Work, Energy, & Power Chapter 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/s 2 ) 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/s 2 ) 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/s 2 ) 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 7  V f 2 = V i 2 + 2ad Substitute F/m in for a (F=ma)

Kinetic Energy Therefore  V f 2 = V i 2 + 2Fd/m Solve for Fd  V f 2 - V i 2 = 2Fd/m  1/2V f 2 – 1/2V i 2 = Fd/m  1/2mV f 2 – 1/2mV i 2 = Fd  We change ‘d’ to ‘s’… and make sure in same direction with ‘CosΘ’

Work-Energy Theorem Therefore  1/2m  V 2 = Fs CosΘ Where 1/2m  V 2 is the Kinetic Energy (KE) Where Fs CosΘ is the Work (W) Therefore W=Fs CosΘ=1/2 m∆v 2 = ∆KE

Kinetic Energy Known as energy of motion KE = ½ mv 2 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