hey there my name is kayleigh and this lesson is on action and reaction forces
it will be helpful that you've already completed the modules Newton's first law
and Newton's second law before starting this one
we often think that forces happen to an object when I push against a heavy box
I'm exerting a force onto the box but the box is also exerting a force onto me
and this is because forces come in pairs called action reaction force pairs let's
do a quick example take your hand and exert a downward force by pushing it
down against the table what does your hand look like it gets a bit distorted
right a little smushed when you're pushing down on the table and this is
because while you're pushing a force down onto the table the table is also
exerting a force back up on you you're pushing down on the table and the table
pushing back on you is an example of action reaction force pairs we can't
feel the force that we're exerting on to the table but we do feel the force the
table is pushing back onto us now let's focus on a more complex example of
action reaction force pairs when a hammer strikes a nail to push it into a
piece of wood we know the hammer is exerting a force onto the nail the
striking of the hammer is downwards and exerts a downward force onto the nail
now what happens to the nail from Newton's second law we know that an
unbalanced force on the nail will cause it to move and the nail accelerates
downward downward and goes into the wood but that same downward force that the
hammer exerts on a nail is exerted upwards on the hammer and this might
feel hard to understand since the hammer doesn't accelerate back up in the air
but in fact it does Newton's second law effects everything and the upward force
exerted by the nail on the hammer will cause the hammer to accelerate upwards
in the direction of the unbalanced force but the hammer is very heavy and the
nail not very heavy so even though they are experiencing the same force they
will expand different accelerations since the hammer
is so much heavier than the nail the hammer will barely even notice if at all
the acceleration so what seems like a big force on a little object like the
nail will seem like a small force on a big object like the hammer now we're
going to play a game where we find the action reaction force pairs in different
pictures the first picture this is the Apollo 11 spacecraft at the moment of
liftoff do you see where there would be an
action reaction force pair that causes the rocket to lift off the ground the
spacecraft exerts a downward force on the ground by pushing fuel exhaust out
of the bottom of the rocket that's what all that white stuff is that looks like
smoke but this downward forest from the exhaust pushes back up onto the
spacecraft and this causes the spacecraft to have an unbalanced force
pointing upwards and if we use Newton's second law we know that an unbalanced
force will cause the rocket to accelerate and this is how the rocket
has liftoff and the same laws affect the earth but luckily for us the earth is
much heavier than the rocket so we on earth don't notice the acceleration due
to the downward force of the fuel exhaust nice job
now let's try another picture this is a cannon that has just fired a cannonball
it is clear that the cannon has exerted a left and upward force on the
Cannonball and that's why it's now hurtling through the air but the
Cannonball also exerted a force in the opposite direction on the cannon itself
and this force causes something that is called recoil it's the backward movement
of a thing that outputs a projectile or launches something the cannon will
actually roll a small ways back from the unbalanced force
great job studying action and reaction forces now see if you notice any of
these types of force pairs in your life some ideas to use
started our jumping off a diving board skateboarding and kayaking have fun
remember to always be clever