Transcript Force and Motion {PowerPoint}

The Force Is with You
TAKS Objective Four
TAKS Objective 4 – The student will
demonstrate an understanding of motion,
forces, and energy.
TEKS 7.6 B
• Demonstrate that an object will remain at
rest or move at a constant speed and in a
straight line if it is not being subjected to
an unbalanced force.
TEKS 8.7
The student knows that there is a relationship
between force and motion. The student is
expected to:
A. demonstrate how unbalanced forces
cause changes in the speed or direction
of an object's motion.
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.
Students will use
Hover Pucks® to produce near frictionless
motion.
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).
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.
ENGAGE
• Bill Ding
• Push Me, Pull Me
– Flying Saucer story
– Videos of cars jumping- name all forces acting
upon car
• Penny Drop
– Cart with Rocks
EXPLORE
• Dare You to Stop Me
– How did you make one puck go faster?
– How long does it take to travel 1m? 2m? 3m?
Are these different rates?
EXPLAIN
• How do objects behave when there is no
outside force?
• When the puck moves in a straight line,
what forces are acting on it?
• Why doesn’t it move in a perfectly straight
line?
Elaborate
• Knocked off Course
• Flag me Down
EVALUATION
1. How does an object behave when there are no
forces acting upon it?
2. When the puck was moving in activity one
what force(s) were acting upon it?
3. Why didn’t the puck move in a perfectly
straight line?
4. Why did the puck not move each equal
distance in exactly the same time?
5. Graph distance on y-axis, time on x-axis =
speed
6. Graph speed on y-axis, time on x-axis =
acceleration
Special thanks to the Physics Classroom
Website which was used to prepare this
lesson.
http://www.glenbrook.k12.il.us/GBSSCI/PHY
S/Class/newtlaws/u2l2d.html