Work, Energy, & Power
Chapter 8
Let’s start with WORK…
• Work is only done if an object is displaced
by the force, in the same direction as the
force!
Work
• Work is the process of changing the energy of the
system
• Units: Joules
– 1,000 J = 1kJ (1kilo-Joule)
How do we do Work?
1. Changing the speed of an object
–
–
Catching a baseball
Applying brakes of your car
W = F∆d
2. By doing work against another force.
–
–
–
Doing a pull-up
Bench press
Or simply getting up out of your chair
Practice Problems
• Work is only done if the object is displaced
in the same direction as the force.
Solutions
• A: W = Fd = 100N x 5 m = 500J
100N
30°
• B: W = Fd = [100N Cos (30°)] x 5 m = 433J
• C: W = Fd = (mg) d = (15kg x 9.8m/s2) 5 m = 750J
Power
• Power is simply the rate at which work is
done.
• The faster we do Work… the more powerful
our action is
• The slower we do that same Work… the
less powerful our action is
What makes the backhoe
loader more POWERFUL?
Power
• Power is simply the
rate at which work is
done
• P=W/t
• Units: Watt
From Work to Energy
• CAUSE… WORK
• EFFECT… ∆ENERGY
Homework
• Read 8.1 - 8.2
• Complete Problems 1- 5, page 2 of packet
The Many Forms of Energy
• Energy is the ability of an object to cause a
change in itself or the environment
–
–
–
–
–
Thermal
Nuclear
Chemical
Electrical
Motion (KE)
- Radiant
- Gravitational (PE)
- Sound
- Electrochemical
Mechanical Energy
• There are two types:
• Gravitational Potential
Energy
• Kinetic Energy
Can Gravity Do Work?
• Recall…
• Of Course!
– like any force, the
gravitational force can
cause an object to be
displaced.
– We call this work,
Gravitational potential
energy
W = F∆d
Work and Gravitational Potential
Energy
– We know…W = Fd
– Or in the specific case of gravity doing the
work we know F=mg
– So… W = (mg)d
– So, we could say… W = Fd = mgh
– You might recall… PE = mgh
– Work = Fd = mgh = Gravitational Potential
Energy (PE)
Gravitational Potential Energy
(GPE)
• Known as energy of
position
• Measured in Joules
• GPE = mgh
• Example: How much
GPE does a 4500 kg
roller coaster possess
if it is poised on top of
a 48 m high hill?
Kinetic Energy
• Start with equation 7
– Vf2 = Vi2 + 2ad
• Substitute F/m in for a
(F=ma)
Kinetic Energy
• Therefore
– Vf2 = Vi2 + 2Fd/m
• Solve for Fd
– Vf2 - Vi2 = 2Fd/m
– 1/2Vf2 – 1/2Vi2 = Fd/m
– 1/2mVf2 – 1/2mVi2 = Fd
Work-Energy Theorem
• Therefore
– 1/2mV2 = Fd
• Where 1/2mV2 is the Kinetic Energy
(KE)
• Where Fd is the Work (W)
• Therefore
2
W=Fd=1/2 m∆v = ∆KE
Kinetic Energy
• Known as energy of
motion
• KE = ½ mv2
• Example: Same
rollercoaster is
traveling at 20.6
m/s. Kinetic
Energy?
The Law of Conservation of
Mechanical Energy
• Energy can not be created or destroyed. It
can only be transformed.
• One of the “Big Three” Conservation Laws
(the others being Mass and Momentum)
• In other words, if you have 1,000 J of
energy you can change it into other forms of
energy without losing any of it. Every,
single Joule is accounted for.
Transform: Change forms
You Make the Calculations:
……..and these
Homework
• Read 8.3-8.6
– (Mechanical, Potential, and Conservation of Energy)
• Complete Problems
– 1 (a, b, and c only), and 2-4, page 7 of packet