Transcript Newtons Laws of Motion - Winston Churchill High School

Physics C-5
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Objects at rest tend to stay at rest.
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Objects in motion tend to stay in motion.
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Also called the Law of Inertia.
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Force is an action that can change motion.
◦ A force is a push or a pull.
 Change is not required, just the ability. (push on
car)
◦ Forces can be used to increase the speed,
decrease the speed, or change the direction in
which an object is moving.
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Inertia is a term used to measure the ability
of an object to resist a change in its state of
motion.
Large inertia = large force req. to move
Small inertia = small force req. to move
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Because inertia is a key idea in Newton’s first
law, the first law is sometimes referred to as
the law of inertia.
The word inertia comes from the Latin word
inertus, which can be translated to mean
“lazy.”
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Force is the cause of acceleration of an
object or a change in motion.
We typically imagine a force as a “push” or
“pull.
SI unit of Force: Newton (N)
◦ Newton = mass x acceleration
◦ 1N = 1kg • 1m/s2
External Force – a single force that acts on
an object as a result of an interaction
between the object and its environment.
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Contact Force – force that arises from the
physical contact of two objects.
◦ you push a cart to move it forward
◦ pull on a wagon
◦ Friction
◦ Air resistance
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Field Force – force that can exist between
objects, even in the absence of physical contact
between objects. (force at a distance)
◦ Gravity
◦ Attraction or repulsion between electrical or magnetic
charges
Force
Symbol Type/Definition
Direction
Friction
Ff
Contact force – acts
to oppose sliding
motion between
surfaces
Parallel to the
surface and opposite
the direction of
sliding
Normal
FN
Contact force –
exerted by a
surface on an
object
Perpendicular to and
away from the
surface
Spring
Fsp
Contact force –
Opposite the
push or pull a
displacement of the
spring exerts on an object at the end of
object
the spring
Air
Fair
Contact force –
Resistanc
e
frictional force which
acts upon objects as
Opposite to the force
of gravity.
* frequently neglected due to its
negligible magnitude (and due to
Force
Symbo Type/Definition
l
Tension FT
Contact force – pull
Thrust
Fthrust
Weight
Fg
Direction
Away from the object
exerted by a string,
and parallel to the
rope or cable when
string, rope, or cable
attached to a body and
at the point of
pulled taut.
attachment
Contact force – forces In the same direction
that move objects such as as the acceleration of
rockets, planes, cars, and the object barring
people
any resistive forces
Field Force – the
magnitude of the
force of gravity
acting on an object
Straight down toward
the center of the
Earth
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Can you explain why the long table would
make the trick hard to do?
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The engine
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The brake system
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The steering wheel and steering system
◦ supplies force that allows you to change motion
by pressing the gas pedal.
◦ is designed to help you change your motion by
slowing down.
◦ is designed to help you change your motion by
changing your direction.
• An object at rest remains at rest and an object in
motion continues in motion with constant velocity
unless it is acted upon by an unbalanced force.
• The tendency of an object to not accelerate or resist
change is inertia.
oInertia is directly proportional to an
object’s mass.
• greater mass = less acceleration
• smaller mass = more acceleration
• The unbalanced force is when there is an individual
force which is not being balanced by a force of
equal magnitude and in the opposite direction.
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Net External Force – the total force
resulting from a combination of external
forces on an object.
If the net external force on an object is
zero, then the acceleration (or change in
its motion) is zero and the object is in
equilibrium.
Equilibrium – the state of a body in which
there is no change in its motion.
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If you apply
more force to
an object, it
accelerates at
a higher rate.
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If the same force
is applied to an
object with
greater mass,
the object
accelerates at a
slower rate
because mass
adds inertia.
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The simplest concept of force is a push or a
pull.
On a deeper level, force is the action that has
the ability to create or change motion.
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In the English system,
the unit of force, the
pound, was originally
defined by gravity.
The metric definition
of force depends on
the acceleration per
unit of mass.
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A force of one newton is exactly the amount of
force needed to cause a mass of one kilogram
to accelerate at one m/s2.
We call the unit of force the newton (N).
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The force F that appears in the second law is
the net force.
There are often many forces acting on the
same object.
Acceleration results from the combined
action of all the forces that act on an object.
When used this way, the word net means
“total.”
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A force of one pound is equal to about 4.448
newtons.
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To solve problems with multiple forces, you
have to add up all the forces to get a single
net force before you can calculate any
resulting acceleration.
A cart rolls down a ramp. Using a
spring scale, you measure a net force
of 2 newtons pulling the car down. The
cart has a mass of 500 grams (0.5 kg).
Calculate the acceleration of the cart.
1.
2.
3.
4.
You are asked for the acceleration (a).
You are given mass (m) and force (F).
Newton’s second law applies: a = F ÷ m
Plug in numbers. (Remember: 1 N = 1 kg·m/s2)
Three forms of the second law:
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The word dynamics refers to problems
involving motion.
In dynamics problems, the second law is often
used to calculate the acceleration of an object
when you know the force and mass.
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Speed increases
when the net force
is in the same
direction as the
motion.
Speed decreases
when the net force
is in the opposite
direction as the
motion.
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We often use positive and negative numbers to
show the direction of force and acceleration.
A common choice is to make velocity, force,
and acceleration positive when they point to
the right.
Three people are pulling on a wagon
applying forces of 100 N, 150 N, and
200 N. Determine the acceleration and
the direction the wagon moves. The
wagon has a mass of 25 kilograms.
1.
You are asked for the acceleration (a) and direction
2.
You are given the forces (F) and mass (m).
3.
4.
The second law relates acceleration to force and mass: a = F
÷m
Assign positive and negative directions. Calculate the net
force then use the second law to determine the acceleration
from the net force and the mass.
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Wherever there is acceleration there must
also be force.
Any change in the motion of an object results
from acceleration.
Therefore, any change in motion must be
caused by force.
An airplane needs to accelerate at 5 m/sec2
to reach take-off speed before reaching the
end of the runway. The mass of the airplane
is 5,000 kilograms. How much force is
needed from the engine?
1.
2.
3.
4.
5.
You asked for the force (F).
You are given the mass (m) and acceleration (a).
The second law applies: a = F ÷ m
Substitute. (Remember: 1 N = 1 kg·m/s2.)
F = 5,000kg x 5 m/s2 = 25,000 N
A tennis ball contacts the racquet for much less than
one second. High-speed photographs show that the
speed of the ball changes from -30 to +30 m/sec in
0.006 seconds. If the mass of the ball is 0.2 kg, how
much force is applied by the racquet? (hint: Calc
acceleration first ---(a=vf-vo / t)
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Mass and weight are not the same thing.
Mass is measured in kg and is the same
everywhere in the universe.
Weight is a force which depends on gravity
(9.8 N/kg on earth)
F = ma or W = mg
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The condition of zero acceleration is called
equilibrium.
In equilibrium, all forces cancel out leaving
zero net force.
Objects that are standing still are in
equilibrium because their acceleration is
zero.
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Objects that are moving at
constant speed and direction are
also in equilibrium.
A statics problem usually means
there is no motion.
Suppose you are walking two
dogs which pull on the leash with
80 N each. How much force do
you have to use to keep the dogs
from moving?____________
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“For every action there is an
equal and opposite reaction.”
This statement is known as
Newton’s third law of motion.
Newton’s third law discusses
pairs of objects and the
interactions between them.
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The astronauts working on the space station have a
serious problem when they need to move around in
space: There is nothing to push on.
One solution is to throw something opposite the
direction you want to move.
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The two forces in a pair
are called action and
reaction.
Anytime you have one,
you also have the other.
If you know the strength
of one you also know the
strength of the other since
both forces are always
equal.
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Newton’s third law states
that for every action force
there has to be a reaction
force that is equal in
strength and opposite in
direction.
Action and reaction forces
act on different objects,
not on the same object.
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Newton’s third law states that for every
action force there has to be a reaction force
that is equal in strength and opposite in
direction.
Action and reaction forces act on different
objects, not on the same object.
The forces cannot cancel because they act on
different objects.
Three people are each applying 250 newtons of force to try to
move a heavy cart. The people are standing on a rug.
Someone nearby notices that the rug is slipping. How much
force must be applied to the rug to keep it from slipping?
Sketch the action and reaction forces acting between the
people and the cart and between the people and the rug.
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The act of moving or the ability to move from
one place to another is called locomotion.
Any animal or machine that moves depends on
Newton’s third law to get around.
When we walk, we push off the ground and
move forward because of the ground pushing
back on us in the opposite direction.
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Jets, planes, and
helicopters push air.
In a helicopter, the
blades of the propeller
are angled such that
when they spin, they
push the air molecules
down.