Transcript speed - Scituate Science Department

FORCES AND MOTION
AND STUFF (AND WHATNOT)
EVERYTHING MOVES!
• I mean e-v-e-r-y-t-h-i-n-g
• From sub atomic particles to the entire
galaxy itself is in motion
• mountains, stars, clouds and every single
living organism on earth is in some kind of
motion...ALL THE TIME.
Aristotle
• This famous philosopher classified motion into
two groups natural and unnatural
• natural motion would be the motion of a boulder
falling, smoke rising or the movement of planets
and such through the sky
• He believed that natural objects seeked out natural
resting places
– because of this, he felt that natural motion DID NOT
require a force
Unnatural motion...
• ...was the motion of a cart or some other
man-made object and DID require a force
because we would have to push or pull it to
make it move
SORRY ARISTOTLE...
• ...but you were wrong!!!
• Well, half wrong.
• Why only half wrong?
I TRIED
MY
BEST.
IT’S OKAY
BUDDY. NOT
EVERYONE
CAN BE AS
AWESOME AS
ME.
stupid
son of a...
Galileo
• Aristotle’s theories were taken as fact for
about 2000 years
• Until Galileo blew them away
• Galileo’s experiment at the Leaning Tower
of Pisa destroyed Aristotle’s belief that
larger objects fall faster
• he also showed that Aristotle was wrong
about forces too
MAN...I CAN’T
WIN!
BOOYAH!!
Galileo’s simple experiments
were quite powerful
• He simply rolled balls down flat planes at
different angles and observed
• He noticed that a ball rolling down an angle
increased speed and then decreased speed as
it rolled up an incline
• He also noticed that a ball rolling on a flat
surface neither increased nor decreased
speed
His conclusion...
• A force is required to start an object
moving, but once moving, no force is
required to keep it moving-except for the
force needed to overcome friction
• He also believed that if there was no friction
the ball moving on the flat surface would
roll forever (he was right)
Thanks for the history lesson
but...
• ...what is a force?
• Forces are pushes or pulls on objects
• There are four forces in the universe
–
–
–
–
gravity
electromagnetic force
strong nuclear force
weak nuclear force
• most forces that we think of are
electromagnetic
Let’s Talk About Motion and
Speed and Velocity
• MOTION IS RELATIVE
• What...what does that mean?
• I’ll show you
What happened?
• What changed?
• How do you know?
• If you didn’t see it happen, how do you
know?
“Motion is relative” means...
• ...motion is an object’s change in position in
relation to other objects
• so, you know Spongebob moved because he
is in a different position in relation to
Patrick
• So, is the school moving?
Speed
•
•
•
•
What is speed?
Speed is the distance moved per unit time
We can thank Galileo for this too
Before him people thought of speed as fast
or slow
• Galileo was the first to define and measure
speed
• speed = distance/time (the first equation
you’re responsible for)
Average vs. Instantaneous
• Imagine that you are doing this
Are you travelling at the same
speed the entire journey?
• NO!!!
• Why not?
• Correct.. So as you go the speed varies from
one instant to the next
• This means that there is an average speed
which is equal to :
– the total distance travelled/the total time it took
There is also a bunch of
instantaneous speeds
• These are the speeds you are travelling at
any given second during the trip
• instantaneous speed is what a speedometer
measures
• The SI unit for speed is meters/second or
m/s
• Ummmmm......
Back to physics...
•
•
•
•
•
We know speed now. What is velocity?
The unit for velocity is m/s
Does that help?
No, the unit fo speed is m/s too.
What is the difference?
Velocity tells a direction also
• Velocity is a vector.
• A vector describes a direction as well as a
magnitude
• If we say “a car travels 60 km/h”, that is
speed
• The car’s velocity is 60 km/h to the north
• get it?
• Oh yeah, force is a vector too.
Newton’s First Law
of Motion
• Called the Law of Inertia
• But first, what’s inertia?
• Inertia is a property of matter. All matter
has inertia. It is the object’s resistance to
change in motion.
Now back to Newton’s 1st Law
• “Every object continues in a state of rest or
of uniform speed in a straight line unless
acted upon by a nonzero net force.”
• What?
• All this means is that one needs to apply a force to
an object to change it’s motion, or put another
way: Object’s at rest tend to stay at rest unless
acted upon by an unbalanced force. Object’s in
motion tend to stay in motion unless acted upon
by an unbalanced force.
Net force
• When we say “force” we mean the net force
or the total of all forces acting on an object
Look at this guy go. He is pushing the box across
the floor. What is the net force? To find out we
need to know ALL of the forces acting on the box.
Air pressure
Natural force (normal force)
Air
The push
resistance
friction
gravity
Air pressure
Natural force (normal force)
The push
friction
gravity
If this guy is applying a force
of 10 N (ten newtons) to the
left, but all the other forces
add up to 5 N in the opposite
direction then the net force is
5 N to the left
The Newton
• The Newton is the SI unit for force
• It is named after Sir Isaac Newton
• If force is equal to the mass of an object
times its rate of acceleration or: F = ma or
force = kg x m/s2 then the unit for force is
the kg m/s2
• nobody wants to say “kilogram meter per
second squared” so we call it a Newton and
we just write a “N”
Balanced vs. Unbalanced forces
• Change in motion occurs when forces are
unbalanced
• the little boy can move the box to the left
because he is applying more force in that
direction than friction is applying in the
opposite direction. However...
• If the forces are balanced there is no motion
5N
5N
F = 0
• 5 N to the left opposing 5 N to the right is just like
saying 5 - 5, which we know equals 0
• so 5N - 5N = 0N
• No force equals no motion
• When the net force is zero the object is in
equilibrium
• it can be represented mathematically like this: F
= 0.  this Greek symbol stands for “the vector
sum of” and F is force
The Support Force (the normal
force or the natural force)
• This will be quick
• Put simply the normal force is the force that
the ground or the floor pushes up onto the
bottom of your feet
• It acts directly against gravity, it is equal to
gravity but in the opposite direction
• Without it, we would be pulled into the
center of the planet where we would die a
horribly painful death
Equilibrium for moving objects
• We discussed equilibrium already. What
was it again?
• It is when the net force acting on an object
is zero or ΣF = 0
• So what if the object is moving?
• If it is in motion with a CONSTANT
VELOCITY then it is also in equilibrium.
This just means that the velocity is neither
increasing nor decreasing
Newton's 2nd Law of Motion
• “The acceleration produced by a net force
on an object is directly proportional to the
net force, is in the same direction as the net
force, and is inversely proportional to the
mass of the object.”
• What?
• Let's define some terms first
Acceleration
•
•
•
•
•
•
It is simply the rate of change of velocity
We can thank Galileo for this too
So a = v/t
What the heck is v?
It means “the change in” velocity
so...acceleration is equal to the change in
velocity divided by the time it took to
change
The 2nd Law of Motion
re-defined
@All this law is saying is that an object
accelerates in the direction that the force is
applied
@so...the big equation that relats to this is
@F = ma or force = mass x acceleration
@keep in mnd that you will now be
responsible for not only F = ma but also
m=F/a and a = F/m
Friction
¥ Friction is the force that arise between two
objects that are sliding across each other
¥ It can occur between solids, liquids or gases
¥ PLEASE KEEP IN MIND friction
ALWAYS opposes the direction of motion
¥ this is what cause movement when you
walk, ride a bike, drive a car or boat
Friction
Force applies by tire to
the rock
Why?
œ MICROWELDS!
œ The surfaces of solid objects, no matter how
smooth, are not smooth.
œ Microwelds are microscopic bumps and
valleys
œ When surfaces interact, these microwelds
interact this cause friction
œ the sum of the forces on the molecular level
is equal to the total force of friction
Freefall
We know that acceleration rate DOES NOT
depend on mass
Thanks Galileo!
DON’T MENTION IT.
YOU EVER WONDER
WHAT I’M LOOKING
AT?