Unlike your everyday experience, where you go to an after school job and earn money, work in physics does not pay for your car insurance. Work has a special meaning in physics.
Work is done on an object when a force is applied to an object and the object is displaced. Units: Newton x Meter (Nm) Work is a scalar quantity Do not confuse applying a force with doing work. Work is only done if the object moves a distance. Pushing or pulling on an object may make you sweat but you do no work unless it moves. Only the force parallel to the direction of the displacement does work. Work = Force x displacement (cosine of angle for the portion of the force parallel to the distance traveled)
A teacher applies a force to a wall and becomes exhausted. A book falls off a table and free falls to the ground. A waiter carries a tray full of meals above his head by one arm across the room. (Careful! This is a tricky question.) A rocket accelerates through space. Read the following five statements and determine whether or not they represent examples of work. NO YES
Power
In Physics Power is the Rate of Doing Work
A watt is a small unit If you lift a glass of water (~2N) to your mouth (~0.5 m) in 1 second you do one joule of work and you are doing work at rate of one watt Since a watt is relatively small, power is often measured in kilowatts (kW)
Simple Machines
Simple Machines A simple machine is an object with few moving parts that redirects an applied force to perform work. Usually a simple machine is used to reduce the force needed to perform a task.
Simple Machines and Rube Goldberg
Work and Machines Force of Effort - Fe - The amount of force that is applied to the machine. Force of Resistance - Fr - The amount of force that the machine applies. Distance of Effort - de - The distance the force of effort is applied. Distance of Resistance - dr - The distance the machine applies the force.
Force of Effort Force of Resistance
Force of Effort Force of Resistance Distance of Effort Distance of Resistance
L e v e r
F 1 = F r = Force of Resistance F 2 = F e = Force of Effort D 1 = d r = Distance of Resistance D 2 = d e = Distance of Effort Fulcrum = Pivot Point Lev er
Lever Classes First Class Lever Second Class Lever Third Class Lever Fulcrum - Resistance - Effort Effort - Fulcrum - Resistance Fulcrum - Effort - Resistance
First Class Lever
Pulleys redirect the force of effort. The number of pulleys and their arrangement can reduce the amount of effort force needed to raise a particular load. The number of lifting strands will determine the advantage to using a pulley system
1 Lifting Strand 2 Lifting Strands 3 Lifting Strands Pulleys How many lifting strands for each pulley system?
W i = Work Input – The Amount of Work applied to the machine W o = Work Output – the Amount of Work transferred by the machine W i = F e d e W o = F r d r F e =Force of Effort F r =Force of Resistance d e =Distance of Effort d r =Distance of Resistance