physics_ddps1713_ chapter 4-work, energy, momentum and power
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7/27/2019 Physics_DDPS1713_ Chapter 4-Work, Energy, Momentum and Power
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Chapter 1 - 1
DDPS 1713
PhysicsCHAPTER 4
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Chapter 1 -
WORK, ENERGY,
MOMENTUM & POWER
4.1 Definition of work, work energy theorem.
4.2 Potential energies, kinetic energies, law of
conservation of total mechanical energy
4.3 Impulse, Momentum, impulse-momentum
theorem,
4.4 Conservation of linear momentum, elastic andinelastic collisions
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Chapter 1 -
WORK
Work is done when a force produces motion. An engine pulling a train does work; so does a crane when it
raises a load against the pull of gravity.
Work is said to be done when the point of application of a force
moves and is measured by the product of the force and the
distance moved in the direction of the force or its displacement.
Unit: Joule (J) = Newton x meter = Nm.
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ntdisplacemeforceWork
sFW
cosFFx
cosFsW
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Chapter 1 -
POWER
Poweris the rate at which work is done. If the amount of work, W is done in time interval t, the average
power, P is defined as
The work done on an object contributes to increasing the energyof the object. A more general definition ofpower isthe rate of
energy transfer.
We find from equation W = F x s , therefore equation can be
written as
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t
WP
x vFt
FxsP
Unit : Joules per second (J/s), also called a watt(W)
1 W = 1 J/s = 1 kg.m2/s3
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Chapter 1 -
ENERGY
In mechanics, there are two types of
energy :
Kinetic Energy (KE)
Potential Energy (PE)
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Chapter 1 -
KINETIC ENERGY (KE)
Kinetic energy is the energy of motion. An object that has motion - whether it is vertical or horizontal
motion - has kinetic energy.
There are many forms of kinetic energy
vibrational (the energy due to vibrational motion),
rotational (the energy due to rotational motion), and
translational (the energy due to motion from one location to
another).
Kinetic energy is a scalar quantity
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Chapter 1 -
KINETIC ENERGY (KE)
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Chapter 1 -
Example
A 145-g baseball is thrown so that it acquires a speed of 25 m/s.(a) What is its kinetic energy?
(b) What is the net work done on the ball to make it reach this
speed, if it started from rest?
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Chapter 1 -
POTENTIAL ENERGY
Potential energy is the energy of an object or asystem due to the position of the body or the
arrangement of the particles of the system.
The SI unit for measuring work and energy is the
joule, J.
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Chapter 1 -
Example
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Chapter 1 -
Conservation of Mechanical Energy
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The sum of kinetic energy and potential energy is called
mechanical energy(E).
The principle of conservation of energy states that the sum of
kinetic and potential energies of a system is always constant
E = KE+ PE
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Chapter 1 -
Problem
Height (m) PE = mgh (J) KE = mv2 (J) PE + KE (J)20 117.72 0 117.72 ( E )i15
88.29
29.43
117.72
1050 117.72 ( E )f
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A simple example of conservation of mechanical energy is afalling ball.
Consider a 0.6-kg ball falling from a height of 20 m. As the ball
falls, the increase in kinetic energy means a decrease in potential
energy but the total energy (mechanical energy) remains the
same.
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Chapter 1 -
Problem
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Chapter 1 -
Example
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Chapter 1 - 15
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Chapter 1 -
MOMENTUM
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Chapter 1 -
MOMENTUM
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Chapter 1 -
IMPULSE
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Chapter 1 -
COLLISIONS
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Chapter 1 - 20
Problem
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Chapter 1 - 21
Elastic Collision
Besides having momentum, moving objects have kinetic energy
(KE).
In a collision, kinetic energy is generally not constant because
some of it is converted to heat, sound and into internal elastic
potential energy when the objects are deformed.
Therefore the kinetic energy, before and after collision is not the
same. In ideal condition collision, it is assumed that: KEbefore = KEafter
This type of collision is called elastic collision. During collision
both objects are deformed by the impulsive force acting on
them. After collision the objects return to their original forms.
Elastic collisions must satisfy two conservation principles: the principle ofconservation linear momentum
the principle ofconservation of kinetic energy
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Chapter 1 -
Inelastic Collisions
Inelastic collisions are those in which only the linear momentum
is conserved while the kinetic energy before and afterthecollisions are not constant. KEbefore = KEafter
If the colliding objects stick together after the collision and move
as single mass, the collision isperfectly inelastic collision
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Chapter 1 -
A 3-kg object that is moving at 8 m/s to the left
strikes another object of mass 8 kg that is moving to
the right with a speed of 10 m/s. The two objects
stick togetherafter the collision to form a single unit
and moves with velocity v. Find:
Draw the diagram before and after strikes.
The magnitude and direction ofvelocity v?
The kinetic energy loss in the collision?
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Problem
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Chapter 1 - 24
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Chapter 1 - 25
Explosion
Momentum is conserved in explosion in an isolated systemwhere no external forces act.
Momentum before the explosion is the same as that after it.
Give some other examples which use the idea of 'recoil', e.g.:
firing a cannon ball
firing a bullet from a rifle pushing a boat away from a bank.
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Chapter 1 -
A man fires a rifle which has mass of 2.5 kg. If the mass of the
bullet is 10 g and it reaches a velocity of 250 m/s after shooting,
what is the recoil velocity of the pistol?
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Problem
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