Transcript Newton`s Laws & Momentum

AMY SHANTA BABOOLAL
PHYSICS PROJECT:
MECHANICS
ARISTOTLE’S ARGUMENTS
• One of his well known arguments is: to understand
•
change, a distinction must be made between the form
and matter of a living thing. For example a sculpture has
the form of a human but the matter of bronze. If the
bronze is molded into a new form, then a change has
just occurred.
He also thought of movement as a type of change, by
observing what occurs when something is created or
destroyed.
NEWTON’S THREE LAWS OF MOTION
• FIRST LAW: States that an object at rest tends to stay at
rest and an object in motion continues to move with the
same velocity, unless the object is acted upon by an
unbalanced force. Inertia is the property which causes
this change in motion.
• SECOND LAW: When a force acts on a body the rate of
change of momentum is proportional to the applied force
and takes place in the direction which the force acts,
giving the equation: F = ma
• THIRD LAW: The statement used to describe this law is:
For every action there is an equal and opposite reaction.
WHAT IS A FORCE?
 A force
can be simply described as a push
or a pull that causes a change in the state
of motion of and object. When a force is
applied to an object it may cause the
object to change in: shape, size or motion.
 Forces are represented using arrows and
can be either contact or non – contact.
 The unit for force is Newton.
DYNAMICAL SYSTEMS




When a an object is at rest, it takes a force to get it moving. Likewise, when
an object is moving it takes a force to stop.
To explain Newton's first law, we can use the example of the X and brakes in
a car. For the car to move from rest, a force has to be applied to the X
similarly, for the car to stop a force has to be applied to the brakes.
In Newton’s second law, we see that multiplying the acceleration and mass of
an object, we can get the force needed to move the object. For example to
move a roller coaster with mass 4500kg moving at an acceleration of
1000ms-1, the force needed is 4500kg*1000ms-1 = 4500000N
When we walk, we exert a force on the ground, and the ground exerts an
equal but opposite force on our foot, causing it to come back up. This proves
Newton’s third law; each action has an equal and opposite reaction.
WHAT IS LINEAR MOMENTUM?
• Momentum is the physical quantity that
takes into account both the mass and
velocity of an object.
• It can be associated with Newton’s first
and second laws.
• The linear mass of an object can be
described as the mass of the object
multiplied by its velocity i.e. p=mv
• It is measured in kgms-1.
LINEAR MOMENTUM
• An example used to demonstrate linear
momentum is: A ball A of mass 5kg is
traveling north at 6ms-1. Another ball B of
mass 3kg is traveling south at 4ms-1. To
find the momentum of each, we multiply
the mass by the velocity. The momentum
of ball A was found to be 30kgms-1 and the
momentum of ball B was found to be
12kgms-1.
THE LAW OF CONSERVATION OF
LINEAR MOMENTUM.
 The statement used to describe
the law of conservation of linear
momentum is: Provided that the
vector sum of the external
forces acting on a system is
zero, the total linear momentum
of that system remains constant
during collisions.
 Momentum can be conserved
for all interactions in which the
vector acting on the force is
zero.
 When we use collisions, we see
that the momentum before and
after a collision are equal.
After collisions, one of the following may
occur:
 Both objects come to a complete
stop
 Both objects move together in the
same direction.
 Both move off in the opposite
direction.
EXAMPLE: Two trains are on the same
track. Train B of 200kg is stationary
and train A is moving at 80ms-1 with
mass 150kg. If they collide train A
pushes train be off to a spade 30ms-1.
What is the speed of train A?
M1U1 + M2U2=M1V1 + M2V2
(150*80) + (200*0)=(150*V1) + (200*30)
12000 = 150V1 + 6000
12000 -6000 = 150V1
6000/150 = V1
V1 = 40 MS-1
CIRCULAR MOTION AND FORCES
• A force is needed when an object is
moving in circular motion.
• The force keeps the object in place.
• Without the force, the object may move out
of orbit.
• The force provides attraction between the
two objects present.
• If a force is not present there will not be
an attraction between the objects.
CIRCULAR MOTION AND FORCES





When an object moves in uniform circular motion, it means that the
object moves in a circle with constant speed, but not constant
velocity. The velocity is not constant because the direction is
continuously changing.
When an object, in circular motion is accelerated towards the
centre of the circle a centripetal force causes the acceleration and
changes the direction without changing the speed.
The formula for finding the centripetal force in a circle is: Fc =
mv2/r, where r is the radius of the circle.
EXAMPLE 1:When the Earth orbits the Sun, an unbalanced force,
gravity, is present. This gravitational force of attraction between the
Earth and the Sun, provides the centripetal force needed for circular
motion.
EXAMPLE 2: When a car is going around a bend, the frictional
force acting on the tires of the car provides the centripetal force
needed.