Work and Energy ENERGY The ability to do
Work and Energy
ENERGY The ability to do work
WORK A change in energy
Work Energy
Circular Definition!
Radiant Thermal Electrical Nuclear Chemical Sound Mechanical Magnetic
At home – check out http: //www. youtube. com/watch? v=3 ff_ AXVlo 9 U for the Singing Tesla Coil!
Potential energy + Kinetic energy = Mechanical energy
Potential energy + Kinetic energy = Mechanical energy Example of energy changes in a swing or pendulum.
2 Kinds of Mechanical Energy • Kinetic energy (KE) • Potential Energy (PE) –Gravitational PE (PEg) –Elastic PE (PEelastic)
Kinetic Energy due to motion KE = ½ 2 mv
Potential Energy due to position
Bowling Ball – Conservation of Energy
Conserving Energy
Gravitational PE PEg = mgh
A 25 kg boy runs with a speed of 6. 5 m/s. How much kinetic energy does the boy have? 530 Joules
How much potential energy does a 0. 50 kg apple have if it hangs from a tree 13 m above the ground? 64 Joules
A 220 gram fly has a kinetic energy of 55 Joules. How fast is the flying? 22 m/s
ENERGY IS CONSERVED!!
WORK W = FDs (Work = Force x displacement) Units: Joule (J) = N· m
Work is done only when components of a force are parallel to a displacement.
Force helps motion Force opposes motion Force 90° to motion Object not in motion Positive Work Negative Work No work
A student drags his book bag 15 meters across the classroom by pulling on it with a force of 5. 0 Newtons. How much work does the student do on the bag? 75 Joules
Larry carries his 35 N physics book across the room 25 m away. How much work does he do on the book? 0 Joules
Work-Kinetic Energy Theorem Wnet = DKE
POWER The rate at which work is done
W P= Dt Or…
DKE P= Dt Or…
DPE P= Dt Or…
P = Fv = Force * velocity
The SI unit of power is the Watt, named in honor of James Watt. One Watt, W, of power is the power achieved when 1. 0 J of work is done or 1. 0 J of energy is transferred in a time of 1. 0 s.
Unit of Power Watt = 1 J/s Horsepower (hp) = 746 W
How much power is generated by a motor that can lift a 5300 kg crate of bricks with a velocity of 15 m/s? 780, 000 W
How much power is generated by a car engine that does 5500 Joules of work in 35 seconds? 160 W
How long will it take a 36 Watt motor to pull a box across the floor 25 m with a force of 16 N? 11 s
Homework: Chapter 8, Hewitt: Read 8. 1 -8. 6 Do RQ 1 -13; P&C 1 -3, 5 -7; T&E 3 -8; T&S 1 -4
- Slides: 52