Transcript FORCE & MOTION

FORCE
&
MOTION
Why do Newton’s Laws Matter to Me?
• Sir Isaac Newton discovered answers to many
questions about motion.
• Many historians believe Newton’s ideas about
motion were the beginning of modern science.
• The laws of motion explain the relationships
between forces acting on an object, the object’s
mass, and its motion.
FORCE
• A soccer ball sits on the ground, motionless,
unless you kick it.
• Your binder sits on the table unless you pick it
up.
• If you hold your pencil above the ground and
let it go, gravity pulls it to the floor.
• IN ALL OF THESE CASES, THE MOTION OF THE BALL, BINDER,
OR PENCIL WAS CHANGED BY SOMETHING PUSHING OR
PULLING ON IT.
FORCE CONTINUED
• An object will speed up, slow down, or turn
only if something is pushing or pulling on it.
• FORCE is a push or a pull.
• A force can be exerted in different ways.
Example
Imagine throwing a ball. Your hand exerts a
force on the ball. After the ball leaves your
hand, gravity’s force on it causes its path to
curve downward. When the ball hits the
ground, the ground exerts a force, stopping
the ball.
How Do Scientists Measure and
Describe Forces?
• More than 1 force can act on an object at the
same time.
• Example: You hold a paper clip near a magnet.
You, the magnet, and gravity all exert forces
on the paper clip.
• The combination of ALL the forces acting on
an object is called the net force.
• When more than one force is acting on an
object, the net force determines the motion of
the object.
• In our example, the paper clip is not moving,
so the net force is zero.
Combining Forces
• Forces in the same direction add together to
form the Net Force.
• The net force is the difference between 2
forces that are in the opposite direction. The
net force is also in the direction of the larger
force.
EXAMPLE
You and friend are standing on opposite sides
of a door. You are both pushing against the
door, but the door does not move. The Force
that each of you is exerting on the door is
equal and in the opposite direction. The Net
Force is therefore zero.
EXAMPLE
You and your friend are on opposites sides of
the door again. Only this time the door starts
to close. This is because the force you are
exerting on the door is greater than the force
your friend is exerting to try to keep the door
open. The Net Force is the difference between
the force that you and your friend are
exerting. Yours is the larger force that is why
the door is closing.
BALANCED & UNBALANCED FORCES
• A Force can act on an object without causing it
to accelerate if the other forces cancel the
push or pull of the force.
• EXAMPLE: You and your friend pushing on the
door and it didn’t move. The two forces are
equal in opposite directions and therefore,
cancel each other out.
• BALANCED FORCES: Two or more forces
exerted on an object that cancel each other’s
effects and do not cause a change in the
object’s motion. The Net Force is zero.
• UNBALANCED FORCES: The forces do not
cancel each other and the motion of an object
changes. The Net Force does not = zero.
EXAMPLE
You and your friend pushing on a
door. The door starts to close even
though your friend is pushing to hold
it open.
NEWTON’S LAWS OF MOTION
• NEWTON’S FIRST LAW OF MOTION: An object
at rest remains at rest unless an unbalanced
force acts on it and causes it to move.
• EXAMPLE: If you stand on a skateboard and
someone pushes you, you and the skateboard
start moving. You began to move when the
force was applied.
• Newton’s First Law says if there in NO net
force acting on an object the object remains at
rest, OR, if the object is already moving, it
continues to move in a straight line with
constant speed.
• A Force does not need to be constantly
applied to an object to keep it moving.
• The motion of an object does not change until
an unbalanced force acts on it.
Friction
• FRICTION: Rubbing force that acts against the
motion between two touching surfaces and
always slows an object down.
• Friction is the force that brings nearly
everything to a stop.
• EXAMPLE: A baseball or softball player sliding into a
base. The friction between the ground and the player
slows them down.
• EXAMPLE: When you’re riding your skateboard, you have
to keep pushing because of the friction between the road
or sidewalk and the wheels.
• Friction is why you never see objects moving with
constant velocity unless a net force is applied.
• Friction always acts on objects that are sliding or moving
through air or water.