Work, Power, & Simple Machines

Work Work is done when a force causes an object to be displaced.  The object must be displaced (moved) for work to take place. W = Fd SI Unit Nm = Joule (J)

Force & Displacement The force must be parallel to the displacement of the object for work to be done. Picking up a Football Force is Up Displacement is up. FAFA d Work is done by you! Dropping a Football Force is Down Displacement is Down FgFg d Work is done by Gravity! Running with a Football Force is Up Displacement is horizontal FAFA d No Work is done because F and d are perpendicular!

Work Work can be positive or negative.  Work is Positive when it helps the motion of the object. Ex: An applied force helps an object move; force and displacement are in the SAME direction.  Work is Negative when it opposes the motion of the object. Ex: Friction does work to slow an object down; force and displacement are in OPPOSITE directions.  No Work is done when: The force is perpendicular to the displacement. There is NO displacement. + Work - Work F d F d

Power the rate at which work is done SI Units J/s = Watts (W) Another Unit for Power is Horsepower (hp) 1 hp = 746 Watts Machines do the same work as us, just faster.

Machines - devices that make work easier by changing the magnitude and/or the direction of your force. 2 parts of a machine 1. Input – what the person does to the machine 2. Output – what the machine does to the load Load – object being moved Machines

Work Input W i = F i d i  Work Input – the work a person does on the machine.  Input Force – the force exerted by the person on the machine  Input Distance – the distance the person has to push or pull the machine

Work Output W o = F o d o  Work Output – the work the machine does on the load  Output Force – the force exerted by the machine on the load  Output Distance – the distance the machine moves the load

Do machines do more or less work than people? Machines do not change the amount of work done, they decrease the force. Ideal Machine – No work is lost W i = W o Ideally: W i = W o All the work you put in, you get out. Actually: W i > W o Ideal Machines Some work is used to overcome friction within the machine, you will always do more work

Mechanical Advantage – a ratio of how much a machine increases the input force. Actual Mechanical Advantage (AMA) (takes into account friction) Ideal Mechanical Advantage (IMA) (does not take into account friction) Mechanical Advantage No Units!

Efficiency – how close a machine comes to being IDEAL. Efficiency of an ideal machine = Two ways to find it Mechanical Efficiency These are the same equations How? 100%

6 Simple Machines

Pulley m FiFi FoFo dodo didi Person Pulls Rope down; Pulley Lifts Object Up. The force the machine is providing is equal but opposite to the object’s weight (load).

Incline Plane m FiFi FoFo didi dodo Person pushes up the incline; Machine lifts the object up the vertical height. The force the machine is providing is equal but opposite to the object’s weight (load).

Lever m FoFo FiFi Output Arm Person Pushes lever down; Pulley Lifts Object Up. The force the machine is providing is equal but opposite to the object’s weight (load). Fulcrum – fixed point the lever pivots around Input arm – distance from fulcrum to input force Output arm – distance from fulcrum to load Input Arm

A person sitting in a wheelchair goes up a ramp that is 15 m long and 5 m high. The person and the wheelchair together weigh 65 N. He applies 30 N of force on the wheelchair up the ramp to get to the top. Calculate everything! W i = ? W o = ? AMA = ? IMA = ? ME = ? Calculating Everything 450 J 325 J %