reindeer pulls on a
sleigh Newton's third law tells us that
the sleigh is pulling back on it with an
equal force but the reindeer can still
move the sleigh forward because it's
standing on the ground when it takes the
step it's pushing backward on the ground
with its foot and the ground is pushing
it forward meanwhile the reindeer is
also pulling on the sleigh while the
sleigh is pulling right back but the
force from the ground pushing the
reindeer forward is stronger than the
force from the sleigh pulling it back so
the animal accelerates forward and so
does the slate so one takeaway here is
that there would be no Christmas without
physics but now we have an idea of some
of the forces we might encounter let's
describe what's happening when a box is
sitting on the ground the first thing to
do which is the first thing you should
always do when you're solving a problem
like this is to Jawas known as a free
body diagram basically you draw a rough
outline of the object
put a dot in the middle and then draw
and label arrows to represent all the
forces we also have to decide which
direction is positive in this case we'll
choose up to be positive for our box the
free body diagram is pretty simple
there's an arrow pointing down
representing the force of gravity and an
arrow pointing up representing the force
of the ground pushing back on the box
since the box is staying still we know
that it's not accelerating which tells
us that those forces are equal so the
net force is equal to zero but what if
you attach a rope to the top of the box
then connect it to the ceiling so the
box is suspended in the air your net
force is still zero because there's no
acceleration on the box and gravity is
still pulling down in the same way it
was before but now the counteracting
upward force comes from the Rope acting
on the box and what we call the tension
force to make our examples simpler we
almost always assume that ropes have no
mass and are completely unbreakable no
matter how much you put on them they'll
pull right back which means that the
tension force isn't fixed if the Box
weighs five Newtons and the tension in
the Rope is also five Newtons but if we
add another five newtons of weight the
tension in the rope will become 10
Newton kind of like how the normal force
changes with the grapes on the foil but
in this case is in response to a pulling
force instead of a push Shiki is that no
matter what you can add the forces
together to give you a particular
force even though that net force might
not always be zero like in an elevator
so let's say you're in an elevator or as
I call them a lift the total mass of the
lift including you is a thousand
kilograms and it's movement is
controlled by a counterweight attached
to a pulley the plan is to set up a
counterweight of 850 kilograms and then
let the lift go once you let go the lift
is going to start accelerating downwards
because it's heavier than the
counterweight and the hope is that the
counterweight will keep it from
accelerating too much so how will we
know if it's safe how quickly is the
lift going to accelerate downward to
find out let's first draw a Freebody
diagram for the lift making up the
positive direction the force of gravity
on the lift is pulling it down and it's
equal to the mass of the lift times
small G 9810 newtons of force in the
negative direction and the force of
tension is pulling the lift up in the
positive direction which means that for
the lift the net force is equal to the
tension force minus the mass of the lift
times small G now since Newton's first
law tells us that F net equals MA we can
set all of that to be equal to the lifts
mass times and downward acceleration
minus a that's what we're trying to
solve for so let's do the same thing for
the counterweight gravity is pulling it
down with eight thousand three hundred
and thirty eight point five Newton's of
force in the negative direction and
again the force of tension is pulling it
up so that the net force is equal to the
tension force minus the mass of the
counterweight times small G and again
because of Newton's second law we know
that all of that is equal to the mass of
the counterweight
times that same acceleration a which is
positive this time since the
counterweight is moving upwards so
putting all of that together we end up
with two equations and two unknowns
we don't have a value for the tension
force and we don't have a value for
acceleration but what we're trying to
solve for is the acceleration so we use
algebra to do that when you have a
system of equations like this you can
add or subtract all the terms on each
side of the equal sign to turn them into
one equation for example if you know
that one plus two equals three and that
four plus two equals six you can
subtract the first equation
from the second to get three equals
three and in our case with the lift
subtracting the first equation from the
second gets rid of the term that
represents the tension force we now just
have to solve for acceleration meaning
we need to rearrange the equation to set
everything equal to a we end up with an
equation that really just says that a is
equal to the difference between the
weights or the net force on the system
divided by the total mass essentially
this is just a fancier version of F net
equals MA
and we can solve that for a which turns
out to be zero point seven nine five
meters per second squared which is not
that much acceleration at all so as long
as you aren't dropping too far down you
should be fine even if the landings a
little bumpy in this episode you learned
about Newton's three laws of motion
how inertia works that net force is
equal to mass times acceleration
how physicists define equilibrium and
all about the normal force and the
tension force crash course physics is
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