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