Work and Power WorkEnergy Theorem l Energy is

Work and Power

Work-Energy Theorem l Energy is the ability to do work. l Work is applying a force to an object that causes displacement of the object. l The work-energy theorem states that without energy, work cannot be done.

Work l. A force acting upon an object to cause a displacement (movement in a direction) l For a force to qualify it must 1. Displacement MUST happen l 2. Force MUST cause the displacement l What are some examples of work?

Examples a horse pulling a plow through the fields l a father pushing a grocery cart down the aisle of a grocery store l l l (Hmmmm… work or a miracle? ) a freshman lifting a backpack full of books upon her shoulder l (would a junior do work like this? ) a weightlifter lifting a barbell above her head l a shot-put launching the shot, etc. l

Work or NOT? Discuss with your partner, alter the cases in which work is not done to illustrate work l A teacher applies a force to a wall and becomes exhausted. l A book falls off a table and free falls to the ground. l A waiter carries a tray full of meals above his head by one arm across the room. l A rocket accelerates through space.

Answers l No. The wall is not displaced. l Yes! The is a downward force (gravity) which acts on the book to displace it. l No. There is an upward force, and there is a horizontal displacement but the force does not cause the displacement l Yes the expelled gas is the force which accelerates the rocket through space.

Describing Work Mathematically W = fd l W = force x distance l l Symbol: W l Unit: J, Joule

Units of Work (and Energy) l The l One joule = l l Joule 1 Newton of force displacement of 1 meter 1 Joule = 1 Newton*meter Each set of units is equivalent to a force unit times a displacement unit.

Example Problem 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?

Power l Power is the rate at which work is done or the rate at which energy is transformed. work Force x distance P = time For example: If you and a friend push two boxes (identical in mass) for the same distance = YOUR WORK DONE IS EQUAL BUT: If your friend pushes the box faster than you, your friend is MORE POWERFUL!

Power l Symbol: P l Unit: W, Watt l 1 W = 1 J/s

l l Example Problem You do 900 J of work in pushing a sofa. If it took 5 sec to move the sofa, what was your power? l W = 900 J l t = 5 sec l P = ? ? Watt