Work and Machines Work Power Energy Machines

What is work? Work – what happens when an object changes its position by moving in the direction of the force that is being applied. Work = force x distance Joule – the metric unit of work

Work A woman rides a bike using 14 Newtons of force and travels a distance of 100 meters. How much work was done? W = F x d W = (14 N)(100 m) W = 1400 Nm or 1400 J

Work A man moves a box 50 meters using 250 Joules of work. How much force did he use? W = F x d F = W/d F = (250 J)/(50 m) F = 5 N

Work A truck does 500 Joules of work and carries 250 Newtons of force. How far did the truck travel? W = F x d d = W/F d = (500 J)/(250 N) d = 2 m

Power Power – the amount of work a person does within a given period of time Watt – the unit used to measure power Power = Work/time

Power How much power is completes 220 Joules of work in 11 seconds? P = W/t P = (220 J)/(11 s) P = 20 Watts

Power How much power is done when 500 Joules of work is completed in 25 seconds? P = W/t P = (500 J)/(25 s) P = 20 Watts

Energy – the ability to do work Kinetic Energy Energy of motion Examples – falling book, moving car Depends on the mass of the object and the speed of the object K.E. = ½ mv 2 Potential Energy Stored Energy Examples – book on a table, parked car Depends on the mass of the object, the height of the object, and gravity P.E. = mgh

Forms of Energy Chemical energy – stored in the bonds between atoms Heat energy – created by the movement of particles Mechanical energy – energy in moving objects Nuclear energy – stored in the nucleus Radiant energy – associated with light Electrical energy – causes electrons to move

Energy Generator – a device used to convert mechanical energy to electrical energy Law of Conservation of Energy – energy cannot be created or destroyed

Simple Machines Simple machines – a tool with few parts that makes it easier or possible to do work Change the direction or size of the force applied Change the distance through which the force acts

Levers Lever – a simple machine containing a bar that can turn around a fixed point Fulcrum – a fixed point around which a lever rotates Effort force, F e - the force applied to a machine by the user Resistance force, F r – the force applied to a machine by the object to be moved

Types of Levers First class lever – fulcrum is between the effort force and the resistance force Examples: crowbar, hammer, screwdriver Second class lever – resistance force is between the effort force and the fulcrum Examples: wheelbarrow, paper cutters, nutcracker Third class lever – effort force is between the resistance force and the fulcrum Examples: broom, oar, flyswatter

Efficiency Work input – work put into the machine by the user Work input = F e x d e Work output – work done by a machine against the resistance Work output = F r x d r Efficiency – how well a machine performs Efficiency = (Work output/Work input) X 100%

Efficiency What is the work input done by a person that moves an object 4 meters using a force of 10 Newtons? Work input = F e x d e Work input = (10 N)(4 m) Work input = 40 J

Efficiency What is the work output done by the machine that moves an object 2 meters using a force of 10 Newtons? Work output = F r x d r Work output = (10 N)(2 m) Work output = 20 J

Efficiency What is the efficiency of the machine? Efficiency = (Work output/Work input) X 100% Efficiency = (20 J)(40 J) X 100% Efficiency = (0.5) X 100% Efficiency = 50 %

Mechanical Advantage Mechanical Advantage – factor by which a machines multiples the effort force MA = F r /F e What is the mechanical advantage of a machine that uses an effort force of 15 N to lift a resistance force of 5 N? MA = F r /F e MA = (15 N)/(5 N) MA = 3

Mechanical Advantage MA = effort arm/resistance arm What is the mechanical advantage of a lever with an effort arm of 28 m and a resistance arm of 7 m? MA = effort arm/resistance arm MA = (28 m)/(7 m) MA = 4

Other Simple Machines Pulley – a simple machine made up of a rope, chain or belt wrapped around a wheel A fixed pulley is attached to a surface. A movable pulley is not attached and will rise along with the resistance. The MA of a pulley system is usually equal to the number of ropes that are pulling upwards.

Other Simple Machines Inclined plane – a simple machine made up of a ramp, used to lift an object Inclined planes decrease the force, but increase the distance the object must move. MA = length of the slanted surface/vertical height

Other Simple Machines Screw – a simple machine made up of an inclined plane wrapped around a straight piece of metal Wedge – a simple machine made up of an inclined plane or pair of inclined planes that are moved Wheel and axle – a simple machine made up of a wheel attached to a shaft