physics_ddps1713_ chapter 4-work, energy, momentum and power

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

physics_ddps1713_ chapter 4-work, energy, momentum and power

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