Transcript Newton’s First Law Chapter 4

Newton’s First Law
Chapter 4
Timeline
Note: Aristotle is not on this timeline. He was born in the 4th century BCE (384-322 BCE), about 2000 years
before Copernicus died
Aristotle (384-322 BCE)
• Aristotle's Physics was written in the fourth century BCE. For
more than two thousand years this book served as the basis
of natural philosophy up to the sixteenth century, the time of
Galileo.
• Aristotle had two types of motion: natural motion and violent
motion.
• Natural motion is motion arising from the nature of an object.
– This motion does NOT require an external cause to occur.
– All early object are composed of four elements: earth, water, air,
and/or fire
Motion of things is determined by their natural tendencies to move
towards their proper place (only straight up or down):
• Earthly things towards the center of the Earth.
• Things composed of air float up into the air.
•
Violent motion is contrary to the nature of the object.
– This motion does require a FORCE to cause motion.
• e.g. a stone thrown into air moves in a violent motion.
Aristotle on the Heavens
• Aristotle distinguished between the motions of
earthly and heavenly bodies
• In the heavens there was heavenly (celestial) motion.
• All objects in the heavens move in perfect circles
with the Earth at the center, motionless.
• Aristotle believed that the universe was Earth
centered (geocentric).
• Humanity believed Aristotle for 2000 years.
Copernicus (1473-1543)
• Nicolaus Copernicus was a Polish
astronomer, mathematician, and
physicists
• Major achievements include:
– He studied the heavens and data on the
motion of the heavens and came to the
conclusion that Aristotle’s geocentric
model was not correct
– He wrote the famous book De
revolutionibus about his heliocentric (sun
centered) model of the universe which was
not published until he was on his deathbed
because he feared persecution.
Galileo(1564 – 1642)
• Galileo Galilei was an Italian physicist,
mathematician, astronomer, and
philosopher
• Major achievements include:
– first systematic studies of uniformly accelerated
motion
– Through these studies he came up with his
Principle of Inertia:"A body moving on a level
surface will continue in the same direction at
constant speed unless disturbed."
– improvements to the telescope which allowed
him to observe Jupiter’s moon leading to him to
support the Copernican view of the universe
and disproved Aristotle’s view
Inertia and Mass
• The resistance of an object to acceleration is called
inertia OR Inertia is the resistance an object has to a
change in its state of motion.
• A more massive object is more difficult to accelerate
than a less massive object.
• Mass resists acceleration – the more mass, the more
resistance.
• Mass is the quantity of matter in an object and is
also the measure of the inertia of an object.
Newton
• English mathematician and scientist who
lived from 1643 to 1727
• Newton added physics interpretations to
the mathematical descriptions of
astronomy by Copernicus, Galileo and
Kepler
• Major achievements include:
– Invented calculus as a necessary tool to
solve mathematical problems related to
motion
– Discovered the three laws of motion
– Discovered the universal law of mutual
gravitation
Aristotle, Copernicus, Galileo and Newton Questions
1)
2)
3)
How would Aristotle explain a rock falling to Earth?
How would Aristotle explain a hot air balloon floating in the sky?
What is the difference between violent motion and natural
motion?
4) Is an arrow shot from a bow, natural or violent motion? Explain.
5) Make a list of those who believed in a geo centric view of the
universe and those who believed in a heliocentric model of the
universe.
6) Did Aristotle believe and object required a force to continue
moving? Did Galileo?
7) Who is credited with coming up with the concept of inertia?
8) Who is credited with inventing calculus?
9) What did Galileo observed that convinced him Copernicus was
correct?
10) What did Galileo’s famous experiment at the leaning tower of Pisa
prove?
Newton’s First Law – The Law of Inertia
• An object at rest tends to stay at rest and an
object in motion tends to stay in motion with
the same speed and in the same direction
unless acted upon by an unbalanced force.
Force
• A force is a push or pull acting upon an object as a
result of its interaction with another object.
• The unit for force is Newton, N.
• Forces may be placed into two broad categories
based on whether the force resulted from the
contact or non-contact of the two interacting objects
Contact Forces
Action-at –a distance Forces
Frictional Force
Gravitational Force
Tensional Force
Electrical Force
Normal Force
Magnetic Force
Air Resistance Force
Applied Force
Spring Force
Gravity, Normal Force, Friction and Tension
• Fgrav or W: The force of gravity is the force with which the earth,
moon, or other massive body attracts an object towards itself. By
definition, this is the weight of the object. All objects upon earth
experience a force of gravity which is directed "downward"
towards the center of the earth.
• Fnorm or FN:The normal force is the support force exerted upon an
object which is in contact with another stable object. The normal
force is always perpendicular (90 degrees) to the surface.
• Ffrict or f: The friction force is the force exerted by a surface as an
object moves across it or makes an effort to move across it. The
friction force opposes the motion of the object.
• Ftens or T: Tension is the force which is transmitted through a string,
rope, or wire when it is pulled tight by forces acting at each end.
The tensional force is directed along the wire and pulls equally on
the objects on either end of the wire.
Balanced and Unbalanced Forces
Vectors and Scalars
• In studying motion and forces in 2D it is helpful separate quantities into
those that depend on direction and those that do not
• Vectors are quantities which are fully described by both a magnitude and
a direction.
• Scalars are quantities which are fully described by a magnitude alone.
Quantity
Vector
Scalar
Distance
Displacement
Comments
X
refers to "how far out of place an object
is"
X
Speed
X
Velocity
X
Acceleration
X
X
Time
X
X
how fast something is moving
how fast and in what direction
Temperature
Force
Refers to "how much ground an object
has covered" during its motion.
Vectors
• Physical quantities that have both magnitude and direction are
conveniently represented by vectors.
• Vectors are represented graphically by arrows indicating their direction
and having lengths proportional to their magnitudes (size).
• All vectors have a tail and head.
• The resultant is the vector sum of two or more vectors.
5 m East
5 is the magnitude of this vector, East is the direction
10 m East
These vector is twice as long because
10 m West their magnitude is 10, instead of 5
Equivalent sets of vectors?
• Examine the following sets of vectors and
decide if they are equivalent or not. Justify
your answer in each case.
5 m East
a)
10 m East
10 m East
b)
10 m West
5 m East
c)
d)
5 m East
5 m East
5 m South
Vector Addition
• If vectors are in the same direction, simply add the numbers
to get the resultant
• If vectors are in the opposite direction, subtract to get
resultant
• If vectors are at a right angle, use Pythagorean theorem
Adding Velocity Vectors
1) Find the resultant velocity of an airplane flying at
100 km/hr with a 25 km/hr tail wind. 100 km/hr +
25 km/hr = 125 km/hr
2) Find the resultant velocity of an airplane flying at 100
km/hr with a 25 km/hr head wind. 100 km/hr - 25
km/hr = 75 km/hr
25 km/hr
100 km/hr
Resultant
75 km/hr
Adding Velocity Vectors
• The resulting flight path is a result of both velocity vectors and
can be found using a2 + b2 = c2 or a scale diagram.
• 1002 + 252 = c2 c = 103.1 km/hr
• http://www.physicsclassroom.com/mmedia/vectors/plane.ht
ml
RESULTANT
25 km/hr
crosswind
100 km/hr direction
Vectors Question
• Which angle of
attack will bring a
kayaker to the
other bank at a
point directly
opposite her entry
point?
Free Body Diagram
• Free-body diagrams are diagrams used to show the relative
magnitude and direction of all forces acting upon an object in
a given situation. A free-body diagram is a special example of
the vector diagrams discussed previously.
• It is important to remember that only forces, not motion are
included in free body diagrams and arrows should show size
and direction of force.
• Examples of free-body diagrams:
An object resting
on a table OR an
object sliding
across a
frictionless
surface
An object
hanging
from two
ropes
An object
in free fall
An object
falling with
air
resistance
An object accelerating
across a table where
friction is smaller than
the applied force –
forces are unbalanced
An object moving at
constant velocity
across a table where
friction is equal to the
applied force –forces
are balanced
Newton’s First Law in Action
• If a car were to abruptly stop and the seat belts were
not being worn, then the passengers in motion would
continue in motion.
http://www.physicsclassroom.com/mmedia/newtlaws
/cci.html
• If a truck were to abruptly stop and the straps holding
the ladder were no longer functioning, then the
ladder in motion would continue in motion.
http://www.physicsclassroom.com/mmedia/newtlaws
/il.html
Can you think of a few more examples which further
illustrate applications of Newton's first law?
• blood rushes from your head to your feet when riding on a
descending elevator which suddenly stops.
• the head of a hammer can be tightened onto the wooden
handle by banging the bottom of the handle against a hard
surface.
• to dislodge ketchup from the bottom of a ketchup bottle, the
bottle is often turned upside down, thrust downward at a high
speed and then abruptly halted.
• headrests are placed in cars to prevent whiplash injuries
during rear-end collisions.
• while riding a skateboard (or wagon or bicycle), you fly
forward off the board when hitting a curb, a rock or another
object which abruptly halts the motion of the skateboard.
Newton’s First Law Questions
1)
2)
3)
4)
5)
Imagine a place in the cosmos far from all gravitational and frictional
influences. Suppose an astronaut in that place throws a rock. Describe
what happens to the rock.
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 with the same speed and
in the same direction?
Mac and Tosh are arguing in the cafeteria. Mac says that if he throws his
jello with a greater speed it will have a greater inertia. Tosh argues that
inertia does not depend upon speed, but rather upon mass. With whom
do you agree? Why?
If you were in a weightless environment in space, would it require a
force to set an object in motion?
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.
Sources
• Conceptual Physics by Paul Hewitt
• www.physicsclassroom.com