Transcript Applications of Newton`s first law of motion

PPMF102 – Lecture 1
Newton’s Laws of Motion
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Newton
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Newton’s First Law of Motion
Every object continues in its state of
rest or of uniform speed in a straight
line unless acted on by a nonzero
force
The tendency of a body to maintain
its state of rest or of uniform motion
in a straight line is called inertia.
Newton’s first law is also called the
law of inertia.
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Applications of Newton's first
law of motion
1. “Wait until the bus stopped”
2. “Fasten your seat belt” in cars,
aeroplanes, or roller coasters.
3. Headrests are placed in cars to
prevent whiplash injuries during
rear-end collisions.
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Car seat belt
Seatbelt for roller
coaster
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Applications of
Newton's first law of motion
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The head of a
hammer can be
tightened onto the
wooden handle
by banging the
bottom of the
handle against a
hard surface.
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Applications of Newton's first law of
motion
5. 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.
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Applications of Newton's first
law of motion
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Blood rushes from your head to
your feet when riding on a
descending elevator which
suddenly stops.
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Applications of Newton’s 1st law
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A tablecloth is
whipped from
beneath dishes
placed on a
table top.
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Applications of Newton’s 1st law
8. Flip a coin in highspeed car, bus, or
plane and catch the
vertically moving coin
as if the vehicle were
at rest.
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Applications of Newton’s 1st law
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Conceptual Question: Newton’s first law.
A school bus comes to a sudden
stop, and all of the backpacks on the
floor start to slide forward. What
force causes them to do that?
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Force
•A force is a push
or pull.
•An object at rest
needs a force to
get it moving.
•A moving object
needs a force to
change its
velocity.
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Force
•Force is a vector, having both
magnitude and direction.
•The magnitude of a force can be
measured using a spring scale.
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Various names of force
Force
Weight
Friction
Normal force
Tension in a
string
Symbol
W
f
N
T
Unit
N
N
N
N
These are all forces with the same unit
Newton (N).
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Force causes velocity
of object to change
•It doesn’t take a force to keep an
object moving in a straight line—it
takes a force to change its motion or
velocity.
•It takes a force to change either the
direction or the speed of an object.
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Newton’s Second Law of Motion
The net force acting on an object
equals the mass times the
acceleration of the object.
F = ma
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Newton’s Second Law of Motion
F = ma
F  net force acting on an object
[F  net force
 summation of all forces
 resultant force
 total force]
m  mass of the object
a  acceleration of the object
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Newton’s Third Law
Whenever one object exerts a force
on a second object, the second
exerts an equal and opposite force
on the first
To every action there is an equal and
opposite reaction
 “action” force and “reaction” force
are acting on different objects
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Newton’s Third Law
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Newton’s Third Law
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Newton’s Third Law
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Newton’s Third Law of Motion
Conceptual question: What exerts the force to
move a car?
Response: A common answer is that the engine
makes the car move forward. But it is not so
simple.
The engine makes the wheels go around. But if
the tires are on slick ice or deep mud, they just
spin. Friction is needed. On firm ground, the tires
push backward against the ground because of
friction. By Newton’s third law, the ground pushes
on the tires in the opposite direction, accelerating
the car forward.
Force = mass x acceleration
F = ma
Unit: Newton (N)
1 N = 1 kgm/s2
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Mass
A property of a body
Quantity of matter
A measure of the inertia of a body
SI unit is kilogram (kg)
Does not depend on location
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Weight—the Force of Gravity;
and the Normal Force
Weight is the force exerted on an
object by gravity.
Close to the surface of the Earth,
where the gravitational force is
nearly constant, the weight of an
object of mass m is:
where
Weight—the Force of Gravity;
and the Normal Force
An object at rest must have no net force on it. If it is
sitting on a table, the force of gravity is still there;
what other force is there?
The force exerted perpendicular to a surface is called
the normal force. It is exactly as large as needed to
balance the force from the object.
N
W
Weight—the Force of Gravity;
and the Normal Force
Example 1: Weight,
normal force, and a
box.
A box of mass 10.0 kg
is resting on a smooth
(frictionless) horizontal
surface of a table.
(a) Determine the
weight of the box and
the normal force
exerted on it by the
table.
(b) Now the box is pushed down on
with a force of 40.0 N. Again
determine the normal force exerted
on the box by the table.
(c) If the box is pulled upward with a
force of 40.0 N, what is the normal
force exerted on the box by the
table?
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Weight—the Force of Gravity;
and the Normal Force
Example 2: Accelerating
the box.
What happens when a
person pulls upward on
the box in the previous
example with a force
greater than the box’s
weight, say 100.0 N?
Weight
A force
A force of gravity acting on a body
Weight = mass x gravity
SI unit is Newton (N)
Depends on location i.e. strength of
gravity
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Friction – standard model
Force of resistance to the relative
motion of two solid objects when the
two surfaces are in contact.
Friction is directly proportional to
normal force.
Static friction – friction without
motion.
fs = s N
s  coefficient of static friction
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Kinetic friction – friction when there
is relative motion between surfaces.
fk = k N
k  coefficient of kinetic friction
Friction is in fact a very complex
phenomenon which cannot be
represented by a simple model.
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