Work, Energy and Power

Has Work Been Done?? A teacher applies a force to a wall and become 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 A rocket accelerates through space

Work Work is defined as a force acting upon an object to cause a displacement. There are three key words in this definition – force, displacement, and cause. In order for a force to qualify as having done work on an object, there must be a displacement and the force must cause the displacement

Work = Force x Distance The unit for Work is Nm or J(Joule) 1 Joule is the work done by a Force of 1 Newton in moving an object a Distance of 1 meter. If an object does not move then no work has been done. Force and displacement must both be in the same direction for work to have been done (eg; Flag in a parade goes horizontally but the force is vertical).

Example # 1 What Work is done by pushing a physics text book with a Force of 20N a Distance of 1m to avoid doing homework. W=Fd =20N x 1m =20J

Example # 2 Mrs. Sanford lifts a 65kg student 0.5m. What Force does Mrs. Sanford use to lift the student? What work does she do?

Solution a) Weight=F=mg =(65kg)(-9.8 m/s2) =-637N Mrs. Sanford exerts an upward force of +637N b) W=Fd =(637N)(0.5m) =318.5J

Has Work Been Done?? A teacher applies a force to a wall and become 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 A rocket accelerates through space

Has Work Been Done Revisited This was an example in which work was not done since the wall was not displaced This is an example of work since the force of gravity acts on the book and causes the book to be displaced in a downward direction This one was a trick, there was no work done since the force did not cause the displacement Yes there has been work done since the force of the rocket causes the rockets displacement

Demonstration Work

Power The quantity of work has to do with a force causing a displacement. Work has nothing to do with the amount of time that this force acts to cause the displacement Power is the rate a which work is done. It is the work/time ratio. Mathematically, it is computed using the following equation. Power = Work Time

Power Continued The standard metric unit of power is the Watt As is implied by the equation for power, a unit of power is equivalent to a unit of work divided by a unit of time Thus, a Watt is equivalent to a Joule/second For historical reasons, the horsepower is occasionally used to describe the power delivered by a machine. One horsepower is equivalent to approximately 750 Watts.

Check your Understanding Two physics students, Jacob and Ryan are in the weightlifting room. Jacob lifts the 100-kg barbell over his head in 5.0s; Ryan lifts the same barbell over his head in 2.9 s. Which student does the most work? Which student delivers the most power? Explain your answer.

Solution They both will do the same amount of work since each must lift the same barbell the same distance over their heads Ryan will be the most powerful since he does the same amount of work but in less time.

Efficiency Energy can be, and is, converted into forms that do no work or do not serve the intended purpose. Transforming energy involves some “loss” of useful energy. Efficiency is the ratio of useful energy or work output to the total energy or work input.

Efficiency = Woutput X 100% Winput OR Efficiency = Eoutput X 100% Einput

Example A 100 W light bulb produces 90000J of light energy in 30 minutes. What is the efficiency of the light bulb?

Demonstration Power