Download 06 FORCES

Motion and Forces
What is force?
SECTION
Passing a basketball to a team member or kicking a soccer
ball into the goal are examples of applying force to an object.
A force is a push or pull that one body exerts on another.
EQUILIBRIUM
These students are pushing on the box with an
equal force but in opposite directions. Because
the forces are balanced, the box does not move.
ENVIRONMENT
SYSTEM
These students are pushing on the box with unequal forces
in opposite directions. The box will be moved in the
direction of the larger force.
These students are pushing on the box in the same direction. The
combined forces will cause the box to move.
When two or more forces act on an object at the same time,
the forces combine to form the net force.
The net force on the box is zero because the two forces cancel
each other.
Forces on an object that are equal in size and opposite in
direction are IN EQUILIBRIUM.
EQUILIBRIUM
EQUILIBRIUM
CONTACT
FORCES
These students are pushing on the box with an
equal force but in opposite directions. Because
the forces are balanced, the box does not move.
LONG RANGE FORCES - ACT ON OBJECTS
FROM A DISTANCE WITHOUT TOUCHING
THEM.
GRAVITY
MAGNETISM
ELECTRIC FORCES - POSITIVE ATTRACTS
NEGATIVE
Forces have agents
Each force has a specific, identifiable, immediate
cause called the agent.
Whatever is causing the force is called the agent.
You should be able to name the agent of each force,
for example, the force of the desk or your hand on
your book.
The agent can be animate, such as a person, or
inanimate, such as a desk, floor, or a magnet.
The agent for the force of gravity is Earth’s mass.
WHAT CAUSES AN OBJECT TO MOVE?
WHAT CAUSES MOTION?
WHAT HAPPENS WHEN YOU THROW A
TENNIS BALL?
Force and Acceleration
What’s different about throwing a ball as
hard as you can and tossing it gently?
When you throw hard, you exert a much
greater force on the ball.
How is the motion of the ball different in
each case?
Newton’s Second Law
Newton’s second law of motion describes how
force, mass, and acceleration are connected.
According to Newton’s second law of motion:
A net force acting on an object causes the
object to accelerate in the direction of the net
force.
FORCE = MASS X ACCELERATION
F=MxA
FORCES CAUSE AN OBJECT TO ACCELERATE.
FORCES CAN ACT IN ANY DIRECTION – A FORCE
IS A VECTOR BECAUSE IT HAS MAGNITUDE AND
DIRECTION
MORE THAN ONE FORCE CAN ACT ON AN
OBJECT
WHEN MORE THAN ONE FORCE ACTS ON
AN OBJECT, WE MUST COMBINE ALL THE
FORCES INTO ONE VECTOR.
THIS ONE VECTOR REPRESENTS THE NET
FORCE.
WE USE FREE BODY DIAGRAMS TO
REPRESENT HOW A FORCE OR FORCES
ACTS ON AN OBJECT.
TO DRAW A FREE BODY DIAGRAM, WE USE A DOT
TO REPRESENT THE OBJECT AND DRAW THE
FORCE VECTORS WITH THE TAIL ATTACHED TO
THE DOT.
The vector sum of two or more forces on an object is
called the net force.
FORCE IS MEASURED IN NEWTONS
F=mxa
F = kg X m / s2
1 Newton = 1 kg - m / s2
NEWTON’S FIRST LAW OF MOTION
Inertia (ihn UR shuh) is the tendency of an object to
resist any change in its motion.
If an object is moving, it will keep moving unless an
unbalanced force acts on it.
If an object is at rest, it tends to remain at rest unless
an unbalanced force acts on it.
Equilibrium
If the net force on an object is zero,
then the object is in equilibrium.
An object is in equilibrium if it is at
rest or if it is moving at constant
velocity.
NEWTON’S FIRST LAW OF MOTION
Inertia (ihn UR shuh) is the tendency of an object to
resist any change in its motion.
If an object is moving, it will keep moving unless an
unbalanced force acts on it. In
If an object is at rest, it tends to remain at rest.
Its velocity is zero unless a force makes it move.
Forces
Newton’s Second Law
Force, Mass, and Acceleration
Newton’s first law of motion states that the motion of an
Object changes only if an unbalanced force acts on it.
Force and motion are connected.
How does force cause motion to change?
Force and Acceleration What’s different
about throwing a ball as hard as you can
and tossing it gently?
When you throw hard, you exert a much
greater force on the ball.
How is the motion of the ball different in
each case?
Newton’s Second Law
Newton’s second law of motion describes how
force, mass, and acceleration are connected.
According to Newton’s second law of motion,
the net force acting on an object causes the
object to accelerate in the direction of the net
force.
FORCE = MASS X ACCELERATION
F=MxA
A FORCE CAUSES AN OBJECT TO ACCELERATE IN
THE DIRECTION OF THE FORCE
According to Newton’s second law of motion, the
net force acting on an object causes the object to
accelerate in the direction of the net force.
The acceleration of an object is determined by the
size of the net force and the mass of the object
according to the equation:
THE MORE THE MASS, THE MORE FORCE IS
REQUIRED TO ACCELERATE IT
The acceleration of an object is determined by
the size of the net force and the mass of the
object according to the equation:
Using Newton’s Laws
Newton’s second law describes the connection
between the net force exerted on an object and
its acceleration.
F = ma
We know that the acceleration due to the earth’s
gravity is 9.8 m/s2
We know that
F = ma
So what is the force on an object due to
the earth’s gravity?
F = m x 9.8
2
m/s
F = m x 9.8
2
m/s
This is the force that the earth’s gravity exerts on
every thing on the earth. This is the force that pulls
us down in our chairs. This is the force that pulls a
tennis ball down out of the air.
Hold a big steel mass and a small steel mass
in your hands.
Do you feel the force of the earth pulling
down more on the large mass?
The more mass, the more force.
What is WEIGHT?
Weight is the force due to gravity.
Weight is the same as
Is weight the same thing as MASS?
We use Fg to represent the force due to
gravity.
Fg is the same as the weight of an object.
For an object free-falling
through the air, the net
force on the object is Fg
Scales – what is being measured –
mass or weight?
Weight and Mass
Weight and mass are not the same. Weight is a
force, and mass is a measure of the amount of
matter an object contains.
Mass never changes. It always stays the same.
If you go to the moon, your mass is the same. If you
go to Jupiter, your mass is the same. If you are
floating weightless in space, your mass is the same.
Weight and Mass
If you go to the moon, your mass is the same. BUT,
THE MOONS GRAVITY IS ONLY 1/6 OF EARTH’S
GRAVITY.
EARTH GRAVITY = 9.8 m/s2
MOON GRAVITY = 1.6 m/s2
If your mass is 100kg
Weight = m x a
Earth Weight = 100kg x 9.8 = 980 Newtons
Moon Weight = 100kg x 1.6 = 163 Newtons
Apparent weight What is weight? What does a bathroom
scale measure? Suppose you stood with one foot on the scale
and one foot off? Or what if a friend pushed down on your
shoulders or pushed up on your elbows? Then there would be
other contact forces on you, and the scale would not read
your weight.
What happens if you are standing on a scale in an elevator?
As long as the elevator is in equilibrium, that is, at rest or
moving at constant speed, the scale reads your weight. But if
the elevator accelerates upward, then the scale reads a larger
force. What does it feel like to be in an elevator like this? You
feel heavier; the floor presses harder on your feet. On the
other hand, if the acceleration is downward, then you feel
lighter, and the scale reads less. The force exerted by the scale
is called the apparent weight.
Imagine that the cable holding the elevator breaks.
The scale with you on it would accelerate with a = -g.
The scale would read zero! Your apparent weight
would be zero. That is, you would be weightless.
However, weightlessness doesn’t mean your weight
is zero, but that there are no contact forces pushing
up on you. Weightlessness means that your apparent
weight is zero.
Weighing Yourself in an Accelerating Elevator
Your mass is 75 kg. You stand on a bathroom scale in an
elevator.
Going up! Starting from rest, the elevator accelerates at 2.0
m/s2 for 2.0 s, then continues at a constant speed. What is
the scale reading during the acceleration? Is it larger than,
equal to, or less than the scale reading when the elevator is
at rest?
Sketch the Problem
• Sketch the situation as in Figure 6–6b.
• Draw the motion diagram. Label v and a.
• Choose a coordinate system with the positive direction up.
• The net force is in the same direction as the acceleration,
so the upward force is greater than the downward force.
Friction
Suppose you give a skateboard a push with your hand.
According to Newton’s first law of motion, if no forces are
acting on a moving object, it continues to move in a
straight line with constant speed.
What happens to the motion of the skateboard after it
leaves your hand? Does it continue to move in a straight
line with constant speed?
You know the answer. The skateboard gradually slows down
and finally stops.
Friction
The force that slows the skateboard and brings it
to a stop is friction.
Friction is the force that opposes motion
between two surfaces that are touching each
other.
What causes friction? Would you believe the
surface of a highly polished piece of metal is
rough?
If two surfaces, such as two pieces of silver, are pressed
tightly together, welding, or sticking, occurs in those areas
where the highest bumps come into contact with each other.
These areas where the bumps stick together are
called microwelds and are the source of friction.
These areas where the bumps stick together are called
microwelds and are the source of friction.
There are three types of friction…
Static Friction
Kinetic (Sliding) Friction
Rolling Friction
Static friction is the friction between two surfaces that are
not moving past each other. In this case, your push is not
large enough to break the microwelds, and the box remains
stuck to the floor.
Kinetic friction is the force that opposes the motion of two
surfaces sliding past each other. Sliding friction is caused by
microwelds constantly breaking and then forming again as
the box slides along the floor
The friction between a rolling object and the surface it rolls
on is rolling friction.
Rolling friction is due partly to the microwelds between a
wheel and the surface it rolls over. Microwelds break and
then reform as the wheel rolls over the surface.
Rolling friction is usually much less than static or sliding
friction.
This is why it’s easier to pull a load in a wagon
rather than dragging it along the ground.
The Normal Force is the force that is perpendicular
to the surface. The normal force is the Weight of
the object.
Normal Force
The static friction force can vary from 0 to the maximum static friction force just before
motion begins.
Air Resistance
When an object falls toward Earth, it is pulled downward
by the force of gravity.
However, another force called air resistance acts on
objects that fall through the air.
Because of its greater
surface area, the
spread-out bag has
much more air
resistance acting
on it as it falls.
Air resistance, not the
amount of mass in the
object, is why feathers,
leaves, and pieces of paper
fall more slowly than
pennies, acorns, and
apples.
If no air resistance is
present, then a feather and
an apple fall at the same
rate.
The force of air resistance on an open parachute is large. This
causes the sky diver to fall slowly
Terminal Velocity
The force of air resistance increases with speed. As
an object falls, it accelerates and its speed
increases. So the force of air resistance increases
until it becomes large enough to cancel the force of
gravity.
This terminal velocity is the highest velocity that a
falling object will reach.
Fa on b
= - Fb on a
THE FOUR FUNDAMENTAL FORCES
GRAVITATIONAL ATTRACTION – ALL MASSES ATTRACT
EACH OTHER THROUGH GRAVITATIONAL ATTRACTION
MAGNETIC AND ELECTROSTATIC FORCES
NUCLEAR FORCES IN THE NUCLEUS OF AN ATOM –
STRONG NUCLEAR FORCES ACT BETWEEN PROTONS
AND NEUTRONS TO HOLD THE NUCLEUS TOGETHER
WEAK NUCLEAR INTERACTIONS IN SOME FORMS OF
RADIOACTIVE DECAY