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12.1
Forces
Chapter 12
Forces and Motion
What are we going to learn?
A soccer ball sits at rest. You come along and
kick it, sending it flying across the field. It
eventually slows to a stop. You applied a force
to start it moving, and then it stopped all on its
own, right?
No!
Forces cause all changes in motion. Just as
you applied a force to the ball to speed it up
from rest, the ground applied a force to slow it
down to a stop. If the ground didn’t apply a
force to the ball, it would keep rolling forever
without slowing down or stopping.
Forced to Change
What’s the BIG Idea?
She jumps with enough force to stay in the air
long enough to turn before she lands
We will find out the answer as we go along...
What is a Force?
How are Forces Described?
Force – a push or pull
When one object pushes or pulls another
object, the first object exerts a force on the
second object
Ex: You exert a force on a computer key when
you push it
Ex: You exert a force on a chair when you pull
it away from a table
Force is described by its strength and by the
direction in which it acts
Pushing to the left is a different force from
pushing to the right
The direction and strength can be represented
by an arrow
The arrow points in the direction of the force
The arrow length tells you the strength (ex: the
longer the arrow, the greater the force)
The strength of the force is measured in the SI
unit called a newton (N), after the scientist Sir
Isaac Newton
1 newton is the force that causes a 1-kilogram
mass to accelerate at a rate of 1 meter per
second each second (1 m/s2)
A force can cause a resting object to move, or
it can accelerate a moving object by changing
the object’s speed or direction
We will find out the answer as we go along...
How do Forces Affect Motion?
Often more than one force acts on an object
at the same time
The combination of all the forces on an object
is called the net force
The net force determines if and how an object
will accelerate
When the forces on an object are balanced,
the net force is zero and there is no change in
the object’s motion
When an unbalanced force acts on an object,
the object accelerates
You can find the net force on an object by
adding together the strengths of all the
individual forces acting on the object
Look at the figure below
The big dog pushes on the box with a force of
16 N to the right
The small dog pushes on the box with a force
of 10 N to the right
The net force on the box is the sum of these
forces
In the situation, there was a nonzero net force
A nonzero net force causes a change in the
object’s motion
What if the Forces on an Object are
not acting in the same Direction?
The big dog pushes with a force of 20 N
The small dog pushes with a force of 10 N
The dogs are pushing against each other
When the forces on an object act in opposite
directions, the strength of the net force is found
by subtracting the strength of the smaller force
(think negative) from the strength of the larger
force (think positive)
When forces act in opposite directions, the net
force is in the same direction as the larger force
In the example, the box will accelerate to the
right
Let’s try a few examples...
Use what you know about net force to describe
the motion of the box in the figure. Assume the
box starts at rest.
1. You pull on your dog’s leash to the right with
a 12 N force. Your dog pulls to the left with a 6
N force. Sketch this situation, including labeled
force arrows.
1.
2. What is the net force on the leash? Calculate
it. Draw and label it.
2. 6 N to the right
Tug of War Clip
http://techtv.mit.edu/videos/850-mitphysics-demo----no-win-tug-of-war
What's the definition of balanced forces?
What's the definition of balanced forces?
Equal but opposite forces in strength and
direction resulting in a zero net force
What's the definition of unbalanced forces?
What's the definition of unbalanced forces?
Direction and strength of forces acting on an
object result in a non-zero net force which
causes the object to accelerate
When forces are unbalanced and in opposite
directions, what is the combined force equal
to? What direction is it?
When forces are unbalanced and in opposite
directions, what is the combined force equal
to? What direction is it?
The combined force is equal to the difference
between the 2 forces
It will be in the direction of the larger force
Use your answers above to explain what
happens in a tug-of-war stand-off.
Use your answers above to explain what
happens in a tug-of-war stand-off.
The 2 sides each exert an opposite force.
The side that exerts the most force will cause
a movement of the other side toward them.
Use your answers above to explain what
happens in a arm-wrestling stand-off.
Use your answers above to explain what
happens in a arm-wrestling stand-off.
Each arm wrestler is exerting a force
opposite the other wrestler. The person with
the most force will cause the other’s arm to
move in the direction of the greater force.
How is a tug-of-war stand-off similar to an
arm-wrestling stand-off?
How is a tug-of-war stand-off similar to an
arm-wrestling stand-off?
Same: both involve opposite forces, if there is
a net zero force there is no winner, if there is
a net non-zero force the side/person with the
greater force will cause the other to move in
the direction of the greater force
How are the two stand-offs different?
How are the two stand-offs different?
Different: team compared to single person;
when there is a net non-zero force for the
arm wrestler, the winner’s force will stop
when their hands hit the table; direction of
force is straight line for tug of war but angled
for the arm wrestler
Catching a baseball
In space
30 N to the right
The net force becomes 0 N so it won’t
accelerate
5 N to the left
Their strength and direction
Non zero net forces
Friction
The force that 2 surfaces exert on each other
when they rub against each other
Ex: If you slide a book across a table, the
surface of the book rubs against the surface of
the table
Factors that Affect Friction
2 Factors:
types of surfaces involved
how hard the surfaces are pushed together
Ex: The football player is pushing on a
blocking sled
If his coach wanted to make it harder to move
the sled, the coach could change the surface
of the sled. Covering the bottom of the sled
with rubber would increase friction and make
the sled harder to move.
Smooth surfaces produce less friction than
rough surfaces
Ex: if you rubbed your hands together forcefully,
there would be more friction than if you rubbed
your hands together lightly
Friction increases when surfaces push harder
against each other
Friction acts in a direction opposite to the
direction of the object’s motion (opposes the
motion of objects that touch as they move past
each other)
Without friction, a moving object will not stop
until it strikes another object
How could you make it easier moving heavy
objects?
How could you make it easier moving heavy
objects?
Put sliders, dolly, or slick surface under the
object
How could you make it easier moving heavy
objects?
Put it on a dolly or put sliders under it
What would happen to the football player if he
switched to a much heavier sled?
What would happen to the football player if he
switched to a much heavier sled?
He would find it harder to push because it
pushes down harder against the ground
Types of Friction
There are 4 main types of friction:
Static friction
Sliding friction
Rolling friction
Fluid friction
Static Friction
The friction force that acts on objects that are
not moving
Sliding Friction
A force that opposes an object’s direction of
motion as it slides over a surface
Rolling Friction
The friction force that acts on rolling objects
Fluid Friction
This force opposes an object’s motion through
a fluid
Ex: Air resistance that acts on an object
moving through the air
Why is it more difficult to walk on ice than on
the floor?
Why is it more difficult to walk on ice than on
the floor?
Ex: Because your feet slide on the ice (There is
more friction between your shoes and the floor
than there is between your shoes and ice)
Which type of friction keeps your shoes from
slipping on the floor?
Which type of friction keeps your shoes from
slipping on the floor?
Static friction
Which type of friction keeps you skating on an
icy pond?
Which type of friction keeps you skating on an
icy pond?
Sliding friction
Question...
How is friction between people like friction
between objects?
Question...
How is friction between people like friction
between objects?
The strength of the friction depends on how
intensely 2 people dislike each other, just as the
strength of a frictional force depends on how
hard the surfaces are pushed together
The strength of the friction depends on the
personalities of the 2 people, just as the
strength of the frictional force depends on the
types of surfaces involved
Name 4 types of friction and give an example of
each.
Name 4 types of friction and give an example of
each.
Static: pushing a car that is not moving
Sliding: a sled sliding down a hill
Fluid: a plane flying through the air
Rolling: skateboard wheels on cement
What types of friction occur between your bike
tires and the ground when you ride over
cement, ride through a puddle, and apply your
brakes?
What types of friction occur between your bike
tires and the ground when you ride over cement
(rolling), ride through a puddle (fluid), and apply
your brakes (sliding)?
What is friction affected by?
What is friction affected by?
The surfaces involved and how hard they press
against each other
Gravity
A force that acts between any two masses
It is an attractive force
Earth’s gravity acts downward toward the
center of Earth
What forces are acting on the skydiver?
What forces are acting on the skydiver?
Gravity and friction
Which force is greater?
Which force is greater?
Gravity
How can you tell?
How can you tell?
The net downward force causes them to fall
toward Earth
How will the skydiver land safely?
How will the skydiver land safely?
He will deploy his parachute which will slow his
fall
Sir Isaac Newton concluded that a force pulled
objects straight down toward the center of
Earth
We are so used to objects falling that we may
not have thought about why they fall
Newton published his work on gravity in 1687.
What observations might you make today that
would lead you to the same conclusions about
gravity?
What observations might you make today that
would lead you to the same conclusions about
gravity?
Rain falling, apples falling from trees, thrown
baseballs eventually hitting the ground
Universal Gravitation
Newton realized that gravity acts everywhere in
the universe, not just on Earth
Ex: makes skydivers fall to the ground
keeps the moon orbiting around Earth
keeps all the planets in our solar system
orbiting around the sun
Law of Universal Gravitation
The force of gravity acts between all objects in
the universe that have mass
This means that any 2 objects in the universe
that have mass attract each other
You are attracted not only to Earth but also to
the moon, the other planets in the solar system,
and all other objects around you
Earth and the objects around you are attracted
to you as well
You do not notice the attraction among small
objects because these forces are extremely
small compared to the force of Earth’s attraction
The gravitational force between some objects
is stronger than the force between others
You observe only the effects of the strongest
gravitational forces
Ex: You don’t see your pencil fly toward the
wall the way you see it fall toward Earth
Factors Affecting Gravity
2 factors: Mass
Distance
Mass is a measure of the amount of matter in
an object (SI unit is the kilogram)
The more mass an object has, the greater the
gravitational force between it and other objects
Ex: Earth’s gravitational force on nearby objects
is strong because the mass of Earth is so large.
The more massive planets interact with a
greater gravitational force than the less massive
planets
Gravitational force also depends on the
distance between the objects’ centers
Ex: As distance increases, gravitational force
decreases. That’s why Earth can exert a visible
gravitational force on a pencil in your room and
not on a pencil on the moon.
Ex: Athletes in space would be able to perform
feats that are impossible on Earth...
Gravitational forces are weaker so golf balls
and baseballs can float or fly farther in space
Give an example of another sport which would
be affected by reduced gravitational force
Give an example of another sport which would
be affected by reduced gravitational force
Gymnastics, jumping events in track
What’s that Again?
The more mass an object has, the greater its
gravitational force
The shorter the distance is between one object
and another, the stronger the gravitational
force between the objects
Between which 2 objects is the gravitational
force stronger, a robin and a worm or a bear
and a cub? Why?
Between which 2 objects is the gravitational
force stronger, a robin and a worm or a bear
and a cub? Why?
Assuming the distances are the same,
gravitational force is stronger between a bear
and cub because they have greater mass
Between which 2 objects is the gravitational
force stronger, 2 apples in a fruit bowl or 2
apples on different branches of an apple tree?
Why?
Between which 2 objects is the gravitational
force stronger, 2 apples in a fruit bowl or 2
apples on different branches of an apple tree?
Why?
Between 2 apples in a fruit bowl because the
apples in the bowl are closer, and gravitational
force decreases as distance increases
In the solar system, which body exerts the most
gravitational force? Why?
In the solar system, which body exerts the most
gravitational force? Why?
The sun since it has the greatest mass
1. Circle the object in the outermost orbit which
experiences the greatest gravitational pull from
the sun.
2. Planet B’s force arrow from the gravitational
pull of the sun should be (longer/shorter) than
planet A’s arrow.
3. Draw what a planet would look like it is was
the same distance from the sun as Planet C but
experienced a smaller gravitational pull from the
sun.
Mass and Weight
Mass is sometimes confused with weight
Mass is the measure of the amount of matter in
an object
Weight is a measure of the force of gravity on
an object
Ex: Your bathroom scale displays the
gravitational force Earth is exerting on you
What happens to the gravitational force
between 2 objects when their masses are
increased?
What happens to the gravitational force
between 2 objects when their masses are
increased?
When mass increases, the force increases
What happens when the distance between the
objects increases?
What happens when the distance between the
objects increases?
When distance increases, the force decreases
If the mass of Earth increased, what would
happen to your weight?
If the mass of Earth increased, what would
happen to your weight?
My weight would also increase. The
gravitational force would be stronger.
What about your mass?
What about your mass?
My mass would stay the same.
At any time, your mass is the same on Earth
as it would be on any other planet
But your weight varies with the strength of the
gravitational force
The dog has a different weight at different
places in the solar system
On the moon, he would weigh about 1/6 of what
he does on Earth
On Mars, he would weigh just over 1/3 of what
he does on Earth
The Ups and Downs of It...
Gravity causes objects to accelerate
downward
Air resistance acts in the direction opposite to
the downward motion of gravity and reduces
acceleration
Terminal Velocity
The constant velocity of a falling object when
the force of air resistance equals the force of
gravity
Projectile Motion
The combination of an initial forward velocity
and the downward vertical force of gravity
causes the ball to follow a curved path
Projectile motion is a falling object’s motion
after it is given a forward velocity
Projectile Motion