Upload
geona
View
42
Download
2
Embed Size (px)
DESCRIPTION
Newton’s Laws 2nd. acceleration of an object is directly proportional to the net force on it and inversely proportional to its mass (defines the newton). Newton’s second law of Motion. F=ma Force = mass x acceleration Mass = kilogram Acceleration = m/s2 Force = 1 N (Newton) - PowerPoint PPT Presentation
Citation preview
Newton’s Laws 2nd
• acceleration of an object
is directly proportional
to the net force on it and
inversely proportional to
its mass
• (defines the newton)
Newton’s second law of Motion
• F=ma
• Force = mass x acceleration
• Mass = kilogram
• Acceleration = m/s2
• Force = 1 N (Newton)
• Force = 1 kg-m/s2
Newton’s second law of Motion
• A larger force acting on an object causes a greater acceleration
• A larger mass requires a greater force than a smaller mass would require to achieve the same acceleration.
• Hitting a ball harder causes a greater acceleration
• If you hit a ping pong ball and a tennis ball with the same force, would they have the same acceleration?
The greater the mass, the greater the force must be for a given
acceleration.• the amount of
acceleration depends not only on the force but on the mass being pushed.
•For a given force, the acceleration produced is inversely proportional to the mass.
Apr 20, 2023 Physics 1 (Garcia) SJSU
More Mass, Less Acceleration
The greater the mass of an object, the less it accelerates when acted on by a force.
Newton’s second law of Motion
• A net force acting on an object causes the object to accelerate in the direction of the force.
• Acceleration is determined by the size of the force and the mass of the object.
• F= ma
Apr 20, 2023 Physics 1 (Garcia) SJSU
Mass & Acceleration
For a given force, greater mass, smaller the acceleration
Double the Mass
Triple the Mass
Half the Mass
impliesHalf the Acceleration
Third of the Acceleration
Double the Acceleration
implies
implies
Acceleration goes as
the inverse of mass. Mathematically, we write,
Acceleration ~ 1/(Mass).
Apr 20, 2023 Physics 1 (Garcia) SJSU
Demo: Hammer Head
Hammer a nail into a piece of wood placed on top of massive object (gold brick or huge book).
Inertia of massive object keeps it from moving; can place on top of head and not feel it.
Force is a scalar property
depends on 2 things;
Size or strength of the
force
and the
Direction of the force.
• Vector.
• A quantity with size
and direction
• Scalars
• are described by
size only
Force• You can measure force with
a Spring scale.
Mass and Weight should not be confused with...
• Volume• the quantity of space an
object occupies
• Density • the quantity mass per
unit volume
• Mass• the quantity of matter
in an object
• the measurement of the inertia
• measured in kilograms (kg)
Apr 20, 2023 Physics 1 (Garcia) SJSU
Mass: Quantity of matter in an objectWeight: Force of gravity on an object
Mass is a universal property.Weight depends on gravity
(different on Moon).
Mass is the measure of inertia.Metric unit for mass is the kilogram.Metric unit for weight is Newton (since it’s a force)
Mass & Weight
Earth Moon
Weight
Mass Vs. Weightmass vs. force
Mass Weight
• is a measure
of the force of
the Earth’s
gravity on the
mass of the
object.
is a measure of
the total amount
of matter
contained within
an object.
In metric units, the unit of force is the newton,
• . A 1-kilogram brick weighs about 10 newtons (more precisely, 9.8 N).
• The books offer the same resistance to speeding up or slowing down regardless of whether it’s on Earth, on the Moon, or on any other body attracting it.
• You’d have to provide the same amount of force to accelerate a huge truck to a given speed on a level surface on the Moon as on Earth.
An anvil in outer may be weightless, but it is not
massless.• The astronaut in space finds
that it is just as difficult to shake the “weightless” anvil as it would be on Earth.
• If the anvil were more massive than the astronaut, which would shake more—the anvil or the astronaut?
Newton’s 2nd Law of Motion
• F=ma
• N = Kg*m/s2
• Newton was the first to discover the
relationship between—acceleration,
force, and mass.
• It states; an object is directly proportional
to the net force acting on the object, is in
the direction of the net force, and is
inversely proportional to the mass of the
object.
NEWTON'S 2nd LAW OF MOTION
Fa
or amF
F am
F am
m
F a
m
m
m
F a
F a
F aM
m
a1
M
M
Acceleration is directly
proportional to force.
Apr 20, 2023 Physics 1 (Garcia) SJSU
Demo: Elevator Cable
Tension in elevator cable depends on acceleration
10 N
1 kg
Zero acceleration
15 N
1 kg
5 m/s2 upward( ½ g upward)
5 N
1 kg
0 N
1 kg
5 m/s2 downward( ½ g downward)
10 m/s2 downward(Free fall)
WeightWeight• the force upon an object due to gravity
• Weight = Mass Acceleration of gravity
W = mg
• measured in Newtons (N) in the metric system or pounds (lb) in the British system
Mass and Weight
• On the Moon the gravitational force is only 1/6 as strong as on the Earth.
• In space you are “weightless” but not “massless”.
• Your mass does not depend on where your are.
• (e.g. Earth, Moon, or space).
WeightLocation Mass
Earth
Moon
Space
18.4 kg
18.4 kg
18.4 kg
180 N
30 N
0 N
http://observe.phy.sfasu.edu/images/KC135-Summer2001/
• The weight of a 10 kg brick is...• A) 98 N • B) 10 kg • C) 9.8 kg• D) 10 N • E) 98 kg
Newton’s Cradle
How much force is need to accelerate a 70 kg
rider and her 200 kg motorcycle at 4 m/s2?
• Mass of rider = 70 kg
• Mass of motorcycle =200 kg• Acceleration = 4 m/s2
• Force = unknown
• Equation F=ma
Example QuestionsExample Questions• How much acceleration does a 747
jumbo jet of mass 30,000kg experience in takeoff when the thrust of all of the engines is 120,000N?
• A) 747 N • B) 4 kg • C) 1/4 kg• D) 4 m/s2 • E) 30,000 kg times 9.8 m/s2
Example QuestionsExample Questions
• The same net force is applied to two blocks.
• If the blue one has a smaller mass than the yellow one, which one will have the larger acceleration?
• A) Blue• B) Yellow
F F
Example QuestionsExample Questions
• The same net force is applied to two blocks.
• If the blue one has a smaller mass than the yellow one, which one will have the larger acceleration?
F F
Inertia
• Friction increases with greater force
• Friction is in the opposite direction of the force applied
• Doesn’t want to change what it is doing
• Momentum
Frictionreduces net force & resulting
acceleration
Friction• Force that opposes the motion
between 2 surfaces that are in contact
Friction
• Depends on
• kinds of surfaces • Force pressing
surfaces together
• Life without Friction
• Couldn’t stand• Clothes wouldn’t stay
on
Friction
• is a result of irregularities in the surfaces of
objects.
• The force required to overcome friction is
called the static friction force.
• The force needed to keep a constant speed is
called the kinetic friction force.
Apr 20, 2023 Physics 1 (Garcia) SJSU
FrictionOrigin of friction is molecular interaction between
solid surfaces.
Friction is complicated.
Friction depends on support force and on properties of the surface.
Basic properties of friction first established by Leonardo da Vinci.
Friction depends on 3 things:• The friction force depends on whether or not the
surfaces are moving.
• The friction force depends on the materials of which
the surfaces are made of.
• The friction force depends on how hard the
surfaces are pressed together. This is called the
Normal Force and depends on mass and gravity
also known as Weight
Imagine your car broke down and you have to push it.
Which takes more force, to get it started rolling or to keep it
rolling?
To get it started
Static friction is greater than kinetic friction.
How is a car affected by friction?
Types of Friction
Static
Sliding
Rolling
fluid
sliding friction is less than static friction
• Once tires start to
slide, the frictional
force is reduced and
off you go
• While the tire is
rolling, its surface
does not slide -it is
static friction—and
therefore greater
than sliding friction.
static friction and sliding friction
• It is very important that you not jam on the brakes in an emergency stop. (the tires lock in place), sliding, providing less friction than if they are made to roll to a stop.
Fluid Friction
•Occurs when moving through Air, water & oil
•includes falling through air
Fluid friction is called drag.
• Friction occurs also in liquids and gases, collectively called fluids (because they flow)
• drag in a fluid depends on the nature of the fluid
• drag does depend on speed and area of contact.
the amount of fluid pushed aside by a boat or airplane depends on the size and the shape of
the craft.
• A slow-moving boat or airplane encounters less drag than faster boats or airplanes.
• wide boats and airplanes must push aside more fluid than narrow crafts
• For slow motion
through water, drag
is proportional to
the speed of the
object.
Air Drag
• In air, drag at most speeds is proportional to the square of the speed
• So if an airplane doubles its speed it encounters four times as much drag
• At very high speed,
however, the simple
rules break down when
the fluid flow becomes
erratic and such things
as vortices and shock
waves develop.
• Friction between the tire and the ground is nearly the same whether the tire is wide or narrow.
• The purpose of the greater contact area is to reduce heating and wear.
Check Yourself
• 1. What net force does a sliding crate experience when you exert a force of 110 N and friction between the crate and the floor is 100 N?
• 2. A jumbo jet cruises at constant velocity of 1000 km/h when the thrusting force of its engines is a constant 100,000 N. What is the acceleration of the jet? What is the force of air resistance on the jet?
Lubricants
• reduce friction by keeping the two sliding
surfaces apart with a thin layer of fluid.
• Friction is no longer rubbing the surfaces
but instead rubbing the lubricant.
Another way to reduce friction is to roll an object over a surface.
• This is called rolling
frictional force instead of
kinetic frictional force.
Apr 20, 2023 Physics 1 (Garcia) SJSU
Friction and Air Resistance
Friction and air resistance are forces opposing motion.
Sliding friction
• Force between surfaces in relative motion
• less that static friction
Friction
• The force of friction does not depend on speed.
• Once an object is sliding, the friction force remains approximately the same.
• Unlike the friction between solid surfaces, fluid friction depends on speed.
• A very common form of fluid ?(liquid or gas) friction for something moving through air is air resistance, also called air drag.
friction does not depend on the area of contact.
• If you slide a smaller
surface, all you do is
concentrate the same
weight on a smaller area
with the result that the
friction is the same.
• Stopping distance when
brakes are applied is not
affected by the number of
tires
the force of friction does not depend on speed
• It may be more when
the crate is at rest and
on the verge of
sliding
• Once sliding the
friction force remains
approximately the
same.
friction does not depend on the area of contact
• Slide the crate on its
smallest surface and
all you do is
concentrate the same
weight on a smaller
area with the result
that the friction is the
same
• the friction between a truck and the ground is the same
• # of tires do not matter
• (More tires spreads the load and reduces the pressure and wear per tire).
• stopping distance is not affected by the number of tires
Coefficients of Friction Equations
• Static force=Coefficient of static x Normal
Force (Weight)
• Fs = us x N
• Kinetic Force=Coefficient of kinetic x Normal Force
(Weight)
• Fk = uk x N
A crate made of wood weighs 85 lbs is on a concrete floor.
• What force is required to get it moving?
• Fs = us x N
• us ; Page 176
• us = .6
• Fs = .6 x 85 = 51
Same crate is now moving, what force is needed to keep it moving at a constant speed?
• Fk = uk x N
• uk ; Page 176
• uk = .4
• Fk = .4 x 85 = 34
Three identical blocks are pulled, as shown, on a
horizontal frictionless surface. If tension in the rope held by the hand is 30 N, what is the tension in the other ropes?
• Use F = ma to the whole system• if the mass of a block is "m"
30 = 3mathereforea = 30/3m=10/m
• The tension on the rope between Block 3 and 2 is "T 1“
•using F = ma
• T1 = m *10/mT1 = 10 N
• if the tension between block 2 and 1 is T2
then using F= ma for both the block 3 and 2T2 = 2m * 10/mT2 = 20 N
• If forces are equal but opposite, they are balanced.
• If one force is greater than the opposite force, than it is an unbalanced force.
What happens when forces aren’t balanced—when net forces do not equal zero.
(Mechanical equilibrium, ΣF = 0, where forces are balanced.)
•The net force on a kicked soccer ball, for example, is greater than zero, and the motion of the ball changes abruptly.
•Its path through the air is not a straight line but curves downward due to gravity—again, a change in motion
the combination of forces acting on an object is the net force
• To increase the acceleration of an object, you must increase the net force acting on it.
• Doubling the force doubles the acceleration• The object’s acceleration is directly
proportional to the net force acting on it.• We write acceleration net force ( directly ∼
proportional to) • It means any change in one is the same
amount of change in the other.
Volume The quantity of space an object occupies.
Newton’s second law The acceleration of an object is directly proportional to the net force acting on the object, is in the direction of the net
force, and is inversely proportional to the mass of the object.
Force Any influence that can cause an object to be accelerated, measured in newtons (or in pounds, in the British system).
Free fall Motion under the influence of gravitational pull only.
Terminal speed The speed at which the acceleration of a falling object
terminates because air resistance balances its weight. When direction is specified, then we speak of terminal velocity.
• A large rocks resistance to
a change in motion (its
mass) is 100 times that of
the pebble. The greater
force offsets the equally
greater mass.
The acceleration of free fall is independent of an object’s mass
• The ratio of weight (F) to mass (m) is the same for
all objects in the same locality; so, their
accelerations are the same in the absence of air
resistance.
The ratio of weight (F) to mass (m) is the same for
the large rock and the small feather; similarly, the
ratio of circumference (C) to diameter (D) is the
same for the large and the small circle.
When Acceleration Is g—Free Fall
• The greater the mass of an object, the
greater the gravitational force of
attraction between it and the Earth.
• the acceleration of an object depends
not only on the force—in this case, the
weight—but also on the object’s
resistance to motion, its inertia.
• inertia is a resistance to acceleration.
• use the symbol g, rather a, to denote
that acceleration is due to gravity alone.
Apr 20, 2023 Physics 1 (Garcia) SJSU
Free Fall Acceleration
Newton’s Second Law explains why all objects fall with same acceleration.
Ratio of weight/mass always the same since weight depends on mass.
Analogy with ratio of circumference / diameter always equals pi (3.1415…).
• A 65 kg person stands on a scale, what is the force of gravity on the person?
• FG = m x g
• g = 9.8 m/s2
• FG = 65 kg x 9.8 m/s2
• FG = 637 N
Wind tunnel
• http://www.grc.nasa.gov/WWW/K-12/airplane/wrong1.html