Download Newtons Laws

Motion, Forces, and Inertia
Newton’s Laws
What is a force?
A force is a push or a pull.
It can change the movement of an object.
“A push or a pull can make things move.
A push or a pull can stop things, too.”
Force is measured in SI units of Newton's (N).
The magnitude (size) of a force can be
measured using a “spring scale”.
The English unit for force is pounds (lbs).
Force
1 Newton is the force required to
accelerate 1 kilogram at 1 meter/sec2
Hence: 1 N = 1 kg m/s2
Math attack!
(Just a little
one)
 this is a “derived unit”
The English unit of force is the Pound (lb)
1 lb = 4.45 Newtons
A man whose weight is 165 lbs also weighs 734 N
165 lb x
4.45 N
1 lb
=
734N
Net force
Net force = the combined forces acting on
an object.
Not all forces are obvious. For example, the
floor is pushing against your feet and air is
pushing against your body.
This is because the force is balanced.
Balanced Forces
Forces that are equal in size and opposite in
direction are balanced forces.
No movement occurs if a force is balanced.
An object with balanced forces is in
equilibrium.
For balanced forces Fnet= 0
The normal force is the support
force exerted upon an object that is
in contact with another stable
object.
Unbalanced Forces
If the two forces are not equal, the forces are
unbalanced.
This results in a movement in one direction.
Objects don't accelerate
(speed up, slow down, or change direction) unless
an unbalanced force acts.
Important!: The opposite is also true! If any object
has constant velocity, then Fnet must = 0 (The
object won’t stop unless a force acts against it).
Q: A book is sliding across a table:
Why does it not “move forever at a
constant speed in a straight line”?
Static Equilibrium
A textbook has two forces on it: gravity (FG)
and the support force of the table (Fnormal).
They balance, and the book does not move.
Dynamic Equilibrium
A skydiver has two forces - gravity (his weight) and air
resistance. When they balance, he moves down with
constant velocity (no acceleration).
air resistance
What is Inertia?
The tendency of an object in motion to
remain in motion (and an object at rest to
stay at rest).
Why isn’t the coin moving with the cardboard?
You’re riding in a train,
standing up. The train
comes to a sudden stop?
What happens to you?
Objects in motion tend to stay in
motion!
Inertia is Latin for “LAZINESS”.
Isaac Newton
Sir Isaac Newton (1623-1727) - developed
the laws of motion.
Born in Leicestershire, England.
Isaac Newton published his
masterpiece Philosophiae
Naturalis Principia Mathematica
in 1687.
Knighted in 1705.
Newton’s First Law
(also known as Inertia)
- an object moving will
keep moving
- an object at rest will
stay at rest
(unless acted on by an
unbalanced force).
1. What is the first rule of physics?
2. If you kick a pebble on Earth, what stops it from moving?
3. What would happen if you kicked the pebble in space?
4. What is the latin word for “laziness”? ____________
Apply the Law!
If you are in a car accident are you “thrown”
forward? Explain!
Newton’s Second Law
the acceleration of an object is in the same
direction as the net force on the object.
Can be calculated with this equation:
Acceleration=
net force (in newtons)
Mass (in kg)
Acc. = Net Force ÷ mass
Or...F=MA
Acceleration is directly proportional to the net force (Fnet)
but inversely proportional to the mass of the object.
Mathematically, this means: A = Fnet
m
Acceleration is proportional to the net force (Fnet) but
inversely proportional to the mass (inertia) of the
object.
Mathematically, this means F=MA
F=MA
A = Fnet / m
Recall that mass= how much
matter is in an object.
The more massive an object
is, the more inertia is has.
Recall that Force (F) is measured in Newtons
Mass (m) is measured in kg
Acceleration (A) is measured in m/s2
Remember- if you
know any 2 things in
the triangle you can
always find the 3rd!
Objects have INERTIA in proportion to their
mass
Q: What has more inertia: a freight train
at rest, or a freight train moving at 50
km/hr?
Q: Which has more mass?
Mass and Weight
•Mass (the amount of matter) gives inertia to an object
•Mass: measured in kilograms (kg), or grams
•Mass is not weight!! Weight is the force of gravity and is
measured in Newtons (N) or pounds (lbs)
•If you go to the moon, your weight is less. Your mass, however,
will be the same.
Q: Do you have less
inertia on the moon?
Q: How can you
lose some of your
mass?
Practice (we’re talking about it).
Suppose the Olympic Bobsled racers push
with a net force of 600 Newtons on the
sled (msled = 60kg). What will be the
acceleration?
A = Fnet
m
Answer=
= 600N = 600kgm/s2
60kg
60kg
10 m/s2
Suppose the Olympic Bobsled racers push and
cause the sled (msled = 100kg) to accelerate at 5.6
m/s2. What is the net force exerted by the
Olympians?
F = ma = (100kg)(5.6m/s2)
net
Fnet = 560 kgm/s2 = 560 N
Check Your Understanding
An unfortunate bug splatters against the
windshield of a moving car. Compared to
the deceleration of the car, the
deceleration of the bug is (2nd Law!)
A] larger.
B] smaller.
C] the same.
Friction = a force that opposes the sliding
motion between two objects
Friction tends to resist objects sliding past
each other
Static Friction is what prevents two surfaces from
sliding past each other (nonmoving friction).
Ex. You push a large box, but it just won't move
Sliding Friction is the forces that oppose two
surfaces that are already sliding (moving friction).
Ex. Once you get the box moving, it is easier to
slide.
Air can exert frictional forces on an object
(This is called DRAG)
The feather has a
greater air
resistance than the
elephant, so it
stops accelerating
sooner, and thus
falls slower.
If you jump out of an airplane, you
might want to use a parachute.
When you first jump out
a plane, gravity is the
force acting upon you
(with some air
resistance). Eventually,
the force of the air
resistance and gravity
is equal and you no
longer accelerate - this
is called the terminal
velocity.
Review- Newton’s Laws
Newton’s First Law tells you what happens
to an object if the net force on it is zero.
Newton’s Second Law tells you what
happens to an object if the net force on it
is not zero.
Newton’s Third Law...
Newton's Third Law
Newton's Third Law is often stated:
“For every action there is an equal and
opposite reaction.”
This statement is correct, but terse and
confusing.
The “action” force is
equal to the
“reaction” force.
In Newton’s Third
Law, there are two
forces involved- and
two objects involved.
Newton's Third Law
Forces are interactions between objects, like
conversations are interactions between people.
Forces always arise in pairs, and two objects are always
involved.
A pushes/pulls B.
B pushes/pulls A.
One of these forces is called the action force, and the
other one is called the reaction force
(it doesn't matter which is which)
Action/Reaction Pairs
• Forces are always born as “twins”!
• Both forces are exactly the same size.
They are equal in magnitude.
• Both forces exist at exactly the same time. They
both start at exactly the same instant, and they both
stop at exactly the same instant. They are equal in
time.
• "opposite" means that the two forces always act in
opposite directions - exactly 180o apart.
Action/Reaction Pairs
• Action Reaction Pairs:
– They are equal
– They are opposite
Are they Balanced Forces??
Recall Balanced ForcesThe two forces on this person
are equal and opposite and they
Balance to a Net Force = zero
Action/Reaction Pairs
• Action Reaction Pairs are not balanced and
don’t cancel out because they always act on
two DIFFERENT objects!:
– They are equal
– They are opposite
They are NOT
Balanced! (The Fnet
does not = 0)
(one force is on the
road,
One force is on the tire)
Action/Reaction pairs are
always 2 different objects!
In this case, the air and the
balloon are the different
“objects”.
Balloon
pushes
air down
Air pushes
balloon up
Examples of A/R Pairs
Bat hits ball
Ball hits bat
Ball hit Pin
Athlete pushes weight up
Weight pushes athlete down
Gun pushes bullet
Bullet pushes gun
Pin hits ball
Check Your Understanding:
Newton’s Third Law
Action. Reaction. Got it.
Ohhh! A shiny thing!
Newton’s Third
Law is Easy!
Wait, Newton has 3
laws? I thought we were
still on the first one!
Argh. I like the math
better. I don’t really get
this.
Check Your Understanding
1) The statement "to every reaction there is
an equal and opposite reaction" is
A] the law of conservation of momentum
B] Newton's first law of motion
C] Newton's second law of motion
D] Newton's third law of motion
Check Your Understanding
2) If a 300-N action force is exerted to the
right, the reaction force will be
A] 300 N to the right
B] 600 N to the right
C] 300 N to the left
D] 600 N to the left
Check Your Understanding
3) When a force is exerted on an object, an
equal and opposite force is exerted by the
object. These forces are referred to as
A] centripetal forces
B] friction forces
C] gravitational forces
D] action-reaction forces
Check Your Understanding
4) Forces always occur
A] when velocities are constant.
B] as single quantities.
C] in pairs
D] in triplets.
Check Your Understanding
5) An unfortunate bug splatters against the
windshield of a moving car. Compared to
the force of the car on the bug, the amount
of force of the bug on the car is (3rd Law!)
A] larger.
B] smaller.
C] the same.
The old story of the Horse and Wagon:
The horse has been studying physics at night
in the barn, and now refuses to pull!
HORSE: "Newton's Third Law says that if I pull on the wagon, the
wagon exerts an equal and opposite force on me.”
FARMER: “Hmm, OK, true”
HORSE: "If these two forces are equal and opposite, they will
cancel, so that the net force is zero, right?"
FARMER: "Well, I suppose so.”
HORSE: "The net force is always the
important thing. If the net force is zero,
then Newton's Second Law (F=ma) says
that the acceleration of the wagon must be
zero.”
“No matter how hard I pull on the cart,
It will never move!”
Let’s look at the Action Reaction
pairs:
The two forces colored yellow in
the diagram are an A/R pair –
"horse pulls wagon"
"wagon pulls horse"
They are equal in magnitude
and opposite in direction.
The two forces colored blue in
the diagram are a A/R pair –
"horse pushes ground"
"ground pushes horse"
They are also equal in
magnitude and opposite in
direction.
(For simplicity, lets say there is no
friction between the wheel and the
ground)
Can action/reaction forces cancel out to zero?
What about the forces colored yellow in the diagram? (an A/R pair)
"horse pulls wagon" & "wagon pulls horse"
They are equal in magnitude and opposite in direction.
Why don’t they cancel out to zero?
The forces don’t cancel out to zero because they
are on two different objects!
-only two forces on the same object can
cancel so that Fnet = 0
The wagon and the ground are
not a force pair.
Why does the wagon accelerate?
Newton's 2nd Law says that an object accelerates if there is a net
(unbalanced) force on it.
There is just one force exerted on the wagon - the force that the horse
exerts on it. The wagon accelerates because the horse pulls on it! The
amount of acceleration equals the net force on the wagon divided by its
mass (Newton's Second Law)
A = Fnet/mwagon
Why does the horse accelerate?
There are 2 forces that push or pull on the horse in the diagram below.
The wagon pulls the horse backwards, and the ground pushes the
horse forward. The net force is determined by the relative sizes of
these two forces.
If the ground pushes harder on the horse than the wagon pulls, there
is a net force in the forward direction, and the horse accelerates
forward.
Why does the horse accelerate?
If the Horse Accelerates Forward:
If the Horse Accelerates Backward:
If the ground pushes harder on
the horse than the wagon pulls,
there is a net force in the forward
direction, and the horse
accelerates forward.
If the wagon pulls harder on the horse
than the ground pushes, there is a net
force in the backward direction, and
the horse accelerates backward.
(This wouldn't happen on level
ground, but it could happen on a hill).
Why does the horse accelerate?
If the force that the wagon exerts on the horse is the same size as the force
that the ground exerts, the net force on the horse is zero, and the horse does
not accelerate.
(horse remains at rest OR moves with constant velocity)
Copyright
© 2011 www.biology-roots.com (Vanessa
Jason)
For single classroom use only.
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