ppt. - work and energy

total amount of energy in the universe

• conserved

• energy can be transformed from one time to another, or even from one object to another

• total amount is always the same

• principle of conservation of energy - no exceptions

  • remains constant as long as the same amount of energy is added and removed

  • as long as no energy enters or leaves the system, and no work is done on or by the system

principle of conservation of energy

• no work is created or destroyed

• no exceptions

energy

• the ability to do work; a scalar quantity

• Joules (J) or Newton-meters (N-m)

Joules

N-m

kinetic energy (K or KE)

• associated with the state of motion of an object

• stable energy: 0 __

potential energy symbol

• PE or U

potential energy (PE or U)

• energy associated with an object’s position relative to another object or the arrangement of a system of objects

• sometimes referred to as stored energy

types of potential energy, U

• gravitational potential energy

• elastic potential energy

• electric potential energy

• chemical potential energy

elastic potential energy

• energy stored in a stretched (or compressed) spring

• a system such as a spring that has a defined spring constant - k - that is extended a distance x from a position of equilibrium

gravitational potential energy

• U_{G, - is the PE of a system due to interactions of gravitational fields}

  • such as the change in U when you lift an object from the floor to a height, h, from the floor - DU = mgDh

mgh vs -GmM/r

• UG = mgh near the surface

• UG = -GmM/r “farther afield)

• it is negative b/c U is at infinity and gets larger and larger negative as a mass moves closer to the earth

electric potential energy

• such as the energy stored in the capacitor of a camera flash unit

chemical potential energy

• charged car battery

mechanical energy

• includes kinetic and potential energy of an object or system due to its motion or position

falling object mechanical energy neglecting air friction

• the gravitational potential energy is converted to kinetic energy as the object falls

• maintaining constant total mechanical energy

falling object mechanical energy including air friction

• total mechanical energy is not constant as the object falls

• some is converted to thermal energy in the molecules of the object in the air

  • means a temperature increase in both the object and the air

work is a

• scalar quantity

• measured in joules

work is negative or porsitive

• force and displacement in same direction: posiitive

• different direction: negative

• perpindicular: zero

(+/-) work on a system (increases/decreases) the mechanical energy of the system

• positive work on the system increases the mechanical energy of the system

• negative work on a system decreases the mechanical energy of the system

• friction usually does NEGATIVE work on systems

ball rolling across a floor mechanical energy

• gravitational potential energy does not change

• friction does negative work on the energy (in opposite direction that the ball rolls)

  • eventually decreasing its kinetic energy to zero as it converts the balls mechanical energy to thermal energy

mechanical energy is

• potential and kinetic

work and energy

• scalar qualities that can be measured in joules or calories

1J = 1 kg-m²/s²

^{1 cal = the amount of energy needed to raise the temp of 1 g of water by 1°C}

^{4.186 J = 1 cal}

^{1 nutritional calorie = 1,000 cal or 1 kcal}

if there is no work ON the system

• its total mechanical energy remains constant

• work done On the system will change the energy of the system

work-energy theorem

• work done on an object or system changes the mechanical energy of the system

• •W = DK + DU

• work done BY a system decreases the energy of the system

• On = +

• by = -

spring

• work done in stretching a spring is = to the amount of Us stored in the sprng

• ½ kx²

• ^{you can find the spring constant by graphing force vs position}

• ^{slope is k}

determining work by graphing

• the work done on an object is equal to its change in mechanical energy

• work is the area under a curve of a force vs position graph

total mechanical energy of a pendulum

• remains constant throughout the pendulum’s motion (neglecting friction)

power

• the rate at which energy changes or the rate at ehich work is done

• measured in WATTS