Chapter 13Work and Power

13.1: Work is the use of force to move an object

We have learned about: Acceleration Newton Force Speed Velocity Vectors

Now we will learn: How are force and work related? How moving objects do work

Force is necessary to do work Work:

The use of force to move an object some distance

You do work only when you exert a force on an object and move it

If you want to do work, you have to use force to move something

Force, Motion, and Work Work:

Must involve a push or pull resulting in motion

Work done by force is related to the size of the force and the distance over which the force is applied: Longer distance = more work More force = more work

Work is done only by the part of the applied force that acts in the same direction as the motion of the object. (pg 420)

Calculating work: Work = force times distance

W = fd A measure of how much force is

applied over a certain distance. The distance involved is the distance

the object moved in the direction of the force

Units for work: Force = Newtons Distance =

meters Work = Newton-meter or the joule

(J)

Objects that are moving can do work:

Moving objects such as bowling balls, air particles, and water do work

Early inventions / machines used moving water or air to do work Water wheel windmills

Work transfers energy Energy:

The ability of a person or an object to do work or to cause a change

By doing work on an object, you are transferring some of your energy to the object.

Work Changes potential and kinetic energy

Kinetic energy: The energy of motion Any moving object has some kinetic energy The faster an object moves, the more kinetic

energy it has. Potential energy:

Stored energy Energy of position or shape

When you think of “shape” think about a spring being compressed

Calculating gravitational potential energy

Potential energy caused by gravity

GPE = mgh g = acceleration due to gravity

(9.8m/s2)

Units are still J (joules) because it is a measurement of energy

Calculating Kinetic Energy Kinetic energy = mass x velocity2 ÷ 2 KE = ½ mv2

Order of operations: 1st: square the velocity 2nd: multiply mass x the squared velocity 3rd: divide by 2

Units are still J (joules) because you are calculating energy

Calculating Mechanical Energy Mechanical energy:

The energy possessed by an object due to its motion or position.

The combination of an object’s potential and kinetic energy

Any object that has mechanical energy can do work on another object.

ME = PE + KE

The total amount of energy is constant

Law of conservation of energy: No matter how energy is transferred or

transformed, all of the energy is still present somewhere in one form or another

Pg 431: Skater has potential energy where? As she rolls down the ramp, what happens

to the potential energy? Kinetic energy? What energy does she have at the bottom

of the ramp?

Forms of energy: Thermal:

Energy an object has due to the motion of its molecules

Chemical: Energy stored in chemical bonds that hold

chemical compounds together Nuclear:

Potential energy stored in the nucleus of an atom

Electromagnetic: Energy associated with electrical and

magnetic interactions

13.3: Power is the rate at which work is done

ME = KE + PE Calculations for energy Work transfers energy How power is related to work and time How power is related to energy and

time Common uses of power

Power can be calculated from work and time

Power: The rate at which you do work

Calculating power: Power = Work / time P = W / t

Power is measured in watts (W) 1 watt = one joule of work done in one

second

Horsepower The amount of work a horse can do

in a minute The average horse could move 150

pounds 220 feet in 1 minute 1 horsepower is = 745 watts which

means that the horsepower is a much larger unit of measurement than the watt

Calculating power from energy

Power = energy / time P = E / t

Power is also the amount of energy transferred over a period of time

Energy = power x time Unit is still the watt (J / s)