Forces, Motion, &
Gravity

Objectives
 Identify
some forces that act in everyday
situations.
 Determine
the force of gravity in SI units.
 Generalize
about how friction affects
motion.
 Observe
objects.
projectile motion in everyday
 How
do you think force and motion are related?
 Why
is gravity called a force?
 When
why?
you throw a ball, what happens to it and
A
force is a push or pull that starts, stops, or
changes the direction of an object.
 Force
transfers energy to an object.
• To lift the
wheelbarrow, the man
must first apply force
to lift it.
• He must then apply
force in another
direction to push the
wheelbarrow.
 The
amount and
direction of most
forces can be
measured.
 A spring scale is one
kind of instrument
used to measure
force.
 The spring scale,
shown on the right,
measures the force of
gravity on an object.
 The
SI unit for force is called a newton.
 The newton is named for Sir Isaac Newton, the
scientist who explained how force and motion
are related.
One newton (N) is the amount of
force needed to cause a 1-kg mass
to accelerate at a rate of 1 m per
second for each second of motion.
 GRAVITY
DETERMINES WEIGHT!
 The
weight of an object
depends on the force that pulls
the object toward the earth.
 You
know this force as gravity!
 Since
the force of gravity on the
earth is 9.8 m/s2, the weight of
a 1-kg object is 9.8 N.
 If
you drop a feather and a
coin at exactly the same
time from a second story
window, which one will
reach the ground first?
 Recall
that the
acceleration of gravity is
9.8 m/s2 …
 When
an object moves
through air, its motion is
opposed by the force of
friction.
 Friction occurs when the
surfaces of any kind of
matter move past each
other.
 Friction from the air
affects the motion of a
falling object by acting
against the force of gravity.
 What
kind of force might
act differently on the
feather and the coin?
FRICTION!
 Measuring
with a Spring Scale - Go to page 37
 Record your predictions & measurements in your
graph.
Measurement in Newtons
OBJECT
1
2
3
4
5
PREDICTED FORCE (N)
MEASURED FORCE
(N)
A
falling object gains
speed until the force from
air friction, which acts
upward, equals the
downward force of
gravity.
 When
the upward and
downward forces are
equal, the object reaches
terminal velocity.
 At
terminal velocity, an
object's velocity becomes
constant.
What forces are acting on both
objects that affect the speed of
the falling feather & coin?
• All falling objects reach terminal velocity
if given enough time.
• However, the time required to reach
terminal velocity varies.
• Dense objects with little
surface area, such the coin,
must fall for several
seconds before reaching
terminal velocity.
• Less dense objects with a
lot of surface area, such as
the feather, reach terminal
velocity much faster.
 HOW
WOULD CHANGING
THE SHAPE OF AN OBJECT
AFFECT ITS TERMINAL
VELOCITY?
 Decreasing
the surface
area of an object will
decrease the force of air
friction acting on it, and
increase its terminal
velocity.
Try it with a tissue…
 If
you throw a ball for your dog to fetch, the
ball follows a curved path to the ground.
 This path is called projectile motion.
http://www.brainpop.c
om/science/motionsfo
rcesandtime/force/
1.
Give two examples of force.
pushing a lawnmower, pushing a door open,
pulling a wagon, throwing a ball
2.
Describe how you can use a spring scale to
measure the force of gravity on an object.
The weight of an object is a measure of the force
of gravity on an object – measured in Newtons.
The force pushes on the object which stretches
the spring. The spring scale has calibrations on it
that measures the force in Newtons as well as its
weight in grams.
First Law of Motion

Objectives
 State
Newton's first law of motion.
 Identify
inertia and explain how to
overcome it.
 Define
operationally how friction
occurs in an everyday situation.
 Observe
Newton's first law of motion.
 Newton
observed that an
object at rest stays at rest
until an outside force
causes it to move.
 He
also observed that an
object in motion continues
to move in the same
direction until a force
stops it or changes its
direction.
 Newton
stated his
observations as the first
law of motion.
Why do the disc boxes at the
top of the stack stay in place
when the girl pulls out the
lower one?
AN OBJECT AT REST WILL
REMAIN AT REST AND AN
OBJECT IN MOTION WILL
REMAIN IN MOTION UNLESS
ACTED ON BY AN OUTSIDE
FORCE !!
 Your
body is at rest,
and it will remain at
rest until some
outside force moves
it.
 Your
To overcome your inertia
and move out of the chair,
you must apply some kind
of force to your body. This
force must be greater than
the forces that are acting
on your body.
body resists
change; you have
inertia.
 Inertia
is the
tendency of an
object to resist any
change in motion.
The first law of motion
and inertia also applies
to objects in motion.
 Inertia can be
overcome only by the
application of some
type of force.

When you pedal a bike, you
accelerate to make the bike move
forward. When the biker stops
pedaling, the bike and the rider keep
moving forward in the same
direction. This is due to inertia.
 …When
matter rubs against other matter, the
motion creates friction.
 Friction works in the opposite direction to
the force of motion.
 Friction is an outside force that resists
motion when two surfaces come in contact.
If you kick a soccer ball on
flat ground, friction between
the ball and the ground
slows and stops the ball.
 Sliding
Friction
 When two solid surfaces slide over each
other, sliding friction acts between
them.
Sliding friction
keeps the
baseball player
from passing the
base.
 Fluid
Friction
 When a boat sails
through the water,
fluid friction from
both the air and
the water opposes
its motion
*
Fluid friction also
occurs when an
object falls through
the air
 Rolling
Friction
 When an object rolls over a surface, the
friction produced is called rolling
friction
The tread on a
tire affects the
amount of
rolling friction
to be overcome.
Which tire will
create more
rolling friction?
1.What is Newton’s first law of motion?
Newtons first law of motion is also the law of inertia – an
object will remain at rest or in motion unless acted upon
by an outside force.
2.Explain how a dog overcomes inertia when it gets up after
napping on the floor.
The dog uses its muscles to apply a force needed to
overcome its napping position.
3.Explain how friction works to keep bicycle tires from
sliding on the road.
The weight of the rider and bike force the tires against
the road surface which creates friction and keeps the
tires from slipping.
Second Law of
Motion
 Objectives

State Newton's second law of motion

Calculate the force of a moving object by
using Information about its mass and
acceleration.

Identify the relationship between
momentum, mass, and velocity.

Make a model to show how force acts during
circular motion.

Newton understood that
force, mass, and acceleration
are related.

He showed that the motion of
an object changes, or
accelerates, when a force
acts on it.

If you know the mass and the
acceleration of an object,
you can determine its force.

This relationship is stated in
Newton's second law of
motion.
THE NET FORCE ON AN
OBJECT EQUALS ITS MASS
TIMES ITS ACCELERATION.
A
direct relationship exists between force (f)
mass (m) and acceleration (a).
 The boy in the picture applies the same
amount of force to both carts.
For both carts to have the
same acceleration, what must
we change about the cart on
the left?
Because more mass was
added to the cart on the
right, it's acceleration is less.
The graph shows the relationship between force
and acceleration during a dog-sled race.
Notice that as the
force the dogs apply
to the sled increases,
the acceleration of
the sled also
increases.
How does the acceleration of the 200 kg
sled compare to the acceleration of the
100 kg sled?
Even without doing calculations, Newton's
second law helps you understand how force,
mass, and acceleration are related.
Recall Newton's formula for the second law of motion.
Sample Problem:
A dog has a mass of 20 kg. If the dog is pushed
across the ice with a force of 40 N, what is its
acceleration?
F=MA
Force = 40N
mass = 20kg
40 = 20 A
20 = 20
A = 2m/s2
DO PRACTICE PROBLEMS 1 & 2 pg. 66
PRACTICE PROBLEMS 1 & 2 pg. 66
F=MA
F=(40kg)(2.5m/s2)
F=100 N
-------------------------------------40kg + 10kg + 10kg = 60 kg
90N=(60kg)A
60
60
A=1.5m/s2
A
rolling marble can be stopped more easily
than a bowling bowl moving at the same
velocity. Both objects have inertia of motion or
momentum.
 The
bowling bowl has more momentum…why?

All objects have inertia of motion, or momentum.

The momentum of a moving object is related to its
mass and velocity.

A moving object has a large momentum if it has a
large mass, a large velocity, or both.
 Momentum
doesn't change unless the
velocity or mass changes.
 However, momentum can transfer from one
object to another.

Choose a sports activity to
demonstrate the transfer of
momentum.

In each instance how was momentum transferred
from one object to another??

Balls change direction and or speed…players may
fall after colliding with another player or ball…

You learned that an object in circular motion
has centripetal acceleration.

For example, if you twirl an eraser on a string,
the tension on the string pulls the eraser
inward, toward the center…The inward
tension on the string is centripetal force.

Centripetal force is any force that causes an
object to follow a circular path
http://www.youtube.com/
watch?v=yyDRI6iQ9Fw
1.
How does the force applied to an object affect
its acceleration?
The greater the force applied to an object, the greater the
acceleration if the mass remains constant.
2.
If an eagle and a bumblebee are both traveling at
16 km/hr., which has more momentum? Explain.
The eagle because the eagle has more mass.
2.
Calculate: A 28 kg meteor hits the surface of the
moon at 130 km/s. What is the meteor’s
momentum? M=mv
3, 640, 000 kg*m/s (remember to convert km/s to m/s because
velocity is measured in m/s)
Third Law of
Motion
 Objectives
 Describe
Newton's third law of motion.
 Distinguish between balanced and
unbalanced forces.
 Predict how forces interact in
everyday situations.
 Infer how the third law of motion is
applied.
When one object exerts a
force upon a second object,
the second object exerts an
equal and opposite force
upon the first object.
• For example, The rockets
of the space shuttle force
burning gases downward
through the exhaust vents.
• In response to these
downward forces, the
shuttle system moves
upward.
•When one object exerts a
force upon a second object,
the second object exerts an
equal and opposite force
upon the first object.

When forces on an object are
unbalanced motion occurs.
 This
crew team uses Newton’s
third law of motion to move
it’s boat.
 The
boat moves in the
opposite direction of the oars
with a force that is equal to
that of the oars as they push
against the water.
http://www.brainpop.com/
science/motionsforcesandti
me/newtonslawsofmotion/p
review.weml
1.
What is Newton’s third law of Motion?
When one object exerts a force on a second
object, the second object exerts an equal and
opposite force upon the first object.
3.
Predict: What are the action and reaction forces
acting when someone jumps from a canoe to a
riverbank? Explain why the jumper falls into the
water?
The force exerted by the jumper makes the boat
move instead of the jumper who falls before
reaching the bank.
Universal Forces
 Objectives
 Identify
the four universal forces.
 Describe how each universal force
affects everyday experiences.
 Generalize on the importance of
universal forces.
 Classify everyday forces according to
the types of universal forces.
 While
traveling through the
universe, you experience several
kinds of forces.
 These
four universal forces are:
Gravitational
 Electromagnetic
 Strong..
 and weak nuclear forces


Newton's law of universal
gravitation states that every
object in the universe attracts
every other object.

Objects of any size are pulled
toward each other by
gravitational force.

For example, the earth's
gravitational force on your body
pulls you toward the ground.

You don't notice the
gravitational force between
small objects because it is the
weakest of the universal forces.
Planets, stars, and galaxies
are so large that their
gravitational force affects
other objects in space.
The
strength of
gravitational force
depends on the
amount of mass in
an object and the
distance between
objects.
What would happen to objects on the earth if
the force of gravity was as strong as the
electromagnetic force?
Gravitational force keeps
the earth and other
objects in the solar system
in orbit around the sun.
 Each planet moves in an
elliptical orbit around the
sun.

Newton realized the gravitational force that
applies to objects on the earth also applies to the
moon.
 He proposed that the moon travels around the
earth because the earth pulls on the moon.

The combined gravitational force of the sun and the
moon (above) produces large spring tides.
 Electromagnetic
forces
are really two different
forces that are closely
related – electric force
and magnetic force.
 Both
of these forces
may attract or repel.
 Electric
force exists
between charged
particles.
 Objects
with
different charges
attract each other,
and objects with the
same charge repel
each other.
 The
electric force is
much stronger than
the gravitational
force.
 Magnetic
force acts between two magnets.
 Magnetic
force attracts or repels just as
electric force attracts and repels.
The magnets
moving this train
can be seen along
the outer edges of
the track.
 An
atom is composed
of electrons orbiting
around a nucleus of
protons and neutrons.
 Electric
forces keep
electrons in orbit
around the nucleus.
 The
forces that hold
the particles in the
nucleus together are
nuclear forces.
 One
type of nuclear
force is called the
strong force.
 Protons
and neutrons
are made of even
smaller particles called
quarks.
 The
strong force holds
the nuclear particles
together by holding the
quarks together.
 The
other type of nuclear force is the weak
forces.
 The weak force holds together the particles
within neutrons and protons.
 In neutrons, the weak force is easily overcome,
and the neutrons decay to form different atoms.
 Nuclei that decay in this way are called
radioactive.
 Measurement
of radioactive decay is used to
reveal the age of the materials used in
ancient artifacts.
 Some kinds of nuclear decay release harmful
radiation that can affect people and the
environment.
 Safe, permanent disposal of radioactive
materials is a worldwide problem.
1.
Name the four universal forces.
Gravitational
Electromagnetic
Strong nuclear forces
Weak nuclear forces
2. Explain how each universal force might apply
when you warm food in an oven.
Gravity keeps food in one place; electromagnetic
forces operate the oven; nuclear force holds the
atoms together in the food.
friction
1. The force that opposes motion is _______________.
2. When the upward and downward forces on a falling
object are equal, the object reaches
Terminal
velocity
_____________.
inertia
3. When your body is at rest, you have _____________.
4. A push or pull that starts or stops the motion of an
object or changes its direction is called a(n)
______________
.
force
5. Gravitation, electromagnetic, and strong and weak
Universal forces .
nuclear forces are ________________
6. The forces that hold together the particles in the
nucleus of an atom are called __________________
Strong nuclear forces.
7. The SI unit for force is newton
_______________ .