# Work Work is the transfer of energy through

- Slides: 35

Work • Work is the transfer of energy through motion!!! • In order for work to be done, 2 things must happen: 1. Object must move(displacement). 2. Applied force must be in the direction of the displacement!!!

Work? ? ? A teacher applies a force to a wall and becomes exhausted. This is not an example of work. The wall is not displaced. A force must cause a displacement in order for work to be done.

Work? ? ? A book falls off a table and free falls to the ground. Yes. This is an example of work. There is a force (gravity) which acts on the book which causes it to be displaced in a downward direction (i. e. , "fall").

Work? ? ? A waiter carries a tray full of meals above his head by one arm straight across the room at constant speed. No. This is not an example of work. There is a force (the waiter pushes up on the tray) and there is a displacement (the tray is moved horizontally across the room). The applied force and the displacement are not in the same direction!!

Is any work done in lifting an object? Is a force applied? YES Does object move in the direction of the applied force? YES

Is there any work done on the nail by the hammer?

Work Equation W=Fd Where: W = work in Joules(J) F = force in Newtons(N) d = distance in meters(m)

Ex. A force of 250 N is applied to push a box 10 m across the room. How much work is done on the box? Given: F = 250 N d = 10 m W=? W=Fd W = (250 N)(10 m) W = 2500 J

Ex. On a recent Rodgers’ Family vacation, the engine stalled out and would not restart. Mr. Rodgers made his family(including his wife) get out of the car and try to push the car up the road. The kids(along with the wife) pushed with a combined force of 3500 N, but the car didn’t move. How much work was done? No Work was done!! The car did not move!!

Ex. The Pizza Hut delivery man carried 2 Extra large pizzas weighing 15 N each into a house 20 m from his car. How much work did the delivery man do? It is awesome to get paid and I don’t even do work!!!!

Ex. A force of 525 N is applied to push a box across the room. If 4900 J of work is done in pushing the box, how far was the box pushed? Given: F = 525 N W = 4900 J d = ? ? ?

A force of 250 N is applied to a box to push it across the room 15 m. How much work was done on the box? A. B. C. D. OJ 16. 7 J 3750 J 37, 500 J

A machine does 5000 J of work in pulling an object across the ground 10 m. What force did the machine exert? A. 50, 00 O N B. 5000 N C. 500 N D. 0 N

Mrs. Hatfield carries a 10 N tray of food 25 m to a customer at her second job as a “Singing Waitress”. How much work does Mrs. Hatfield do? A. B. C. D. OJ 2. 5 J 2500 J

I am lifting 150 kg. I am going for the record!! Am I doing any work? Do I apply a force? What force is required to lift the object? This is not as easy as it may appear!!

If you apply a force less than 500 N, Newton’s 1 st Law says that the box will not be lifted. Forces will be unbalanced downward!! If you apply a force of 500 N, Newton’s 1 st Law still says the box will not be lifted. Forces will be balanced!! 500 N In order to lift the object, the forces must be unbalanced upward. In order to lift the object, the force required must be 500. 0000000000001 N. It must be ~ the weight of the object!!

I am lifting 150 kg. I am going for the record!! What force is required to lift the barbell? Fg = m g Fg = 150 kg(9. 80 m/s 2) Fg = 1470 N

Ex. Mr. Eidson lifts a 600 N barbell over his head a distance of 2 m. How much work does he do lifting the barbell? Given: Fg= 600 N d=2 m W = ? ? ? W=Fd W = (600 N) (2 m) W = 1200 N

Ex. An Olympic weightlifter lifts 250 kg over his head a distance of 1. 5 m. How much work did the weightlifter do? Given: m = 250 kg d = 1. 5 m W = ? ? ? Fg = m g Fg = 250 kg(9. 80 m/s 2) Fg = 2450 N W=Fd W = 2450(1. 5) W = 3675 J

Power • Power is the time rate of doing work!!! • It is how fast you are at doing work!! • If you consider the definition of work and now the definition of Power, then power is how fast you transfer energy through motion!!

I am just as strong as you. We are lifting the same amount!! But I am more Powerful, because I do the work faster!!

Power Equation Where: P = Power in Watts(W) W = work in Joules(J) t = time in seconds(s) Sometimes, you may have to find work using W= F d

Ex. An elevator motor does 360, 000 J of work lifting a load of people. If the elevator ride lasts for 9 s, how much power did the motor consume? Given: W = 360, 000 J t = 9 s P=

Ex. How much power is developed by a little girl pushing her brother with a force of 175 N across the living room floor a distance of 5 m? It takes 12 s to accomplish the task! Given: F = 175 N d=5 m t = 12 s P= 72. 9 W

Mr. Garay is pushing his desk across his office a distance of 2. 5 m by applying a force of 350 N. If she does this work in 3. 2 s, how much power did he develop? A. 273. 4 W B. 448 W C. 2800 W D. 3400 W

A car’s engine exerts a force of 2500 N while pushing the car a distance of 20 m in 3 s. What power was used by the engine? A) 150, 000 W B) 16, 667 W C) 375 W D) 42 W

Ex. A weightlifter can lift a 245 kg barbell over his head a distance of 2 m in a time of 1. 7 s. How much power is developed by the weightlifter? Given: m = 245 kg d=2 m t = 1. 7 s P = ? ? What force is required to lift an object? ? ?

“John, I am coming to get you!!” “No Colonel, They drew 1 st Blood!!” What does Rambo have to do in order to prepare the arrow for the shot? Once pulled, the Bow now has what? From Where? ? Once released, the arrow now has what? From Where?

Once the What has to When the happen in order Roller to get this Coaster is Roller Coaster reaches the headed to the top of the down the hill? hill, now hill, itwhat has doeswhat? it have?

What work is required to get this box onto the table? W=Fd W = (50 N) (1. 4 m) W = 70 J

Once the box is on the table, how much potential energy does it have? PEg = Fg h PEg = 50 N(1. 4 m) PEg = 70 J

If the box fell from the table, how much kinetic energy would it have just as it hits the ground? If the Work was 70 J and the PE was 70 J, it only makes sense that the KE is 70 J!!

Law of Conservation of Energy • As long as no unbalanced forces act on a system of objects, the total energy in a system will remain the same. • Energy may change form, but it cannot be created nor can it be destroyed!!

Power Lab When do you do work as you climb stairs?

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