Unformatted text preview: Newton’s Newton’s Laws of Motion
I. Law of Inertia
II. F=ma
III. ActionReaction While most people know what While most people know what Newton's laws say, many people do not know what they mean (or simply do not believe what they mean). Newton’s Laws of Motion
Newton’s Laws of Motion 1st Law – An object at rest will stay at rest, and an object in motion will stay in motion at constant velocity, unless acted upon by an unbalanced force. 2nd Law – Force equals mass times acceleration. 3rd Law – For every action there is an equal and opposite reaction. 1st Law of Motion (Law of Inertia) An object at rest will stay at rest, and an object in motion will stay in motion at constant velocity, unless acted upon by a force. 1 Law
st Inertia is the tendency of an object to resist changes in its velocity: whether in motion or motionless. These pumpkins will not move unless acted on
by an unbalanced force. 1 Law
st Once airborne,
Once
unless acted on
by an
unbalanced force
(gravity and air –
fluid friction), it
would never
stop! 1 Law
st Unless acted
Unless
upon by a force,
this golf ball
would sit on the
tee forever. Why then, do we observe every
day objects in motion slowing
down and becoming motionless
seemingly without an outside
force?
force?
It’s a force we sometimes cannot see –
It’s
friction.
friction. Objects on earth, unlike the
frictionless space the moon
travels through, are under the
influence of friction.
influence What is this unbalanced force that acts on an object in motion? There are four main types of friction: Sliding friction: ice skating
Sliding
Rolling friction: bowling
Rolling
Fluid friction (air or liquid): air or water resistance
Fluid
Static friction: initial friction when moving an
Static
object Slide a book Slide a book across a table and watch it slide to a rest position. The book comes to a rest because of the presence of a force that force being the force of friction which brings the book to a rest position. In the absence of a force of friction, the book
In
would continue in motion with the same speed
and direction  forever! (Or at least to the end
of the table top.) Newtons’s 1 Law and You
Newtons’s
st Don’t let this be you. Wear seat belts.
Because of inertia, objects (including you) resist changes in their motion. When the car going 80 km/hour is stopped by the brick wall, your body keeps moving at 80 m/hour. 2 Law
nd 2 Law
nd The net force of an object is
The
equal to the product of its mass
and acceleration, or F=ma.
and 2 Law
nd When mass is in kilograms and acceleration is
When
in m/s/s, the unit of force is in newtons (N).
in
One newton is equal to the force required to
One
accelerate one kilogram of mass at one
meter/second/second.
meter/second/second. If mass remains constant, doubling the acceleration, doubles the force. If force remains
constant, doubling the mass, halves the acceleration. nd Newton’s 2 Law proves that different masses accelerate to the earth at the same rate, but with different forces. • We know that objects with different masses accelerate to the ground at the same rate.
• However, because of the 2nd Law we know that they don’t hit the ground with the same force.
F = ma F = ma 98 N = 10 kg x 9.8 m/s/s 9.8 N = 1 kg x 9.8 m/s/
s Check Your Understanding
Check 1. What acceleration will result when a 12 N net force applied to a 3 kg
1.
object? A 6 kg object?
object? 2. A net force of 16 N causes a mass to accelerate at a rate of 5 m/s2.
Determine the mass.
Determine 3. How much force is needed to accelerate a 66 kg skier 1 m/sec/sec? 4. What is the force on a 1000 kg elevator that is falling freely at 9.8 m/sec/
sec? Check Your Understanding
Check 1. What acceleration will result when a 12 N net force applied to a 3 kg object?
1.
12 N = 3 kg x 4 m/s/s
12 2. A net force of 16 N causes a mass to accelerate at a rate of 5 m/s2. Determine the
Determine
mass.
mass.
16 N = 3.2 kg x 5 m/s/s
16 3. How much force is needed to accelerate a 66 kg skier 1 m/sec/sec?
66 kgm/sec/sec or 66 N 4. What is the force on a 1000 kg elevator that is falling freely at 9.8 m/sec/sec? 9800 kgm/sec/sec or 9800 N
9800 3 Law
rd For every action, there is an
For
equal and opposite reaction.
equal 3 Law
rd According to Newton,
According
whenever objects A and
B interact with each
other, they exert forces
upon each other. When
you sit in your chair,
your body exerts a
downward force on the
chair and the chair
exerts an upward force
on your body. 3 Law
rd There are two forces
There
resulting from this
interaction  a force on
the chair and a force on
your body. These two
forces are called action
action
and reaction forces.
reaction Newton’s 3rd Law in Nature
Newton’s 3rd Law in Nature Consider the propulsion of a
Consider
fish through the water. A
fish uses its fins to push
water backwards. In turn,
the water reacts by pushing
reacts
the fish forwards, propelling
the fish through the water.
the
The size of the force on the
water equals the size of the
force on the fish; the
direction of the force on the
water (backwards) is
opposite the direction of the
force on the fish (forwards).
force 3 Law
rd Flying gracefully through the air, birds depend on Newton’s third law of motion. As the birds push down on the air with their wings, the air pushes their wings up and gives them lift. Consider the flying motion of birds. A bird flies by
Consider
use of its wings. The wings of a bird push air
downwards. In turn, the air reacts by pushing the bird
upwards.
The size of the force on the air equals the size of the
The
force on the bird; the direction of the force on the air
(downwards) is opposite the direction of the force on
the bird (upwards).
the
Actionreaction force pairs make it possible for birds
Actionreaction
to fly.
to Other examples of Newton’s Third Other examples of Newton’s Third Law The baseball forces the
The
bat to the left (an
action); the bat forces
the ball to the right (the
reaction). 3 Law
rd Consider the motion of
Consider
a car on the way to
school. A car is
equipped with wheels
which spin backwards.
As the wheels spin
backwards, they grip the
road and push the road
backwards.
backwards. 3 Law
rd The reaction of a rocket is an application of the third law of motion. Various fuels are burned in the engine, producing hot gases. The hot gases push against the inside tube of the rocket and escape out the bottom of the tube. As the gases move downward, the rocket moves in the opposite direction. ...
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This note was uploaded on 09/09/2011 for the course PHYS 6a taught by Professor Stanek during the Spring '07 term at UCSB.
 Spring '07
 STANEK
 Inertia, Third Law Of Motion

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