work, energy and power

power

• the rate of energy transfer

• p = fv

conservation of energy

• energy cannot be created or destroyed, only transferred from one store to another

work done

• work is done when energy is transferred by an external force into / out of the system

mechanical work done

• product of force and distance moved in the direction of the force

• when energy is transferred, work is done ( energy change = work done)

• $$W=Fx\cos\left(\theta\right)$$

• where theta is the angle between the force and the displacement

• maximum work done when force is parallel to displacement

energy stores

• kinetic - due to motion

• gravitational potential - due to position in gravitational field

• thermal

• elastic potential

• chemical

• nuclear

• electrostatic

• magnetic

pathways of energy transfer

• mechanical - force of one object acting on another

• electrical - electrons colliding with ions

• heating - conduction, convection

• radiation - EM waves

energy transfers with negligible resistive forces

• total energy = gpe + KE + Ep

why more energy is needed, than that calculated

• work must be done to overcome resistive forces / friction / drag / air resistance

• some of energy from initial store is transferred into internal energy or thermal energy of the surroundings by friction doing work / drag / air resistance doing work

• when a constant speed is being maintained, all of the initial energy stored is transferred to thermal energy- no overall increase in kinetic energy- work done against a resistive force to overcome it

• if kinetic energy is greater than gpe loss - work was done to increase kinetic energy of system

• use work energy principle

trampoline

• elastic potential energy is transformed to kinetic energy / trampoline does work on person

• kinetic energy is transformed into gravitational potential energy

• person must do work / use chemical energy to raise height

• must overcome resistive forces - drag / heat loss / energy lost in trampoline

pole vaulter

• energy dissipated during upward movement

• energy lost in pole

• potential energy given to pole itself as it rises

• kinetic energy required to move forward over bar

climbing stairs

• not all work done goes to potential energy

• kinetic energy gained

• person gets hot as heat generated in muscles

• must do work to overcome resistive forces

force against distance graph

• area under graph represents work done ( change in energy)

efficiency

• useful energy - transferred to where it is wanted, when it is wanted

• wasted energy - energy transferred to not its intended purpose

• efficiency - ratio of useful energy or power to total input energy or power