Potential Energy (Lageenergie)

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Potential Energy (Lageenergie)

If an object with mass m is placed the height h above a point, the potential energy relative to this point is:

Exercise: Estimate the potential energy of your physics book relative to the floor.

h g

m h

F

E

_pot



_G

   

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Units (Einheiten)

ALWAYS use units that fit:

Length: m time: s mass: kg

The units of other quantities are made of these Energy: J=kg∙m²/s²=Nm N=kg∙m/s²

Note that quantities, such as mass, m, are

written in italic, while units, such as metre, m, are written upright.

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Work

In physics the word „work“ is used differently than in everyday speak. Work, W, denotes the energy transfer if a force F is applyed over a distance Δs:

If an object falls the distance h the work done by the gravitational force equals the potential energy at the height h.

s F

E

W     

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Work, Exercises

1) You apply a force of 100 N to lift a box 5

meters. How much work does the force do?

2) You put the box down again still apply a force of 100 N. How much work does the force do?

3) How much work does the forces do in total?

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Kinetic energy (Bewegungsenergie)

When you let an object fall, it gets faster,

because the potential energy is transformed into kinetic energy (Bewegungsenergie), due to

conservation of energy. The kinetic, E_kin, energy is:

Where v is the velocity of the object.

2

2 1 m v

E

_kin

 

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Energy conservation

Remeber that energy is conserved: it cannot be created or distroyed only transformed. So in the case of a free fall, the total mechanical energy, E, is constant:

2

2 1 m v g

h m

E E

E 

_kin



_pot

    

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Example

o

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Example

A stone of 100 g is placed 10 meters above the ground:

It falls to the ground. Just before it hits the

ground the potential energy is 0 and all energy is now kinetic

2

2 J 1

81 .

9 m v

E

_kin

  

J 81 .

9 m/s

81 .

9 m

10 kg

1 .

0  

²





 m h g

E

_pot

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Example

To find the speed just above the ground you rewrite the formula:

m/s 2 14

2 2

1

₂ ₂















kin

kin kin

m E v

v

m

E

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Power

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Power (Leistung)

Power, P, is the amount of energy, E, used per unit time, t:

Do you remember the units for Power?

Exercise: You lift 10 liters of water 1 meter in 5 seconds. What average Power does that

correspond to?

t

P  E

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Note that power is used differently in everyday speech. Make exercise 1 on page 32 in

Netzwerke.

Power (Leistung)

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Efficiency

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Efficiency (Wirkungsgrad)

The efficiency is called η and is the fraction of the energy that is used for the desired purpose

input used

E

 E

