Advanced Physical Science Ms Pollock 2010 2011 Work

Advanced Physical Science Ms. Pollock 2010 - 2011

Work is done when a force causes an object to move. Two conditions must be met in order for work to be done. Applied force makes object move Movement in direction of applied force

The man is lifting the box in the “up” direction. The force is moving in the “up” direction. Work is being done!

The box is being pushed on the left side in the right direction. The box moves to the right. Force is in the direction of motion, so work is being done.

� Work and energy are related because work involves a transfer of energy. � Example: Walking up stairs carrying a heavy box requires a transfer of energy. � Energy is the ability to do work. � Law of Conservation of Energy � Work done on an object increases the object’s energy.

The people carrying the box up the stairs transfer chemical energy from their muscles to the box. They worked to move the box.

� Work is the force applied over a certain distance. � W = Fd � Work = force X distance � Units: Work = joules, force = Newtons, distance = meters

� You push a refrigerator with a force of 100 N. If you move the refrigerator a distance of 5 m, how much work do you do? F = 100 N d=5 m W = Fd = (100 N) (5 m) W = 500 J

� 1) A force of 75 N is exerted on a 45 -kg couch, and the couch is moved 5 m. How much work is done in moving the couch? F = 75 N d=5 m W = Fd = (75 N) (5 m) W = 357 J

� 2) A lawn mower is pushed with a force of 80 N. If 12, 000 J of work are done in mowing a lawn, what is the total distance the lawn mower was pushed? F = 80 N W = 12, 000 J D=W (This is the derived equation. ) F D = 12, 000 J 80 N D = 150 m

� 3) The brakes on a car do 240, 000 J of work in stopping the car. If the car travels a distance of 50 m while the brakes are being applied, what is the total force the brakes exert on the car? F=W (This is the derived equation. ) d F = 240, 000 J 50 m F = 4, 800 N

Work is only being done when a force is being applied to an object. If the force is removed, the object may continue to move, but no work is being done.

When the pitcher releases the ball, no more work is being done. Force is no longer being applied to the ball.

� Power is the rate at which work is done. � Power is measured in Watts, work in Joules, and time in seconds. �P = W t Power = work ÷ time

Since energy is transferred as the runner moves up the stairs, work is done. The time necessary to do the work is recorded, so power can be calculated.

� You do 900 J of work in pushing a sofa. If it took 5 s to move the sofa, what was your power? W = 900 J t=5 s P=W t P = 900 J 5 s P = 180 W

� 1) To lift a baby from a crib 50 J of work are done. How much power is needed if the baby is lifted in 0. 5 s? W = 50 J t = 0. 5 s P=W t P = 50 J 0. 5 s P = 100 W

� 2) If a runner’s power is 130 W, how much work is done by the runner in 10 minutes? P = 130 W t = 10 minutes Convert 10 minutes to seconds. 10 min X 60 s = 600 s 1 min W = Pt = (130 W) (600 s) W = 78, 000 J THIS WAS A DERIVED EQUATION. ADD IT TO YOUR CONVERSION CARD.

� 3) The power produced by an electric motor is 500 W. How long will it take the motor to do 10, 000 J of work? P = 500 W W = 10, 000 J T=W THIS IS A DERIVED EQUATION. P T = 10, 000 J 500 W T = 20 s

� Energy can be substituted for work in power problems, as work and energy are the same thing. � Power = energy ÷ time �P = E t Energy problems will be worked the same way as power problems.

� Pages 835 – 836 # 51 – 52, 54 – 56, 61 � If you need help, email me, please! I miss you. - Ms. P