Transcript Work and Energy

Day #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
Day #2
Total energy before = Total energy after
(Law of conservation of energy)
Practice Problem #1
What is…
• Work
• W = Fd
• Units?
No displacement, no work!
For work...
– Only portion of force that is
in direction of displacement
is considered
Is work being done?
Practice Problem #2
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 #3
Vector or Scalar?
• Energy
• Work
Work is negative when...
• Force acts in the direction
opposite motion
Practice Problem #4
Work-Energy Theorem
• W=KE
• When work is done on an object,
a change in kinetic energy results
Practice Problem #5
• 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
Energy problems – set 2