Transcript Slide 1 - Images

Acceleration by Force
• Your science book sits on the table until
you pick it up. If you hold your book
above the ground, then let go, gravity
pulls it to the floor.
• An object will speed up, slow down, or
turn only if something is pushing or
pulling on it.
2
Force
• A FORCE is a push or a pull.
• A force can be exerted in different ways.
• For instance, a paper clip can be moved
by the force a magnet exerts, the pull of
Earth’s gravity, or the force you exert
when you pick it up.
• These are all examples of forces acting on
the paper clip.
3
4
Net Force
• The combination of all the forces acting
on an object is the NET FORCE.
• When more than one force is acting on an
object, the net force determines the
motion of the object.
Combining forces to form NET FORCE
• If the forces are in the same direction,
then the net force is the sum between the
two forces.
12 N
8N
NET FORCE = 12N + 8N = 20N, left
Combining forces to form NET FORCE
• If two forces are in opposite directions,
then the net force is the difference
between the two forces, and it is in the
direction of the larger force.
12 N
8N
NET FORCE = 12N - 8N = 4N, right
Forces Can Cancel Each Other
• A force can act on an object without
causing it to accelerate if other forces
cancel the push or pull of the force.
• If you and your friend
push on a door with
the same force in
opposite directions,
the door does not
move.
Balanced Forces
• Two or more forces exerted on an object
are BALANCED FORCES if their effects
cancel each other and they do not cause a
change in the object’s motion.
• If the forces on an object are balanced,
the net force is zero.
Unbalanced Forces
• If the forces are UNBALANCED
FORCES, their effects don’t cancel each
other.
• Any time the forces
acting on an object
are unbalanced, the
net force is not zero
and the motion of
the object changes.
Balanced Forces = No Change in Velocity
• A still object will
never move without
unbalanced forces 12N
acting on it
• A moving object will
never stop moving
without unbalanced
forces acting on it
12N
0N
0N
Newton’s First Law of Motion
• Galileo Galilei, who lived from 1564 to
1642, was one of the first to understand
that a force doesn’t need to be constantly
applied to an object to keep it moving.
• Galileo’s ideas helped Isaac Newton to
better understand the nature of motion.
Newton’s First Law of Motion
• Newton was able to explain the motion
of objects in three rules called Newton’s
laws of motion.
• According to NEWTON’S 1st LAW of
MOTION, if the net force acting on an
object is zero, the object remains at rest,
or if the object is already moving,
continues to move in a straight line with
constant speed.
In Other Words…
• An object at rest
remains at rest unless
an unbalanced force
acts on it and causes it
to move.
• An object in motion
remains in motion
unless as unbalanced
force acts on it and
causes it to stop.
Law of Intertia
a) Newton’s first law of motion is often
called the Law of Inertia.
b) INERTIA is the
tendency of all
objects to resist
any change in
motion.
Ball wants to keep moving
Ball wants to keep sitting still
Law of Intertia
• Unless acted
upon by an
unbalanced force,
this golf ball
would sit on the
tee forever.
Law of Intertia
• Once airborne,
unless acted on by
an unbalanced
force (gravity and
air – fluid friction)
the golf ball would
never stop!
Law of Intertia, continued
c)
Mass is a measure of inertia
d) Small mass = less inertia
e)
Large mass = more inertia
Law of Intertia
Don’t let this be you. Wear seat belts!!
Because of inertia, objects (including you) resist
changes in their motion. When the car going 80
km/hour is stopped by the brick wall, your body
keeps moving at 80 m/hour.
Examples of Newton’s
st
1
Law
 Consider the behavior of a soda in a
glass filled to the rim while starting a
car from rest…
 …while bringing a car to rest from a
state of motion?
Examples of Newton’s
st
1
Law
 Consider how blood rushes from
your head to your feet while quickly
stopping when riding on a
descending elevator.
Examples of Newton’s
st
1
Law
 Consider a brick being painlessly
broken over the hand of a physics
teacher by slamming it with a
hammer. (CAUTION: do not attempt
this at home!)
Examples of Newton’s
st
1
Law
 Consider how to dislodge ketchup
from the bottom of a ketchup
bottle, it is often turned upside
down and, thrusted downward at
high speeds and then abruptly
halted.
Examples of Newton’s
st
1
Law
 Consider why headrests are placed in
cars to prevent whiplash injuries
during rear-end collisions.
Examples of Newton’s
st
1
Law
 Consider how while riding a
skateboard (or wagon or bicycle),
you fly forward off the board when
hitting a curb or a rock which
suddenly stops the motion of the
skateboard
Newton’s Laws of Motion
 Today, Newton’s three laws
of motion are considered to
be the backbone of physical
science. These ideas were
major breakthroughs that
set history on a new course.
Because of these ideas we
now have safety restraints in
multiple types of travel.
ANSWER THE FOLLOWING
SELF CHECK QUESTIONS:
Question 1
You try and push a desk across the floor one
direction but a friend is trying to push it the
opposite direction. As a result, the desk
doesn’t move. This is an example of what
kind of forces?
A. balanced forces
B. opposite forces
C. parallel forces
D. unbalanced forces
Question 2
What happens any time there are
unbalanced forces acting on an object—that
is, any time the net force is not zero?
Question 3
Explain Newton’s first law of motion.
Question 4
Newton’s First Law is also referred to as the
Law of _________________. Why?
Question 5
List and give an example of the four types of
friction:
Question 1
You try and push a desk across the floor one
direction but a friend is trying to push it the
opposite direction. As a result, the desk
doesn’t move. This is an example of what
kind of forces?
Answer
The answer is A:
balanced forces. When
forces are balanced,
the net force is zero.
Question 2
What happens any time there are
unbalanced forces acting on an object—that
is, any time the net force is not zero?
Answer
Any time the net force is not zero, the object
moves.
Question 3
Explain Newton’s first law of motion.
Answer
Newton’s first law of motion says that when
the net force acting on an object is zero an
object already in motion will tend to stay in
motion and one at rest will tend to stay at
rest.
Question 4
Newton’s First Law is also referred to as the
Law of _________________.
Answer
Inertia; because inertia is an object’s
tendency to resist any change in motion. In
other words, an object at rest wants to stay
at rest and an object in motion wants to stay
in motion (unless acted upon by an
unbalanced force)
STUDENTS, BE PREPARED
FOR A 5-10 QUESTION POP
QUIZ ON NEWTON’S FIRST
LAW OF MOTION ON
MONDAY!