UCSD Physics 10

Work, Energy and Power

UCSD Physics 10

2Spring 2008

What is Work?

• Work carries a specific meaning in physics– Formula: work = force displacement

W = F · SW = F · S cos θ

where: F is the force done, S is the displacement made and θ is the angle between the F and S

• Work can be done by you, as well as on you• Work is a measure of expended energy• Work represents a measurable change in a system, caused

by a force.• SI Unit of Work: Joule(j) = 1Nm

UCSD Physics 10

3Spring 2008

Power

• Power is simply energy exchanged per unit time, or how fast you get work done (Watts = Joules/sec)

• One horsepower = 745 W• Perform 100 J of work in 1 s, and

call it 100 W• Run upstairs, raising your 70 kg

(700 N) mass 3 m (2,100 J) in 3 seconds 700 W output!

• Shuttle puts out a few GW (gigawatts, or 109 W) of power!

UCSD Physics 10

4Spring 2008

Energy

• Energy is the capacity to do work• A system that has energy has the ability to do

work.• Energy is measured in the same units as work

because energy is transferred during the action of work.

• Energy is the “fuel” used to do work.

UCSD Physics 10

5Spring 2008

Types of Energy1. Mechanical energy - is the energy possessed by an object due to its motion or its position.- is energy than comes from movement.- Examples: a ball rolling, water flowing2. Chemical Energy- is the energy stored in the bonds of atoms and molecules. - if the bonds break then energy is released.- Examples: TNT, baking soda, and a match. Biomass, petroleum,

natural gas, propane and coal 3. Radiant energy - is also called electromagnetic energy. - is the movement of photons. - Examples: radio waves (AM, FM, TV), microwaves, X-rays, and plant

growth

UCSD Physics 10

6Spring 2008

Types of Energy4. Electrical Energy- is the movement of elections.- Electricity is the movement of electrons.- Examples: Lightning, static electricty,5. Nuclear Energy- is the energy stored in the nucleus of an atom. - structure of the atom is changed resulting in energy released. fission or

fussion- Examples: Submarines, power plants, and smoke detectors 6. Thermal Energy- is the internal energy in substances-the vibration and movement of

atoms and molecules within substance. - is created in the movement of atoms. - Examples: Boiling water, burning wood, rubbing your hands,

geothermal powerplants

UCSD Physics 10

7Spring 2008

Types of Energy

7. Sound Energy- is the movement molecules in the air that produces

vibrations. - Examples: Alarms, music, speech, ultrasound medical

equipment 8. Magnetic Energy- is the attraction of objects made of iron. - is energy made from electric fields and magnetic fields.- Examples: Medical equipment, compass, refrigerator

magnets

UCSD Physics 10

8Spring 2008

Main Types of Energy

Kinetic Energy&

Potential Energy

UCSD Physics 10

9Spring 2008

Kinetic Energy (KE or Ek)

• Energy of motion is called kinetic energy. • The kinetic energy of a moving object depends on

two things: mass and velocity.• Kinetic energy is proportional to mass.• Mathematically, kinetic energy increases as the

square of speed.• If the speed of an object doubles, its kinetic energy

increases four times. (mass is constant)

UCSD Physics 10

10Spring 2008

Kinetic Energy

Ek = 1 mv2

2

Velocity (m/sec)

Mass (kg)

Kinetic Energy (joules)

UCSD Physics 10

11Spring 2008

Potential Energy (KE or Ep)• is a type of energy that depends on the elevation of

an object with respect to the ground.• is present even if the object is not moving.• If displacement from the ground increases, the

potential energy also increases.• is also dependent on acceleration due to gravity.

UCSD Physics 10

12Spring 2008

Potential Energy

Ep = mgh Height (m)

Mass (kg)

Potential Energy (joules)

Accelerationof gravity (m/sec2)