Transcript Forces and Motion

FORCES
AND
MOTION
FORCE
A force is any push
or pull from one
object to another.
TYPES OF FORCES
Magnetic force
Gravity
Friction force
Where have you observed magnetic force?
Toys and magnets
Where have you observed the force of gravity?
Falling objects, planets held in orbit by gravitational pull of the
Sun, tides
Where have you observed friction force?
When sliding an object to move it across the floor, skiing and
skating, a car’s tires rubbing against the road
WHAT IS USED TO
MEASURE FORCE?
A Spring Scale
AIRPLANE FORCES
Engines push or pull a plane forward.
This is called thrust.
As a plane moves forward, air moves
around the wings and creates a force
that raises the plane into the air. This
force is called lift.
A pull called drag slows a plane down.
WE USE FORCES IN
MANY WAYS.
Forces are used to crush, stretch, or twist objects and
deform them.
What is an example of how we use crush?
Crushing an aluminum can.
The harder a substance is, the more force it takes to change
its shape.
FORCE
A force can cause an object to start moving, speed up,
change direction, slow down, or stop.
Each of these involve accelerations. Forces accelerate
objects when they affect their motion.
HOW IS A BAT HITTING
THE BALL A FORCE?
The bat contacts the ball, pushing it in the
opposite direction through the air.
WHAT FORCES ARE EXERTED
ON THE AIRPLANE AS IT IS
FLYING?
Thrust, lift, drag, and weight
HOW COULD YOU MAKE
AN AIRPLANE RISE
FASTER INTO THE AIR?
Increase its lifting force by changing the
wings to increase lift.
Design it to have less wind resistance.
Decrease its mass to make it lighter.
GIVE AN EXAMPLE OF A FORCE
THAT CHANGES BOTH AN
OBJECT’S SHAPE AND MOTION.
When a piece of clay is thrown against a hard surface, the
hard surface will apply a force to the clay. This force will stop
the clay (change in motion) and deform it (change of shape).
WHAT CAUSES A
DROPPED OBJECT TO
FALL TO THE GROUND?
Gravity
IF AN OBJECT WERE DROPPED ON
THE MOON’S SURFACE, WOULD IT
FALL TOWARD EARTH OR
TOWARD THE MOON?
Toward the Moon
SOME GRAVITY EXISTS BETWEEN YOU
AND THE PERSON NEAR YOU. WHY ARE
YOU PULLED TOWARD EARTH AND NOT
TOWARD THE OTHER PERSON?
Gravity depends on mass, and Earth has a
much greater mass than a person.
Gravitational attraction is not just between
an object and Earth. Gravity exists between
any two objects.
WHAT IS NEWTON’S LAW
OF UNIVERSAL
GRAVITATION?
Sir Isaac Newton believed that every object in the universe
was pulling on every other object. This theory is called
Newton’s Law of Universal Gravitation.
Newton stated that gravity depends on the masses of the
objects and the distance between them.
Increasing the mass increases the force and increasing the
distance decreases the force.
FRICTION
Friction is the force that opposes the motion of one object
moving past another.
Friction depends on the surfaces of two objects and how
hard the objects are pushed together. Smooth surfaces
usually have less friction than rough surfaces. Friction
increases when surfaces are pressed together with greater
force. Friction also increases with the weight of an object.
Rubbing your hands together is an example of
friction. The friction causes the hands to slow
down as they are rubbed and this generates heat.
Heat is created whenever there is friction.
HOW COULD YOU INCREASE
THE TRACTION (FRICTION) OF
CAR TIRES ON AN ICY ROAD?
1. Increase the weight of the car
2. Make the tires rougher with better tire tread or chains
WHAT WOULD THE
WORLD BE LIKE
WITHOUT FRICTION?
Without friction, you would not be
able to walk or move around
because your feet would slip on
everything!
BALANCED FORCES
When forces act on an object without
changing its motion they are called
balanced forces.
UNBALANCED
FORCES
A force that causes an object to
change its motion is called an
unbalanced force.
NEWTON’S FIRST LAW
OF MOTION
Newton’s First Law of Motion states that objects do not
change their motion unless forced to do so.
“An object at rest tends to stay at rest, and an object in
constant motion tends to stay in motion, unless acted upon
by an unbalanced force.”
It is sometimes referred to as the law of inertia. This law
describes inertia.
HOW COULD YOU KEEP A
BALLOON FROM RISING
OR FALLING IN AIR?
You could use balanced forces. The force of gravity would
have to be balanced by another force. This could be done
using the force of buoyancy. A balloon could be made to
hover in the air without rising or falling if it were filled with
hot or light gas.
HOW DOES NEWTON’S FIRST LAW
EXPLAIN HOW SEAT BELTS
PREVENT INJURIES IN CAR
ACCIDENTS?
During car accidents, a car undergoes
sudden decelerations. Your body, however,
keeps traveling in a straight line, possibly
through the windshield. Seat belts provide
an outside force that changes your motion
and brings you to a stop.
NEWTON’S SECOND
LAW OF MOTION
Newton’s Second Law of Motion states, “The unbalanced
force on an object is equal to the mass of the object
multiplied by it acceleration.”
F=mxa
What do the symbols F, m, and a stand for?
F is for force, m is mass, and a is acceleration.
WHAT COULD YOU DO TO
A RACE CAR TO MAKE IT
ACCELERATE FASTER?
You could increase the total force that is acting on the race
car by reducing friction and drag or by increasing the power
of the engine. You could also decrease the mass of the race
car.
WHAT WILL HAPPEN TO THE
ACCELERATION OF AN OBJECT IF YOU
DOUBLE BOTH THE MASS AND THE
UNBALANCED FORCE ACTING ON IT?
The acceleration will stay
the same.
NEWTON’S THIRD LAW
OF MOTION
Newton’s Third Law of Motion states, “All forces occur in
pairs, and these two forces are equal in strength and
opposite in direction.”
For every action there is an equal but opposite reaction.
ACTION AND
REACTION FORCES
When one object pushes on a second object, the second
pushes back on the first with the same strength.
The push of the first object on the second is called the action
force.
The push of the second object back on the first is called the
reaction force.
HOW COULD A TOY
ROCKET ACCELERATE
AFTER TAKEOFF?
The toy rocket might fire hot gases
downwards out of its engine. The reaction
force from the gases would push the
engine (and rocket) upwards.
WHAT ARE THE ACTION AND
REACTION FORCES ACTING ON
YOUR BODY WHEN YOU WALK?
Feet push down on the road, which in turn
pushes up on the feet. The reaction force
balances gravity. The feet also push
backward on the road and the road pushes
forward on the feet. This reaction force
propels you forward.
SUPPOSE A CAR IS TRAVELING DOWN
THE HIGHWAY AND A BUG HITS THE
WINDSHIELD. WHAT ARE THE ACTION
AND REACTION FORCES?
The bug exerts an action force on the
windshield, and the windshield exerts an
equal but opposite reaction force on the
bug.
SUPPOSE A LARGE ROCK
INSTEAD OF A BUG HIT THE
WINDSHIELD. WHAT WOULD THE
EFFECT BE?
The action and reaction forces would again
be equal, but greater than the forces with
the bug. If the forces were great enough,
the windshield might crack.