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