Transcript Force and Motion - mrhsluniewskiscience

Chapter
4
Forces in One Dimension
In this chapter you will:
Use Newton’s laws to solve
problems.
Determine the magnitude and
direction of the net force
that causes a change in an
object’s motion.
Classify forces according to the
agents that cause them.
*VD Note
Chapter
4
Table of Contents
Chapter 4: Forces in One Dimension
Section 4.1: Force and Motion
Section 4.2: Using Newton's Laws
Section 4.3: Interaction Forces
Section
Force and Motion
4.1
In this section you will:
•
Define force.
•
Apply Newton’s second law to solve problems.
•
Explain the meaning of Newton’s first law.
Newton’s Contributions
• Calculus
• Light is composed of
rainbow colors
• Reflecting Telescope
• Laws of Motion
• Theory of Gravitation
Force
A force is a push or pull. An
object at rest needs a force
to get it moving; a moving
object needs a force to
change its velocity.
The magnitude of a
force can be measured
using a spring scale.
Section
4.1
Force and Motion
Force and Motion
Consider a textbook resting on a table. How can you cause it to
move?
Two possibilities are that you can push on it or you can pull on
it. The push or pull is a force that you exert on the textbook.
If you push harder on an object, you have a greater effect on its
motion.
The direction in which force is exerted also matters. If you push
the book to the right, the book moves towards right.
The symbol F is a vector and represents the size and direction
of a force, while F represents only the magnitude.
Section
4.1
Force and Motion
Force and Motion
When considering how a
force affects motion, it is
important to identify the object
of interest. This object is
called the system.
Everything around the object
that exerts forces on it is
called the external world.
Section
4.1
Force and Motion
Contact Forces and Field Forces
Think about the different ways
in which you could move a
textbook.
You could touch it directly and
push or pull it, or you could tie
a string around it and pull on
the string. These are examples
of contact forces.
A contact force exists when an
object from the external world
touches a system and thereby
exerts a force on it.
Section
4.1
Force and Motion
Contact Forces and Field Forces
If you drop a book, the gravitational force of Earth causes the
book to accelerate, whether or not Earth is actually touching it.
This is an example of a field force.
Field forces are exerted without contact.
Forces result from interactions; thus, each force has a specific
and identifiable cause called the agent.
Without both an agent and a system, a force does not exist.
A physical model which represents the forces acting on a
system, is called a free-body diagram.
Section
4.1
Force and Motion
Contact Forces and Field Forces
Click image to view movie.
Section
4.1
Force and Motion
Combining Forces
When the force vectors are in the same direction, they can be
replaced by a vector with a length equal to their combined
length.
If the forces are in opposite directions, the resulting vector is the
length of the difference between the two vectors, in the direction
of the greater force.
Vector sum of all the forces on an object is net force.
Demo – 1st law
Section
Force and Motion
4.1
Newton’s First Law
•
What is the motion of an object with no net force acting on it? A
stationary object with no net force acting on it will stay at its
position.
•
Galileo did many experiments, and he concluded that in the
ideal case of zero resistance, horizontal motion would never
stop.
•
Galileo was the first to recognize that the general principles of
motion could be found by extrapolating experimental results to
the ideal case, in which there is no resistance to slow down an
object’s motion.
Section
Force and Motion
4.1
Newton’s First Law
•
In the absence of a net force, the motion (or lack of motion) of
both the moving object and the stationary object continues as it
was. Newton recognized this and generalized Galileo’s results
in a single statement.
•
This statement, “an object that is at rest will remain at rest, and
an object that is moving will continue to move in a straight line
with constant speed, if and only if the net force acting on that
object is zero,” is called Newton’s first law.
Section
4.1
Force and Motion
Newton’s First Law
Newton’s first law is sometimes called the law of inertia.
Inertia is the tendency of an object to resist change.
If an object is at rest, it tends to remain at rest.
If it is moving at a constant velocity, it tends to continue moving
at that velocity.
Forces are results of interactions between two objects; they are
not properties of single objects, so inertia cannot be a force.
Section
4.1
Force and Motion
Newton’s First Law
If the net force on an object is zero, then the object is in
equilibrium.
An object is in equilibrium if it is at rest or if it is moving at a constant
velocity.
Newton’s first law identifies a net force as something that disturbs
the state of equilibrium.
Thus, if there is no net force acting on the object, then the object
does not experience a change in speed or direction and is in
equilibrium.
Forces may be balanced or
unbalanced
• Balanced forces – all forces acting on an
object are equal
– There is NO MOTION
• Unbalanced forces – one or more forces
acting on an object are stronger than
others
– There is MOTION
• A NET FORCE
Balanced Force
•Equal forces in opposite
•directions produce no motion
Unbalanced Forces
•Unequal opposing forces
•produce an unbalanced force
•causing motion
If objects in motion tend to stay in motion,
why don’t moving objects keep moving
forever?
•Things don’t keep moving forever because
there’s almost always an unbalanced force
acting upon them.
•A book sliding across a table slows
down and stops because of the force
of friction.
•If you throw a ball upwards it will
eventually slow down and fall
because of the force of gravity.
Section
Force and Motion
4.1
Newton’s First Law
•
Some of the common types
of forces are displayed on the
right.
•
When analyzing forces and
motion, it is important to keep
in mind that the world is
dominated by resistance.
Newton’s ideal, resistancefree world is not easy to
visualize.
Mass & Inertia
Which vehicle has
more inertia?
• Mass is the amount of
matter in an object.
• The more MASS an
object has, the more
INERTIA the object
has.
• Bigger objects are
harder to start & stop
Slide from www.science-
Newton’s 1st Law
(also known as the law of inertia)
• A moving object moves in a straight line with
constant speed unless a force acts on it.
• The tendency of an object at rest to remain
at rest and an object in motion to remain in
motion unless acted upon by an unbalanced
force.
• Objects do not change their motion unless
a force acts on them
•The truck is in motion. What is the
force that causes it to stop?
•The push of the stopped car.
•The car is at rest. What is the force
that causes it to move?
•The push of the truck.
•Slide from www.science-
Newton’s First Law
(law of inertia)
• INERTIA is a property of an
object
that
describes
how
•much it will resist change to
______________________ the
motion of the object
•mass
•inertia
• more _____ means more ____
1st Law
• Unless acted
upon by an
unbalanced
force, this golf
ball would sit
on the tee
forever.
Objectives
• Explain what is meant by the term net
force.
• Apply the Law of Inertia to explain
physical phenomena.
• Investigate in order to determine the
relationship between weight and
mass.
• Define each of the following terms:
mass, inertia, weight and distinguish
between mass and weight.
What is this unbalanced force that acts on an
object in motion?
• There are four main types of friction:
– Sliding friction: ice skating
– Rolling friction: bowling
– Fluid friction (air or liquid): air or water
resistance
– Static friction: initial friction when moving an
object
1st Law
• Once airborne,
unless acted
on by an
unbalanced
force (gravity
and air – fluid
friction) it
would never
stop!
Inertia
Newton’s First Law
When the motorcycle stops, the rider continues his motion.
Terminal Velocity
Exploration
• Mass and Weight Lab (No lab write up.)
Elaboration Computer Activity
for Newton’s First Law
• http://www.physicsclassroom.com/Class/n
ewtlaws/u2l1a.cfm
Closure
• When the pellet fired into the spiral tube
emerges, which path will it follow?
(neglect gravity)
• Kahoot
Answer
• While in the tube, it is forced to curve, but
when it gets outside, no force is exerted
on the pellet and (law of inertia) it follows a
straight – line path…B!
Inertia Evaluation
• Carpenters use different hammers for
different jobs. A tack hammer is a light
hammer used for small nails in delicate
situations. A regular hammer is heavier
and is used for larger nails in building
walls. Compare the two hammers using
Newton’s 1st law in terms of the
hammers’ function, advantages and
disadvantages.