Transcript File

THE FORCE IS WITH
YOU
LEARNING OBJECTIVES
The learner will apply the laws of motion to
real world examples.
The learner will identify size and direction of
a force.
The learner will determine if motion is
constant or accelerated.
The learner will determine if a force is
balanced or unbalanced.
LEARNING OBJECTIVES
The learner will use equipment to measure time and
distance so that the motion of the object can be
determined.
The learner will use data collected to calculate the
speed of an object.
The learner will explain the results of applying a
force to an object.
MISCONCEPTIONS
True or False?
A force is necessary to keep
object moving
Answer: False
Newton’s First Law of Motion states:
Objects in motion stay in motion in a straight
line unless acted upon by an outside force.
Outside forces such as friction, gravity,
magnetic force, electrical, force, and air
resistance slow/stop objects
True or False
If an object is at rest,
no forces are acting upon it
Answer: False
There are many forces acting on it
(gravity, friction, air pressure, etc)
but they are all BALANCED
VOCABULARY
Motion - change in position
Speed - rate of motion = distance / time
Direction of Motion - where an object is going.
Draw a straight line to represent where object
was in the past and in the present.
VOCABULARY
Force – a push or pull on an object
Equilibrium – a condition where all forces are balanced
Friction- force when two surfaces touch. It is always in the opposite direction of motion
(Newton’s 3rd Law)
MORE VOCABULARY
Velocity – the speed and direction of an
object
Acceleration – a change in either the
speed or the direction of an object
(speed up or SLOW DOWN)
Work – force acting upon an object
multiplied by the distance the object
moves
NEWTON’S 1ST LAW
An object in motion stays in motion
in a straight line, unless acted
upon by unbalanced force. A push
or pull will cause object to speed
up, slow down, or change
direction.
FORCES ARE BALANCED
Object at Rest
V = zero m/s
a = 0 m/s2
Stay at Rest
Objects in Motion
V ≠ zero m/s
a = 0 m/s2
Stay in Motion
(same
speed and
direction
Basically, objects just keep on doing
whatever they are doing unless they are
acted upon by an unbalanced force.
COMMON EXAMPLES
Ketchup stays in the bottom (at rest) until you
bang (outside force) on the end of the bottom.
A headrest in a car prevents whiplash injuries
during a rear-end collision ( your head goes
forward and then jerks backward).
Animation 1 – ladder truck
Animation 2 – no seatbelt
INERTIA AND MASS
The natural tendency of any object is to resist
changing state of motion. This is called inertia.
For example, if an object is moving, it likes to
keep on moving. If an object is stationary, it likes
to remain stationary. It takes some measure of
force to change this tendency.
Are some objects capable of resisting change better
than other objects?
YES, the more mass an object
has, the greater its ability will
be to resist change. For
example, a big guy will be
harder to push over than a
small guy.
Check
out my
MASS!
I have a
LOT of
INERTIA!
QUESTION 1
Imagine a place in the cosmos far from all
gravitational and frictional influences.
Suppose that an astronaut in that place
throws a rock. The rock will:
a. gradually stop.
b. continue in motion in the same direction at
constant speed.
ANSWER
b. continue in motion in the same
direction at constant speed.
QUESTION 2
An 2-kg object is moving horizontally
with a speed of 4 m/s. How much net
force is required to keep the object
moving at this speed and in this
direction?
ANSWER
Nothing, zero, nada…Zero Newtons.
An object in motion will maintain its state
of motion. The presence of an
unbalanced force changes the velocity of
the object.
QUESTION 3
Mac and Tosh are arguing in the cafeteria. Mac
says that if he flings the Jell-O with a greater
speed, then it will have a greater inertia. Tosh
argues that inertia does not depend upon
speed, but rather upon mass. Who do you
agree with? Explain why.
ANSWER
Tosh is correct. Inertia is that quantity which
depends solely upon mass. The more mass, the
more inertia. Momentum is another quantity in
Physics which depends on both mass AND speed.
Momentum (p) is expressed as:
mass X velocity
QUESTION 4
Suppose you were in space in a
weightless environment. Would
it require a force to set an object
in motion?
ANSWER
Absolutely yes!
Even in space, objects have mass; and if they
have mass, they have inertia. That is, an
object in space resists changes in its state of
motion. A force must be applied to set a
stationary object in motion. Newton's laws
rule - everywhere!
QUESTION 5
Mr. Wegley spends most Sunday afternoons
at rest on the sofa, watching pro football
games and consuming large quantities of
food. What effect (if any) does this practice
have upon his inertia? Explain.
ANSWER
Mr. Wegley's inertia will increase!
Mr. Wegley will increase his mass if
he makes a habit of eating and
lying on the sofa; and if his mass
increases, then his inertia increases.
QUESTION 6
Ben Tooclose is being chased through the woods
by a bull moose which he was attempting to
photograph. The enormous mass of the bull
moose is extremely intimidating. Yet, if Ben
makes a zigzag pattern through the woods, he
will be able to use the large mass of the moose
to his own advantage. Explain this in terms of
inertia and Newton's first law of motion.
ANSWER
The large mass of the bull moose means
that the bull moose has a large inertia.
Thus, Ben can more easily change his
own state of motion (make quick
changes in direction) while the moose
has extreme difficulty changing its state
of motion. Physics for better living!
BALANCED FORCES
We call this NORMAL FORCE. The book
is said to be at equilibrium since there
are no unbalanced forces acting upon
the book and it maintains its state of
motion (it still sits on the table).
• Balanced forces do not cause
change in motion
• They are equal in size and
opposite in direction
Lets say you are
standing on the
ground. Since the
floor pushes
upward on you
and gravity is
pulling down on
you, you are at
equilibrium.
LET’S SAY YOU GIVE THAT BOOK
ON THE TABLE A PUSH…
The book goes temporarily into motion, but friction from the table and gravity doesn’t allow it
to go far.
Unbalanced Forces
• An unbalanced force always causes a change
in motion
• When unbalanced forces act in opposite
directions you can find the net force
– Net force
• Magnitude
– The difference between the two forces
• Direction
– Direction of the largest force
3 N, right – 6 N, left = 3N, left
Unbalanced Forces
4 N, left – 10 N, right = 6N, right
Unbalanced Forces
5 N, right + 10 N, right = 15N, right