physics energys

8/18/2019 Physics Energys

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Chapter 5: Work, Energy and Power

- Work

- Energy conversion and conservation

- Potential energy and kinetic energy- Power

Show an understanding of the concept of work in terms of the product of a force and

displacement

in the direction of the force

Calculate the work done in a num!er of situations including the work done !y a gas which is

e"panding against a constant e"ternal pressure: W # p$%

Work &one !y a force is defined as the product of the force and displacement 'of its point of application( in the direction of the force

ie W # ) s cos *

+egative work is said to !e done !y ) if " or its compo is anti-parallel to )

f a varia!le force ) produces a displacement in the direction of ), the work done is determined

from the area

under )-" graph .ay need to find area !y /counting the s0uares1 2

c 3ive e"amples of energy in different forms, its conversion and conservation, and apply the principle

of energy conservation to simple e"amples

4y Principle of Conservation of Energy,

Work &one on a system #

E gain 6 3PE gain 6 7hermal Energy generated ie Work done against friction2

d &erive, from the e0uations of motion, the formula Ek #

89

mv

Consider a rigid o!;ect of mass m that is initially at rest 7o accelerate it uniformly to a speed v, a

constant

net force ) is e"erted on it, parallel to its motion over a displacement s


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Since ) is constant, acceleration is constant,

7herefore, using the e0uation: v

9

# u969 a s,

a s #

8

9

'v9

- u

9

(

Since kinetic energy is e0ual to the work done on the mass to !ring it from rest to a speed v,

7he kinetic energy, E # Work done !y the force )

# ) s

# m a s

#

8

9

m 'v9

- u

9

(

e


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f &istinguish !etween gravitational potential energy, electric potential energy and elastic

potential

energy

3ravitational potential energy: this arises in a system of masses where there are attractive

gravitational

forces !etween them 7he gravitational potential energy of an o!;ect is the energy it possesses !y

virtue of

its position in a gravitational field

Elastic potential energy: this arises in a system of atoms where there are either attractive or

repulsive

short-range inter-atomic forces !etween them ')rom 7opic =, E P E # > k "9

(Electric potential energy: this arises in a system of charges where there are either attractive or

repulsive

wwwstudyguidepk 85

- P?3E 8@ -

electric forces !etween them

g Show an understanding of and use the relationship !etween force and potential energy in a

uniform

field to solve pro!lems

7he potential energy, A, of a !ody in a force field whether gravitational or electric field2 is

related to the force ) it

e"periences !y: ) # -

dA

d"

h &erive, from the defining e0uation W # )s the formula Ep # mgh for potential energy changes

near

the Earth s surface‟

Consider an o!;ect of mass m !eing lifted vertically !y a force ), without acceleration, from a

certain height


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h8 to a height h9 Since the o!;ect moves up at a constant speed, ) is e0ual to m g

7he change in potential energy of the mass # Work done !y the force )

# ) s

# ) h# m g h

i


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- for o!;ect moving at const speed: ) # 7otal resistive force e0uili!rium condition2

- for o!;ect !eginning to accelerate: ) # 7otal resistive force 6 ma +D@P8F8D,+GGP8F52

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