Transcript Lesson 3: Unbalanced Forces

Balanced &
Unbalanced
Forces
Newton’s Second Law of
Motion says:
I am
so
smart
…
“To move a mass, you need a
force!”
► More
Force = more Acceleration
► More
Mass = more Force needed!
The greater the mass = greater inertia = more force needed!
Third Law of Motion
► “For
every action there
is an equal and opposite
reaction.”
► Rockets
take off
because of a force
downwards from the
bottom makes them
accelerate in the
opposite direction!
Upwards
reaction
Downwards
force
Discuss: If your results/data didn’t seem to
match these laws; what variables might have
Forces
in
the
Balloon
Lab:
messed up your results/data in the tests?
2nd Law: The balloon will not move without air pushing outward from it.
3rd Law: When the air comes out, the balloon moves in the opposite direction of the air
2nd & 3rd Law: The more air you put in the balloon the faster it travels
2nd & 3rd Law: The more mass we add the slower the balloon
travels (if the air force remains the same)
SO: To make the balloon move the same speed
(after adding mass) we have to use more force (more air)!
Neg. Force
Pos. Force
(Friction)
(Thrust)
Main Principal
#5: The more
mass an object has, the
more force is needed to move the object!
Gravity
Forces and Accelerations
► We
have already learned that forces cause all
accelerations (change of speed and/or direction)
► But they don’t have to…
► What if there are more than one force acting on
an object at the same time (which is more realistic
than just one)?
► The forces might balance each other out!
► How does this affect the motion of our object…?
► Well…let’s
explore that now…go to next slide:
Balanced Forces: The forces in
each direction are “equal”.
 If more than one force is present, it does not
have to cause an acceleration on an object.
 If another force “balances” the first out, there
will be no acceleration at all.
Think: If both guys (who weight the same)
pull on a rope in opposite directions, with
an equal amount of force, how much will they move?
Example:
Ground pushes up
Gravity pulls down
Gravity pulls down on you…
The ground pushes back up…
THIS KEEPS YOU WHERE YOU ARE!
If these football players push
on each other equally as hard,
will either one move?
More Balanced Forces…
5N
5N
5N
5N
Forces may cancel each other and
produce no movement
=No Acceleration!
“Unbalanced” Forces
If the multiple forces
acting at one time are
not balanced out
(equal), then
acceleration can/will
occur on the object!
If one side of the scale has more mass,
then gravity will accelerate it down!
Kicking the ball causes it to move
quickly in a different direction
Multiple
forces
can
combine
to
move
an
Unbalanced
Forces
object that has too much inertia for one
Acceleration
force Cause
alone. The
forces “add together”!
Adding Forces:
What you
5 just
N
saw was like
two people
pushing on the
N
same 5
box.
Random Object
Notice that all the new forces are pointed in the
same direction, and they add together! So,
instead of only 5N of force pushing the object;
now there are 10 N of force pushing!
All by himself, one person might not be able to
push a car. But with extra friends pushing, it
becomes much easier! The forces add together.
Unbalanced Forces
Cause Acceleration
If forces are not equal and are acting in opposite
directions, a negative acceleration can/will
occur. The forces will subtract from each other!
Subtracting Forces:
5N
Object
5N
Notice that the forces are “unequal” and
pointed in the opposite direction. So they are
“unbalanced” and work against each other –
or one partially cancels the other.
The end result is that the forces on the left
are slowed down by the single force on the right
5N
Why would you WANT a second force to
slow down motion?
Football players use force from their
If
you
the
breaks
to slow
down the
These
are
just
three
examples,
ownused
bodies
to
slow
down
a player
car;
maybecreates
you
wouldn’t
drive
off the
cliff
The running
atmosphere
an
opposite
force
on a spaceship
slowing
it
with
the
ball!
The
opposite
but
I
bet
you
can
think
of
a
lot
down
as it heads
back down at…breaks
to Earth. If it didn’t
it would be
almost
you
are
aimed
create
an
force
from
the
first
player
will
at
least
impossible tomore
stop the spacecraft
before
it impacted the ground!
if
you
tried…
opposite
force
to
slow
moving
cars
down.
slow down the player with the
ball.
Friction = When two surfaces “rub” against each
other
the force generated
is called “Friction”.
Breaks on
Speaking
of slowing
down!
a car use friction to slow the spinning of tires down.
►
Friction acts in “opposition” (against) to current forces
and/or accelerations.
►
Main Principal #6: Friction slows velocity down! (Can cause
a negative acceleration!)
►
If we want to overcome friction (for whatever reason) we
need to add even more force!
EX: If you left the Emergency Break on your car while
driving, theWhat
engine hascreated
to work harderfriction
to overcome the
Discuss:
in
friction to make
the Balloon
car move!
the
Lab?
►
(Hint: there are 2 main ones!)
A: The air (gases) in the room and the straw sliding on the string!
Both act to slow down the motion of the balloon!
A: Since the acorn has more mass, it has more inertia!
Friction
can combine
So,Gravity
it’s currentand
motion
(falling) is harder
to change! And
in an
unbalanced
force:
the air friction
doesn’t
generate enough
force to really
slow it down as much as the leaf! Ta-Da! Thank you…Thank you…
The leaf is colliding with the
molecules of gas in the air!
Which slows down the fall of the leaf!
The acorn shown here is also falling
due to gravity…strangely enough, the
acorn falls faster than the leaf!
But, the acorn is also
colliding with the
same molecules of air
that created a lot of
friction on the leaf.
Discuss: If the friction of the air and the
pull of gravity were in perfect balance, what
would the leaf be doing? Think hard…
Discuss: Why does the
acorn still fall faster than
the leaf?
Balance between
Gravity and Friction
Q: If the friction of the air and the
pull of gravity were in perfect
balance, what
would the skydiver be doing?
A: The skydiver would be going a constant
velocity (moving with no acceleration) down
towards the ground until it hit the ground.
The skydiver is colliding with the
molecules of gas in the air!
Which slows down the fall of the skydiver!
Accelerating
Constant
Velocity
Accelerating
Air
Resistance
30 m/s
28
25
m/s
20
10
0 m/s
Speed:
Constant Velocity
0 m/s
How it works
Notice that as the skydiver
leaves the plane, the force of
gravity accelerates him faster
towards the earth; however,
the moment he jumps out of
the plane, another force
begins to oppose gravity –
Air Resistance, (Friction).
This friction with the air
begins to slow down his rate
of acceleration. Meaning,
he’s still speeding up, just not
as fast.
Gravity
Once the force of air friction
equals the force of gravity,
the skydiver steadily
descends towards earth at a
constant velocity.
Review Balanced Forces
10N
10N
10N
Balanced
Push
i.e. Pushing a
Car
No Acceleration
10N
Balanced
Pull
i.e. Tug-o-war
No Acceleration
Review Un-Balanced Forces
Add Together
10N
10N
Subtract from each
other
10N
10N
10N
Un-Balanced
Same
Direction
Faster
Acceleration
Un-Balanced
Opposite
Direction
Slower
Acceleration