10.4 - energy + efficiency

how does a machine lift / move an object?

by applying a force to it

what happens when a machine applies a force to an object?

it moves / lifts it

what is the work done by the machine to move an object?

W = F s

• W = work done by a machine to move an object

• F = force applied to the object by the machine

• s = distance of object movement in the same direction of the force

W = F s

work done by a machine to move an object

if a machine applies a force to an object, in which direction will the object move?

in the same direction as the force

why does an object move in the direction of the force applied to it?

because when a force is applied to an object to move it, work is being done, and work done = force x distance in the same direction as the force

what distance will an object move if moving at constant velocity?

s = v t

• s = distance moved by object in the direction of the force

• v = constant velocity of object due to balanced forces

• t = time the force from the machine is applied

s = v t

distance moved by an object moving at constant velocity when acted on by a force applied by a machine

what enables an object to move at constant velocity when acted on by a force applied by a machine?

forces are balanced (equal and opposite), i.e., push force = frictional force

what is the power output of a machine moving an object?

P_out = F v

• P_out = power output of a machine moving an object

• F = output force of the machine

• v = speed of object

how do we denote power output of a machine moving an object?

P_out

what is P_out?

power output of a machine moving an object

P_out = F v

power output of a machine moving an object

why is the power output of a machine moving an object P_out = F v?

• P_out = work done by machine / time taken

• = F v t / t

• = F v

P_out = F v can be used under what condition?

object speed is constant

what is useful energy?

energy transferred for a purpose

what is wasted / dissipated energy?

energy that is not transferred tor its intended purpose in a component, i,e., not transferred usefully

what is the efficiency of a machine?

• E_useful / E_total

• W / E_total

• P_out / P_in

what is the efficiency of a machine in terms of energy?

E_useful / E_total

• E_useful = useful energy transferred by the machine

• E_total = energy supplied to the machine

E_useful / E_total

efficiency of a machine

what is the efficiency of a machine in terms of work done?

W / E_total

• W = work done by the machine

• E_total = energy supplied to the machine

W / E_total

efficiency of a machine

what is the efficiency of a machine in terms of power?

P_out / P_in

• P_out = the output power of a machine

• P_in = the input power to the machine

P_out / P_in

efficiency of a machine

what is the percentage efficiency?

efficiency x 100%

efficiency x 100%

percentage efficiency

what is the unit for efficiency?

there is none

why doesn’t efficiency have a unit?

because its a ratio

what is energy usually wasted as?

• heat

• sound

what causes energy to be wasted in machines?

friction

what causes energy to be wasted in electrical systems?

how does xx cause energy to be wasted in electrical systems?

how does friction cause energy dissipation in machines?

• work is done against friction, causing heating of the two surfaces

• internal (thermal) energy of the surfaces increase

what is internal energy?

thermal energy store

what happens to wasted energy?

its dispersed into the surroundings