Forces
Chapter 10
Section 10-1
The Nature of Force
What is a force?
- a push or a pull
Unbalanced Forces
Unbalanced forces - can cause an object to start moving
stop moving, or change direction.
- acting on an object will change the
object’s motion.
Unbalanced
forces in the
same direction
Notice the arrows
Unbalanced
forces in
the
opposite
direction
Balanced Forces
Balanced forces-
- acting on an object will NOT
change the object’s motion.
Balanced forces in the opposite
direction
Balanced forces in the opposite
direction
NO motion
Newton’s First Law of Motion
States that:
an object at rest will remain at rest & an
object in motion at a constant velocity
will continue moving at constant velocity
unless acted upon by an unbalanced
force.
INERTIA - the tendency of an object to
resist change of its motion.
Inertia
Inertia
INERTIA
What will happen
here?
How does it
demonstrate
inertia?
INERTIA
How does this
picture
demonstrate
inertia?
Mass - the amount of matter an
object contains
The amount of
INERTIA an object has
depends on its mass.
Which jar would be
harder to move?
Greater mass = greater
inertia
Mass then can be
defined as a measure of
the inertia of an object.
Section 10 -2
Force, Mass, & Acceleration
Newton’s Second Law of Motion
states that: the net force on an object is
equal to the product of its acceleration and
its mass.
Force = mass x acceleration
Newtons
1 N = the force required to accelerate
1 kg of mass at 1 m/s/s
1 N = 1kg x 1m/s/s
Newtons are the unit that measure force.
Newton’s Second Law of
Motion
Mass x acceleration =
Force
N’s 2nd law shows
how mass, force, &
acceleration are
related.
Newton’s Second
Law of Motion
What would happen to the force required to
move the wagon if the people got out?
Section 10 - 3
Friction & Gravity
Friction - the force that OPPOSES motion.
The force that one surface exerts on
another when the two rub against each
other.
Friction explains why objects will roll or
slide to a stop eventually. (Opposes
motion- remember)
Friction
- The strength of friction depends on two
factors:
- types of surfaces
- how hard surfaces push together
Friction
These skiers can travel very fast
because of very little friction between
their skis & the snow. What if they were
being pulled over rocks?
Friction
Is friction a bad thing??
Friction enables us to:
- WALK
- LIGHT A MATCH
- STOP OUR CARS
- WRITE
- KEEP ALL KINDS OF THINGS
FROM SLIDING AROUND!
3 Types of Friction
SLIDING
FRICTION
- when solid
surfaces
slide over
each other.
3 Types of Friction
ROLLING
FRICTION
- when an
object
rolls
over a
surface.
3 Types of
Friction
Ball bearings are
often used in
producing
ROLLING
FRICTION because much less
force is needed to
overcome rolling
friction than
sliding friction.
3 Types of
Friction
FLUID FRICTION - occurs when
an object moves through a fluid.
(A liquid or a gas)
FLUID
FRICTION
Why?
3 Types of
Friction
The force needed to overcome fluid friction
is usually less than that needed to overcome
sliding friction.
We use fluids called lubricants to reduce
contact of surfaces, reducing friction in
machine parts and other things.
GRAVITY
Gravity is a force like friction but this
force induces motion.
Sir Isaac Newton recognized gravity as the
force that pulls objects straight down
towards the center of the earth.
Newton’s Universal Law of
Gravitation
States that:
all objects in the universe attract each other
by the force of gravity. The size of the force
depends on 2 factors: the masses of the
objects and the distance between them.
Gravity acts on all objects in the universe. It is
what keeps our moon orbiting around the earth.
In fact, gravity holds our solar system together
as well as the whole universe!
GRAVITY
Free fall - when the only force acting upon
an object is gravity.
- an object accelerates as it falls
- near the surface of the earth, objects
accelerate at 9.8 m/s/s
GRAVITY
- object’s velocity increases as it falls
so add 9.8 m/s for every second it falls
(after 10 seconds, it is falling at 98m/s)
GRAVITY - Projectile Motion
- happens
when an
object is
thrown or
projected
horizontally.
You have 2 balls, to be released at the
exact same time. One will just be
dropped, the other will be thrown
horizontally. (A projectile object)
So which will hit the ground first?
The balls both hit
the ground at the
same time
because gravity
acts upon them
with the same
force.
What is this picture
showing? Hint - a
very important
concept that
Galileo proved in
the leaning Tower
of Pisa.
So the picture shows that
objects fall at the same
rate here on earth - even
if they have different
masses.
Which one will hit the water first?
Right - the elephant!
(I know you’re arguing that objects fall at
the same rate on earth. So they should hit
at the same time.)
BUT there’s a little something that will
slow one of them down...
Gravity & Air Resistance
Objects fall through the air here on earth.
Even though it is rather thin it can & does
resist an object’s movement through it.
The leaf will fall slower than the acorn because of the
air pushing on it has a greater effect & slows its fall.
If the leaf and the acorn
were dropped in a
vacuum tube, they
would fall at exactly the
same rate.
Explain these pictures.
Terminal Velocity
If a falling object can fall far enough, it
will eventually achieve terminal velocity.
This is as fast as an object can fall through
a fluid.
Air resistance will stop the object’s acceleration. Terminal
velocity for the average human body through air is about 150
mph. What about water? Would an object sink to the bottom
of the ocean?
Weight
Weight is a measure of the force of
gravity on an object, and mass is a
measure of the amount of matter in that
object.
Weight = mass x acceleration due to
gravity
So weight is measured in newtons since it
is a measurement of force.
Our moon
is only a
sixth of the
earth’s size.
So you
would
weigh less
there
because
there is less
GRAVITY
to act upon
your mass.
These
small
black
spots are
from the
Shoemak
er-Levy
comet
impact.
How much would you weigh on Jupiter?
HINT: it’s 2.5 times larger than earth.
REMEMBER:
Gravity is
affected by 2
factors:
- SIZE of objects
- DISTANCE
between objects
Section 10 - 4
Action & Reaction
Newton’s 3rd Law of Motion
states that: that for every action there is an
equal and opposite reaction.
Newton’s 3rd Law of Motion
Newton realized that forces are not one
sided but are paired.
For every action there is a reaction.
Newton’s 3rd Law of Motion
This squid will squeeze water through its siphon
tube (the action). This jet of water will cause it to
move in the opposite direction in reaction.
Newton’s 3rd Law of Motion
If one skater
pushes
against the
other, what
will happen?
What if it’s a
BIG push,
little push?
Newton’s 3rd Law of Motion
(Suzy-Q can’t help but obey Newton’s 3rd
Law of Motion. She should have paid
closer attention in science class!)
Explain what’s happened.
Newton’s 3rd Law of Motion
How does Newton’s
3rd law apply here?
Do Action-Reaction Forces
Cancel each other out?
Remember: the net force
of equal & opposite,
“balanced” forces is 0. (No
motion) The volley ball
players force against the
ball cancels out. The ball
won’t move.
Do Action-Reaction
Forces Cancel each
other out?
Newton’s 3rd Law refers to
forces on 2 objects.
This players arms exert an
upward force on the ball
(action.) The returns a
reaction force.
One force is on the ball,
one is on the player.
Forces can only be added
together if they are acting on
the same object.
Momentum
Newton called the the “quantity of motion”
The momentum of an object is the product
of its mass and its velocity.
Momentum = Mass X Velocity
Momentum
What kind of crash would it be if this car
was going slower? What would be changed
mass or velocity?
Momentum
Imagine a bullet someone throws versus one
shot from a gun.
How about if a dog were walking and bumped
into you? What if an elephant was walking the
same speed and bumped into you.
A VW and a Mack truck are rolling on the
street. Which could you jump in front of and
stop? Why?
Momentum
What is the momentum of the yellow car?
What is the momentum of the red car?
What is the total momentum of the system?
Momentum
If the two cars collide and stick together,
what will the direction of their resulting
motion be?
Momentum
The yellow car has a greater momentum
(because even though the cars have the
same mass the yellow has a greater
velocity) and so will push the red car
back.
Yellow car’s momentum = 14 m/s x 10 kg
140 kg-m/s or 140 N
Red car’s momentum = 10 m/s x 10 kg
100 kg-m/s or 100N
Total system’s momentum = 240 kg/m/s
Momentum
Law of Conservation of Momentum
states that:
the total momentum of any group of objects
remains the same unless outside forces act
on the objects.
Section 10 - 5
Orbiting satellites
A satellite is any
object that
travels around
another object in
space.
The moon is the
earth’s satellite. The
earth is the sun’s
satellite.
A rocket can rise
into the air
because the gases
it expels with a
downward force
exert an equal but
opposite force on
the rocket.
Which of the laws
of motion is this?
The Space Shuttle
becomes a satellite
once it is circling
(orbiting) earth.
Eventually the
shuttle’s path will tilt
until it is parallel but
miles above the earth’s
surface.
Satellite Motion
Newton wondered what would happen if you stood on a
high mountain & were able to throw a stone as fast as you
wanted. The faster you threw it, the further away it would
land.
At a certain speed, the stone’s path would match the curve
of the earth. Although the earth’s gravity would
continually pull the stone towards it, the ground would
curve away at the same rate. So the stone would circle
around the earth.
Satellite
Motion
In order for a
ball to be
“thrown” into
& stay in orbit,
it must achieve
a speed of
about 7,900 m/s
- or 20 x’s as
fast as a pitcher
can throw a
ball.
Circular
Motion
Any force that
causes an object to
move in a circle is
called centripetal
force.
Centripetal means
center seeking.
Remember:
Acceleration is any
change of velocity
or direction.
A satellite
falls around
earth instead
of into it.
The speed of
a satellite is
very
important. It
must be fast
enough to
maintain it
altitude. If it
slows down
what will
happen to it?
Satellite Location
Some satellites
are in low
orbits, like the
space shuttle. It
takes about 90
minutes to orbit
the earth.
Higher orbits (36,000 km) have satellites,
that travel slower.
Ex. - communications satellites high above
the earth’s equator, travel at the same rate
that the earth rotates so the satellite remains
in a fixed position relative to earth.
The End