Download - V work energy power
Work Energy Theorem
Energy is the ability to do work. Without energy, work cannot be done.
Work
A force acting upon an object to cause a displacement
For a force to qualify it must 1. Displacement MUST happen 2. Force MUST cause the displacement
Examples of Work
a horse pulling a plow through the fields a father pushing a grocery cart down the aisle
of a grocery store a freshman lifting a backpack full of books
upon her shoulder a weightlifter lifting a barbell above her head a shot-put launching the shot, etc.
Work = Force (N) x Distance (m)W = Fd
Only the force parallel to the displacement does work
Work Force MUST be applied in the
direction of motion The unit for work is the
Newton-meter which is also called a Joule.
1 joule is equal to a force of 1 N exerted over a distance of 1 m
Work or No Work
Work or No Work? A= Work B= No Work
A teacher applies a force to a wall and becomes exhausted.
A man pushing a stalled car, but the car doesn’t move.
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 or Not?
Work Sample Problems
1. A book weighing 1.0 N is lifted 2 m. How much work was done?
W = ________ F = _________ D = ________2. It took 50 J to push a chair 5 m across the floor. With what force was the chair pushed?
W = ________ F = _________ D = _________ 3. A force of 100 N was necessary to lift a rock. A total of 150 J of work was done. How far was the rock lifted? W = ________ F = _________ D = _________
4. It took a mass of 200 kg accelerating at 4 m/s2 of force to push a car 4 m. How much work was done?
W = ________ F = _________ D = _________
Power Power is equal to the amount of work done
per unit time.
interval time
donework Power
• The unit for power is the Joule/second which is also called a Watt.
measured in watts (W)
One watt of power is used when one joule of work is done in one second.
Power
http://www.cartoonstock.com/newscartoons/cartoonists/pto/lowres/pton114l.jpg
Work/Power Relationship
Work and power are related because power is a measure of how fast work is done
More power, faster work is done Less power, slower work is done
What is horsepower?
1 horsepower = 746 Watts
A horse exerting 1 horsepower can raise 330 pounds of coal 100 feet in a minute, or 33 pounds of coal 1,000 feet in one minute, or 1,000 pounds 33 feet in one minute.
Sample Power Problems
1. A set of pulleys is used to lift a piano weighing 1,000 N. The piano is lifted 3 m in 60 s . How much power is used?
P = ________ W = ________ T= ________ 2. How much power is used if a force of 35 N is used to
push a box a distance of 10 m in 5 seconds?P = ________ W = ________ T= ________
3. What is the power of a kitchen blender if it can perform 3,750 J of work in 15 s?
P = ________ W = ________ T= ________ 4. How much work is done using a 500-watt microwave
oven for 5 minutes. P = ________ W = ________ T= ________
Work and Power Song
http://www.youtube.com/watch?v=5EsMmdaYClQ