Transcript Work and Energy

Lesson #1
What is energy?
Potential Energy
• Gravitational
PE = mgh
Kinetic Energy
KE = ½ m
2
v
Example
• When you stand at the top of
the football stadium you are
12m above the ground. How
much gravitational potential
energy do you have?
Example
• A 15g bullet is traveling at
450m/s. How much kinetic
energy does it have?
Total energy before = Total energy after
(Law of conservation of energy)
Energy Conversion
Lab – Energy of a Tossed Ball
Lesson #2
Tell me what you know…
• PE
• KE
• Lab from last week
Homework Questions
What is…
• Work
• W = Fd
• Units?
No displacement, no work!
For work...
– Only portion of force that is
parallel to displacement is
considered
Is work being done?
Practice Problem
• John pulls a sled over ice with a force of
2350 N at an angle of 25. The sled
travels 25 meters before being stopped
to load wood. How much work does
John do?
What is…
• Power
• W/t
• Rate at which work is done
• Units?
What is horsepower?
FYI…Equivalent to a horse walking a 24
foot diameter circle, pulling with 180
pounds of force, 144 trips per hour
Conversion
• 1 horsepower (hp) = 746W
Practice Problem, Part 2
Vector or Scalar?
• Energy
• Work
Work is negative when...
• Force acts in the direction
opposite motion
Practice...
• Suzy attaches a rope to a box at an angle
of 33º from the horizontal. She pulls on the
rope with 350 N of force over 15 m. There
is a frictional force of 55 N.
• How much work does Suzy do on the box?
• How much work does friction do on the
box?
• What is the net work on the box?
Work-Energy Theorem
• W=KE
• When work is done on an object,
a change in kinetic energy results
Practice...
• In 1990 Roger Hickey of California
reached a speed of 35.0 m/s on his
skateboard. Suppose it took 21 kJ of
work for Roger to reach this speed
from a speed of 25.0 m/s. Calculate
Hickey’s mass.
Power Lab
Energy problems – set 2