Sct. 1 Work and Power

Work Is done when a force causes an object

to move. Carrying books Pushing grocery cart Lifting weights Dodge truck pulling Ford out of mud.

Applying force to do work.

Work is only done in direction of applied force and when object is moving. Lifting books off of desk = work by

arms Carrying books to class = not work by

arms but by legs

Calculating Work

Work = Force x distanceW = F x dJoules = N x meters What is the total amount of work done

when a football player carries the ball with a force of 25 N for 50 meters?

HW: Jot down a note about work that you did between now and tomorrow in class. Explain how having more power would have affected the work.

W = F x d W = 25 N x 50 m W = 1250 N x m W = 1250 J

Power

How quickly work is accomplished.Who is more powerful? A man who can lift 50 N of weight in

10 seconds or a woman who can lift 50 N of weight in 8 seconds?

Woman. Same weight and work accomplished, less time.

Calculating Power

Power = work/timeP = w/tWatts = Joules/second What is your power if you do 200

Joules of work in 20 seconds?

P = W/tP = 200 Joules/20 secondsP = 10 Joules/sec.P = 10 Watts

Work and Energy

When you do work on an object, because it has moved, you increase it’s potential or kinetic energy. Potential Energy = energy of

position/stored energy Height above earth Stretched rubber band

Kinetic Energy = energy of motion All moving objects have kinetic energy Work = objects in motion

Work and Power HomeWORK.

How much work must you do to move 100, 10 N rocks a distance of 50 meters?

How far did you push the grocery cart if it required 500 Joules of work and needed 25 N of force?

How much power is needed for a Dodge truck to pull a Chevy truck from the mud in 10 seconds if it requires 4000 Joules of work?

Who has more power? Mr. Replogle lifts 15 pieces of cake (1 piece = 1 J of work) to his mouth in 60 seconds and Mr. Brode lifts 21 pieces of cake to his big mouth in 80 seconds.

What kind of energy does the cake have as it is being swallowed to the depths of Mr. Brode’s belly?

What kind of energy did the cake have when it was being held high above his mouth ready to plummet to its dark grave?

w=f x dw=1000N x 50 mw= 50,000 J

w=f x d500J = 25 N x dd= 20 m

P=w/tP= 4000J/10 secP= 40 Watts

Barton Power = 15 J/1500 sec. = .o1 WattsBrode Power = 21 J/1740 sec. = .012 WattsBrode more powerful!

Kinetic EnergyPotential Energy

Section 2 Using Machines

Machine A device that makes doing work

easier Pencil Lawn mower Screw driver Car Computer pulley

Machines contd.

Change the way a person does work, not how much work is done.Have: Input work- effort put in Output work- the work done by the machine

No machine can create work Input work always greater than output work. Due to friction

Mechanical Advantage

The number of times a machine multiplies the input force.MA = output force/input forceEX: A car jack can lift a 2000 N car when a person applies 50 N of force.

MA = 2000 N / 50 N = 40

Work in = Work out

In an ideal machine. Not a real machineFin x din = Fout x dout

If a machine produces 50 N of force when 10 N are put in and the distance out is 2 m, how much distance must the machine have put in?

Fin x din > Fout x dout

10 N x din > 50 N x 2 m

din > 10 m

Ways that machines make work easier.

Exert a smaller force over a greater distance to increase output force. Car jack

Exert a larger force over a smaller distance to increase output distance. Rake

Allow you to change the direction of the input force. Pulley

Efficiency

Ability of a machine to use all of the work in.E = output work/input work x 100%Efficiency is reduced by friction.Therefore always less than 100%

Efficiency Calculation

How efficient is a car that produces 100 Joules of work for every 300 Joules of fuel consumed? E = work out/work inE = 100 J / 300 JE = .33 or 33%

Group Work

What is the mechanical advantage (ma = force out/force in) of a wrench that you put 50 N of force into and it loosens a bolt with a force of 250 N?What is the mechanical advantage of a rake that you must use 100 N of force to rake leaves with and the leaves require 20 N of force to move? What is the efficiency of a car jack that lifts a car using 5000 J of work when you put 10000 J of work into the jack?What is the efficiency of a machine that produces 500 J of work when 400 J of work is put in? Does this machine exist, and if so, what is it?

HomeworkFin x din = Fout x dout

At home pick up a hammer and 2 nails.Start nails in a board and have the same amount of nail sticking out of the board.First nail, hold the hammer as close to the head as possible. Second nail, hold the hammer as close to the end of the handle as possible. Compare the force out.