Work and Energy

??Has Work Been Done?? 1) A teacher applies a force to a wall and become exhausted 2) A book falls off a table and free falls to the ground 3) A waiter carries a tray full of meals above his head by one arm across the room 4) A rocket accelerates through space

force, displacement, and cause. 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.

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. Example: A Flag in a parade goes horizontally but the force is vertical. Therefore no work is done.

??Has Work Been Done?? 1) A teacher applies a force to a wall and become exhausted NO 2) A book falls off a table and free falls to the ground YES 3) A waiter carries a tray full of meals above his head by one arm across the room NO 4) A rocket accelerates through space YES

Work Facts The formula for work is simply force times 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.

Example: What Work is done by pushing a physics text book with a Force of 20 N a Distance of 3m in an attempt to avoid doing homework.

Example: Mr. Harper lifts up a 65kg student to a height of 0.50 m. a) What Force does Mr. Harper use to lift the student? F = Weight = mg =(65kg)(9.8 m/s2) = 637 N =640 N b) What work does he do? W=Fd=(637N)(0.5m) = 318.5 J = 320 J

Example: If two physics students are rearranging a room and they decide to move a desk across the room, a total distance of 3.0 m. If they move the desk at a constant velocity by each exerting horizontal force of 200 N. Calculate the amount of work that was done to move the desk across the room.

Example: A child ties a ball to the end of a 1 m long piece of string and swings the ball in a full circle. If the string exerts a continues force on the ball of 10 N, how much work does the string due on the ball during one full revolution? No distance. No work

Do Practice Problems Pg 225 (pdf 34) #’s 4-10

Force vs Distance Graphs If Mr. Harper was to push a square pig 4 m across the floor with a constant force of 10 N. How much work would Mr. Harper have done on the pig?

Draw a force vs Distance graph of Mr. Harper pushing Peter the pig.

Calculate the area under the graph. What does this area represent? The area under a force vs distance graph is equal to the work done by the force.

Determine the amount of work done by the changing force in the given graph. 27 J

Do Practice Problems Pg 229 (pdf 34) #’s 11-12 omit 11(d)

Positive and Negative Work Consider a weightlifter bench pressing a barbell weighing 650 N to a height of 0.55 m. There are two distinct motions, the first is when the barbell goes up and second is when the barbell is lower back down. Calculate the work done by the weightlifter during the two separate motions. Work done going up Work done going down Total work done ( 0 J )

Do Practice Problems Pg 235 (pdf 34) #’s 14-15 Section Review Pg 235 (pdf 34) #’s 1-5