Chapter 5Work, Energy and Power

What You Need to Know

Alta Physics

Problem Types Work Work at an angle Kinetic Energy Gravitational Potential Elastic Potential Conservation Power

Work The Physics definition of work requires

a displacement, i.e. an object must be moved in order for work to be done!

The Applied force which causes the displacement contributes to the work, i.e. in order to contribute to the work, the applied force must be parallel to the displacement.

Work: A Mathematical Definition

Work = (Force)(Displacement) Units of Work = (Newton)(Meter) 1 Newton•Meter = 1 Joule A Joule is a unit of Energy and it

takes energy to do work and work done on an object either causes it to move (kinetic energy) or is stored (potential energy)

Sample Problem What work is done sliding a 200

Newton box across the room if the frictional force is 160 Newtons and the room is 5 meters wide?

W = Fk • d = (160 N)(5 m)

800 Joules

Sample Problem 5A How much work is done on a

vacuum cleaner pulled 3.0 m by a force of 50.0 N at an angle of 30.0o above the horizontal?

Sign of Work is Important Work is a scalar quantity and can be

positive or negative 0o – 90o = (+) work 90o – 180o = (-) work 180o – 270o = (-) work 270o – 360o = (+) work

Net work (+) the object speeds up Net force does work on the object

Net work (-) the object slows down Work is done by the object on the other

object

Energy Facts There are different types of energy Energy of all types is measured in

Joules Law of Conservation of Energy –

Energy can be neither created nor destroyed, merely changed from one form to another

Types of Energy Mechanical Potential Energy

Energy of Position Gravitational Elastic

Kinetic Energy Energy of Motion

If it moves it has kinetic energy Heat Energy

Heat is a form of Energy Transfer Other Forms of Stored Energy

Chemical Fuels - usually release energy by combustion Food – energy released by digestion

Electrical Generated from other forms of energy

Kinetic vs. Potential Energy

Energy of a Rollercoaster

Kinetic Energy Kinetic Energy is energy of Motion

Any moving object has kinetic energy Dependent on the mass of the object

and its velocity. Mathematically expressed as:

KE = ½ mv2

Sample Problem What is the kinetic energy of a car

with a mass of 2000 kg moving at 30 m/s?

KE = ½ mv2 = (½)(2000 kg)(30 m/s)2

= 900,000 Joules

Sample Problem 5B A 7.00 kg bowling ball moves at

3.00 m/s. How much kinetic energy does the bowling ball have?

How fast must a 2.45 g table-tennis ball move in order to have the same kinetic energy as the bowling ball?

Work-Kinetic Energy Theorem Work done by a net force acting on

an object is equal to the change in the kinetic energy of the object.

Wnet = ∆KE

Wnet = KEf - KEi

Must include all the forces that do work on the object when calculating the net work done.

Sample Problem 5C On a frozen pond, a person kicks a

10.0 kg sled, giving it an initial speed of 2.2 m/s. How far does the sled move if the coefficient of kinetic friction between the sled and the ice is 0.10?

Energy of Position:Gravitational Potential

Energy Potential energy is stored energy Occurs due to the accelerating force of

gravity Is determined by the position of the object

in the gravitational field Mathematically determined by: PE = mgh

where m is mass, g is the acceleration due to gravity and h is the height above a determined baseline.

Sample Problem What is the potential energy of a

10.0 kg rock sitting on a cliff 30.0 meters high?

PE = mgh = (10 kg)(9.8 m/s2)(30 m)

2940 Joules

Elastic Potential Energy Bungee cords, rubber bands, springs

any object that has elasticity can store potential energy.

Each of these objects has a rest or “zero potential” position When work is done to stretch or

compress the object to a different position elastic potential energy is stored

Elastic Potential Energy

Top picture is “rest position”; x = 0 This is a point where the elastic potential energy = 0

Bottom picture is “stretched position” Here elastic potential energy is stored in the spring PE = ½ kx2 where k is the “spring constant” in N/m

Spring Constant Flexible spring = small spring

constant Stiff spring = large spring constant

Where Does “K” Come From?

K is measured in Newtons/meter. It is defined as the force required to displace a spring 1 meter. So:

K = F/x Often K is determined by hanging

a known weight from the spring and measuring how much it is stretched from its rest position.

Sample Problem What is the Elastic potential

energy of a car spring that has been stretched 0.50 meters? The spring constant for the car spring is 90. N/m.

PE = ½ kx2 = (½)(90 N/m)(0.5 m)2

=11 Joules

Sample Problem A spring is hung from a hook and a 10.

Newton weight is hung from the spring. The spring stretches 0.25 meters.

What is the spring constant? If this spring were compressed 0.50

meters, how much energy would be stored?

If this spring were used to power a projectile launcher, which fires a 0.20 kg projectile, with what velocity would the projectile leave the launcher? Assume 0.50 m compression. (PE = KE)

SolutionK = F/x

K =10. N/0.25 m = 40 N/m

PE = ½ Kx2

PE = ½ (40. N/m)(0.50 m)2 = 5.0 Joules

PE = KE = ½ mv2

5.0 Joules = ½ (0.20 kg)(v2)V = 7.1 m/s

Sample Problem 5D A 70.0 kg stuntman is attached to a

bungee cord with an unstretched length of 15.0 m. He jumps off a bridge spanning a river from a height of 50.0 m. When he finally stops, the cord has a stretched length of 44.0 m. Treat the stuntman as a point mass, and disregard the weight of the bungee cord. Assuming the spring constant of the bungee cord is 71.8 N/m, what is the total potential energy relative to the water when the man stops falling?

Work is Exchange of Work is Exchange of EnergyEnergy EnergyEnergy is the is the capacitycapacity to do work to do work Two main categories of energyTwo main categories of energy

Kinetic Energy: Energy of motion A moving baseball can do work A falling anvil can do work

Potential Energy: Stored (latent) capacity to do work Gravitational potential energy (perched on cliff) Mechanical potential energy (like in compressed

spring) Chemical potential energy (stored in bonds) Nuclear potential energy (in nuclear bonds)

Energy can be converted between typesEnergy can be converted between types

Conversion of EnergyConversion of Energy

Falling object converts gravitational potential Falling object converts gravitational potential energy into kinetic energyenergy into kinetic energy

Friction converts kinetic energy into Friction converts kinetic energy into vibrational (thermal) energyvibrational (thermal) energy makes things hot (rub your hands together) irretrievable energy

Doing work Doing work onon something changes that something changes that object’s energy by amount of work done, object’s energy by amount of work done, transferring energy transferring energy fromfrom the agent the agent doingdoing the the workwork

Energy is Conserved!Energy is Conserved!

The total energy (in all forms) in a “closed” The total energy (in all forms) in a “closed” system remains system remains constantconstant

This is one of nature’s “conservation laws”This is one of nature’s “conservation laws” Conservation applies to:

Energy (includes mass via E = mc2) Momentum Angular Momentum Electric Charge

Conservation laws are fundamental in physics, Conservation laws are fundamental in physics, and stem from symmetries in our space and timeand stem from symmetries in our space and time

Conservation of Energy Formula KEi + PEi = KEf + PEf

Your book calls this the change in Mechanical Enegy (MEi = MEf)

Kinetic/Potential Energy Before = Kinetic/Potential Energy After

In a wild shot, Bo flings a pool ball of mass m off a 0.68 m high pool table, and the ball hits the floor with a speed of 6.0 m/s. How fast was the ball moving when it left the pool table?

Frank, a San Francisco hot dog vendor, has fallen asleep on thejob. When an earthquake strikes, his 300 kg hotdog cart rolls up anddown a street. It passes the top of hill A, which is 50.0 m high, at aspeed of 8.00 m/s. How fast is the hot dog cart going to pass the top of the next hill ﴾B﴿, which is 30.0 m high when Frank finally wakes up and starts to run after it?

50.0 m30.0 m

A

B

Sample Problem 5E Starting from rest, a child zooms

down a frictionless slide from an initial height of 3.00 m. What is her speed at the bottom of the slide? Assume she has a mass of 25.0 kg.

Energy Conservation Energy Conservation DemonstratedDemonstrated

Roller coaster car lifted to initial height (energy in)Roller coaster car lifted to initial height (energy in) Converts gravitational potential energy to motionConverts gravitational potential energy to motion Fastest at bottom of trackFastest at bottom of track Re-converts kinetic energy back into potential as it Re-converts kinetic energy back into potential as it

climbs the next hillclimbs the next hill

Kinetic EnergyKinetic Energy

The kinetic energy for a mass in motion isThe kinetic energy for a mass in motion isKE = ½mv2

Example: 1 kg at 10 m/s has 50 J of kinetic energyExample: 1 kg at 10 m/s has 50 J of kinetic energy Ball dropped from rest at a height Ball dropped from rest at a height hh (PE = (PE = mghmgh) hits ) hits

the ground with speed the ground with speed vv. Expect . Expect ½½mvmv22 = = mghmgh h = ½gt2

v = gt v2 = g2t2

mgh = mg(½gt2) = ½mg2t2 = ½mv2 sure enough Ball has converted its available gravitational

potential energy into kinetic energy: the energy of motion

Kinetic Energy, cont.Kinetic Energy, cont.

Kinetic energy is proportional to Kinetic energy is proportional to vv22…… Watch out for fast things!Watch out for fast things!

Damage to car in collision is proportional to v2

Trauma to head from falling anvil is proportional to v2, or to mgh (how high it started from)

Hurricane with 120 m.p.h. packs four times the punch of gale with 60 m.p.h. winds

Energy Energy Conversion/Conservation Conversion/Conservation ExampleExample

Drop 1 kg ball from 10 mDrop 1 kg ball from 10 m starts out with mgh = (1 kg)(9.8

m/s2)(10 m) = 98 J of gravitational potential energy

halfway down (5 m from floor), has given up half its potential energy (49 J) to kinetic energy

½mv2 = 49 J v2 = 98 m2/s2 v 10 m/s at floor (0 m), all potential energy is given

up to kinetic energy ½mv2 = 98 J v2 = 196 m2/s2 v = 14

m/s

10 m

8 m

6 m

4 m

2 m

0 m

P.E. = 98 JK.E. = 0 J

P.E. = 73.5 JK.E. = 24.5 J

P.E. = 49 JK.E. = 49 J

P.E. = 24.5 JK.E. = 73.5 J

P.E. = 0 JK.E. = 98 J

Power Power = Work/time

= Joules/Second (Watt)

Mathematically there are two formulas for Power:

tdF

P or since FVP v

td

then

Sample Problem What power is developed by a 55

kg person who does 20 chin ups, h = 3 m, in 45 seconds?

P= w/t = FΔd/t = mgh/t (20(55 kg)(9.8 m/s2)(3 m))/45 sec

= 718.6 Watts= 700 Watts

Sample Problem 5F A 193 kg curtain needs to be

raised 7.5 m, at a constant speed, in as close to 5.0 s as possible. The power ratings for three motors are listed as 1.0 kW, 3.5 kW, and 5.5 kW. Which motor is best for the job?

QOD

1. Mike pulls a sled across level snow with a force of 225 N along a rope that is 35.0o above the horizontal. If the sled moves a distance of 65.3 m, how much work did Mike do?

2. A student librarian picks up a 22 N book from the floor to a height of 1.25 m. He carries the book 8.0 m to the stacks and places the book on a shelf that is 0.35 m high. How much work was done on the book?

3. A 188 W motor can lift a load at the rate of 6.50 cm/s. What is the maximum load (kg) this motor can lift at this rate?