Work, Energy, & PowerChapter 6
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
2. By doing work against another force.– Doing a pull-up– Bench press– Or simply getting up out of your chair
W =F∆sCosΘ
Practice Problems
• Work is only done if the object is displaced in the same direction as the force.
Solutions
• A: W = Fs Cos 0°= 100N x 5 m = 500J
• B: W = Fs CosΘ = [100N Cos (30°)] x 5 m = 433J
• C: W = Fs Cos 0° = (mg) d = (15kg x 9.8m/s2) 5 m = 750J
30°
100N
Solutions
• A: W = Fs Cos 0°= 100N x 5 m = 500J
• B: W = Fs CosΘ = [100N Cos (30°)] x 5 m = 433J
• C: W = Fs Cos 0° = (mg) d = (15kg x 9.8m/s2) 5 m = 750J
30°
100N
Solutions
• A: W = Fs Cos 0°= 100N x 5 m = 500J
• B: W = Fs CosΘ = [100N Cos (30°)] x 5 m = 433J
• C: W = Fs Cos 0° = (mg) d = (15kg x 9.8m/s2) 5 m = 750J
30°
100N
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
The Many Forms of Energy
• Energy is the ability of an object to cause a change in itself or the environment– Thermal - Radiant– Nuclear - Light– Chemical - Sound– Electrical - Electrochemical– Motion (KE) - Gravitational (PE)
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∆s CosΘ
Work and Gravitational Potential Energy
– We know…W = Fs CosΘ– Or in the specific case of gravity doing the
work we know F=mg – So… W = (mg)s CosΘ– So, we could say… W = FsCosΘ = mgh– You might recall… PE = mgh– Work = FsCosΘ = 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 7Vf
2 = Vi2 + 2ad
• Substitute F/m in for a (F=ma)
Kinetic Energy
• ThereforeVf
2 = Vi2 + 2Fd/m
• Solve for FdVf
2 - Vi2 = 2Fd/m
1/2Vf2 – 1/2Vi
2 = Fd/m
1/2mVf2 – 1/2mVi
2 = Fd
We change ‘d’ to ‘s’… and make sure in same direction with ‘CosΘ’
Work-Energy Theorem
• Therefore1/2mV2 = Fs CosΘ
• Where 1/2mV2 is the Kinetic Energy (KE)• Where Fs CosΘ is the Work (W)
• Therefore W=Fs CosΘ=1/2 m∆v2 = ∆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. *Read 6.4*
Transform: Change forms
You Make the Calculations:
……..and these
Homework
Chapter 6.1
•FOC #’s 1-2
•Problems #’s 1-4, 6-12