Work, Power, and Modes of Energy

Work• Scalar quantity defined as the product

of the force applied to an object and the displacement of the object.

– W=Fd

If work is scalar, can it be negative?

Units for work is JoulesJ=N·m

Yes, if the Force acting on the object is in the opposite direction of the object’s motion then

work will be negative.

Identify the following as work/non-work.

A rocket accelerates through space.

A waiter carries a tray full of meals

above his head by one arm straight

across the room at constant speed.

(Careful!)

A teacher pushing on a

brick wall.

A book falls off a table and free falls to the

ground.

Calculating Work• A 10-N force is applied to push a block across a

friction free surface for a displacement of 5.0 m to the right.

W=FdW=(10N)(5.0m)

W=50J

A force of 50 N acts on the block to move the block a horizontal distance of 3.0 m. How much work is done

by the applied force?

Graphing WorkForce vs Displacement

• If a 25 N force is applied to an object for 2 meters, how much work is done on the object?– W = Fd = – (25N)(2m) = – 50 J

• What is the area under the graph at right?

0 0.5 1 1.5 20

10

20

30

d (m)

F (N

) L x W = 2 m x 25 N =50 N·m = 50 J

Power• The rate at which work is done. (Work/Time)

– P=W/t– P=(Fd)/t– P=Fv

Metric unit for power is WattsWatts = J/s

(1000 Watts = 1 kW)

Two physics students, Will N. Andable and Ben Pumpiniron, are in the weightlifting room. Will lifts the 100-pound barbell

over his head 10 times in one minute; Ben lifts the 100-pound barbell over his head 10 times in 10 seconds. Which

student does the most work?

English unit for power is horsepower (Hp)

1 Hp=746 W

What is Energy (E)?• The ability to do work in a system.– Changes in:• Temperature• Motion• Shape

Energy is kinda like a substance:You can measure it. (Joules, calories, BTUs)

It can be stored in a variety of ways.It can be transferred.

It is universal.

System: object(s) we focus on in a problem.

Surroundings: everything else!

Kinetic Energy (K)• “moving” energy– Anything in motion has KE• Wind• Rain• Cars• People

• Dependent on m and v– But….velocity has more of an effect than mass.

Potential Energy (U)• Energy because of position.• Several Types:Gravitational Potential Energy

(UG)Energy due to Fg.

Comes from mass, height, and g.The higher off the ground, the

more UG.No height, No UG.

Elastic Potential Energy (US)Energy due to stretching or

compression.Often transferred to other

objects causing them to move.

Chemical Potential Energy

• Energy stored in the chemical bonds of atoms.– Food– Batteries– Wood– Fuel

• If it can undergo a chemical change then it has PEchem

Dissipated Energy• Energy that produces random movement of

molecules, light, or sound.– Usually due to friction or deformation of objects.– Non-reusable form of energy.

Where does all

that energy

go?