Chapter 6Work and Energy

Forms of Energy

• Mechanical

• Kinetic, gravitational

• Thermal

• Microscopic mechanical

• Electromagnetic

• Nuclear Energy is conserved!

Bowling ball video Conservation of energy

• https://www.youtube.com/watch?v=mhIOylZMg6Q

• Conservation of energy

Roller Coaster Conservation of Energy

• https://www.youtube.com/watch?v=LrRdKmjhOgw

Work examples

• Pushing a heavy suitcase requires a lot of energy or WORK. The heavier the suitcase and the longer distance you push it, the more work is done.

Units of Work and Energy

SI unit = Joule1 J = 1 Nm = 1 kgm2/s2

W F x

Example 1: Work

• You push a 20kg box a distance of 2.5 meters. How much work is done?

• You measured the amount of work you put in after pulling bag of potatoes to be 50 J. You know that the potatoes weigh 10 kg and you accelerated 0.8 m/s. What was the distance you traveled.

Work• Relates force to change in energy

• Scalar quantity

• Independent of time

W rF (

rx f

rxi )

Fx cos

Example 2: Work

• A boat is put into the water from a ramp that makes an angle of 60 degrees. The boat slides a distance of 5.0 meters down the ramp. The boat weighs 4900 kg. How much work does gravity do on the boat?

How about you are pushing something and it doesn’t

move?• Are you doing any work?

• Why or why not?

• Think of the equation…

• Why do you get tired?

Work can be positive or negative

• Work is positive if the force has a component that is in the direction of motion.

• Work is negative if the force has a component that is opposite to the direction of motion.

• Work is zero if there is no motion.

Work can be positive or negative

• Man does positive work lifting box

• Man does negative work

lowering box

• Gravity does positive work when box lowers

• Gravity does negative work when box is raised

The work done by separate forces can be summed

• W total=W1+W2+W3….

• Since the work can be positive or negative, it is possible for the work to be zero (or below) even though individual amounts of work are nonzero and positive.

• Pg. 195 in the textbook

Section 6.2 and 6.3 Work and Energy

• Key terms:

• Kinetic energy

• Potential energy

• Mechanical energy

• Elastic energy

Kinetic energy

• Energy due to motion of an object. When you go down a hill you are creating more kinetic energy as the car speeds up.

• Measured in Joules (J)

• Kinetic energy is increased if mass or velocity increases.

• Think of a child vs. a large adult sliding to first base..

Kinetic Energy

Same units as work

Remember the Eq. of motion

Multiply both sides by m,

1

2mv f

2 1

2mvi

2 max

KE f KEi Fx

v f2

2vi2

2ax

KE 1

2mv2

Example 3A skater of mass 60 kg has an initial velocity of 12 m/s. He slides on ice where the frictional force is 36 N. How far will the skater slide before he stops?

Work and kinetic energy are related

• Total work done on an object is equal to the change in its kinetic energy

• 0.5(m)(vf)2 – 0.5(m)(vi)2 =Work total

Example 4

• How much work is required for a 74 kg sprinter to accelerate from rest to a speed of 2.2 m/s?

• Vi=0

• Vf=2.2m/s

• M=74kg

Potential EnergyIf force depends on distance,

For gravity (near Earth’s surface)

PE Fx

PE mgh

Example 5

• Find the potential energy of a 60kg person standing on a building that is 10meters high?

• A candy bar has a nutritional value of 880,000J. If a 80kg mountain climber eats a bar and converts all the energy to potential energy (not realistic) how much altitude can the climber gain?

Potential Energy of Spring

PE=-Fx

x

F

PE 1

2(kx)x

PE 1

2kx2

Springs (Hooke’s Law)

Proportional to displacement from equilibrium

F kx