Today’s Plan: 1.Tests are graded 2.Energy Lecture 3.Work – Energy Lab 4.Homework: Lab and...

Today’s Plan:

1. Tests are graded

2. Energy Lecture

3. Work – Energy Lab

4. Homework: Lab and Handout

Energy Introduction

• What source/types of energy can you name?

• We focus on “mechanical energy”.

• But first…

Work = force X distance

To do work, you must:

1. Apply a force

2. Move something a distance

Unit of measure of work:

• Joule– N m

I push a box… or does the box push me?

• If I push with a steady force of 12 N for 6.0 meters– How much work did I do?

Energy = the ability to do work

• Also measured in joules.

Power = work done / time interval

That darn box…

• If I push it those 6 meters in 4 seconds…– How much power do I use?

Unit of measure for power:

• Watt1 watt = joule/second

Mechanical Energy = Energy due to position or the movement of something

Two types of Mechanical Energy:

•1. Potential Energy = energy with the potential to do work.

Kinetic Energy:

•KE = ½ mv2 •Energy of motion.

Kinetic Energy

• A 50 kg skier is moving at 10 m/s– Calculate the skier’s kinetic energy

Potential energy & gravity

• When PE is due to its height above ground…

• We call it “gravitational potential energy” (GPE).

GPE: work done against gravity

Work =Force = weight = Work = Potential Energy = Work

Thus… G.P.E. = m g h (height)

Force X distance

Mass X gravity

Mass X gravity X distance

At the gym…

• You lift a 50 kg mass from the ground to 2 meters up.– Calculate the work done and the potential

energy

• 50 kg x 9.81 m/s2 x 2 m = 981 J– Work done and PE are the same!

Power to the People Lab

• A lab for the people, by the people and of the people!

• People Power!!!

December 9 Plan of the Day

Energy and Work Notes continued…

Pulley Lab

Energy and Work Notes continued…

What is work…

force X distance

What is power…

• Work / Time

What is energy…

• Ability to do work

Quick review…

If I push on the wall am I doing any work?

If the cart is moving at a constant velocity with no force acting on it, is there any work being done?

What is the unit for work?

• Joules

What is the unit for power?

• Watts

What is the units for energy?

• Joules

Atlanta Olympics - Richard

Work and power!

If you double the distance that a force is applied…

What happens to the work?

If you cut the time it takes to do a certain amount of work in half…

What happens to the power output?

Kinetic energy

• KE depends on mass and speed of object

• KE = ½ mv2

• If you double the mass…– You double the KE

• If you double the velocity…– You QUADRUPLE the KE

Kinetic Energy

• Throw the cans out the window

• Have they lost their energy?

• KE = ½ mv2

– = ½ mass x speed2

• What are you doing when you give something a push?– What happens to the thing being pushed?

Speed is squared!

• Let’s say you are driving at 10 m/s– And slam on your brakes – – Stopping in 10 meters

• How far would you skid if you were going 20 m/s?– Assuming the same force…

• How about 30 m/s?

Work – kinetic energy connection

• If you do work on an object – you cause it to speed up. – Kinetic Energy is increased

• Work = Change in energy

• (force X distance) = Δ KE

• W = Δ ½ mv2

Work – Kinetic Energy

• Work = ΔKE– F x d = ½ mv2

• If the car has a mass of 1000 kg and is moving at 10 m/s– What is the kinetic energy?– What is the force you need to apply to stop

the car in 20 meters?

Conservation of energy: Energy is not created or destroyed – the

total amount never changes, it only changes form.

Mechanical Energy

• Imagine a rock at the bottom of a valley.– Does it have energy?

• Now imagine that rock poised at the edge of a cliff.– Does it have energy?

• How are they different?

Mechanical Energy

• The 2 most common forms:– Energy due to position– Energy due to motion

• Mechanical Energy = the sum of the 2– Potential energy– Kinetic energy

The total ME stays the same

• How much PE?– KE?

We use machines to multiply force or change direction.

In a closed, ideal system, the work that is input is the same as the

work output.

• Work in = Work out

Thus…

Forcein X Distancein = Forceout X Distanceout

What is the purpose of this pulley?

Pulleys IMA can also be calculated by counting the number of support ropes!!

Lab

• Pulleys – 5 different configurations

• Set them up and determine:– The work you do to lift the mass 0.1 m– Force (that you apply) x distance (that you

pull)

• Compare it to the work done by the work done on the mass– Its weight (force) x distance lifted (0.1 m)

December 11

Mechanical Advantage and Efficiency

Lever Lab

How many strings segments are lifting the object?

#2

And now?

#3

#4

Review – what is energy?

• The ability to do work…

Potential to Kinetic

Or a dart being shot…

In a closed, ideal system

• Work in = Work out

Let’s look at pulleys & levers…

Machines are used to multiply force or change direction of the

force.

Work in is what you put into the system. (Force and Distance)

“Work out”: the object.

• (force (weight) and distance)

Mechanical advantage…

• How much the machine multiplies the force.

• Higher the MA = greater advantage

• MA = force out / force in

Ideal Mechanical Advantage

• Assumes 100% efficient!

• IMA = distance input / distance output

IMA ignores Mr. Friction

• So your Actual MA…– Will be less than ideal

Actual Mechanical Advantage

• Takes into account energy lost…

• AMA = force output / force input

Example problem: pulley

• You pull with a force of 12 N to lift an object that weighs 30 N

• You pull the string 3 meters to lift it 1 meter

• What is your IMA and AMA?

#4

How does a lever work?

• Fulcrum

• Distances to the fulcrum

• This is a class 1 lever5 meters 2 meters

30 Newtons

15 Newtons

Type Two Lever

5 meters 10 meters

20 Newtons15 Newtons

IMA =

AMA =

Type Three Lever

5 meters 10 meters

20 Newtons15 Newtons

Efficiency...

• The percent of work you do that is transformed into useful work output

• Efficiency = work out/work in x 100%

Another way to figure it out…

• Efficiency = AMA X 100%

IMA

In the real world – efficiency must always be less than 100%.

Example Problem:

• You use a pulley to lift a 2000 Newton tarp full of leaves 10 feet.

• You only have to use a force of 800 Newtons but you have to pull the rope 30 feet.

• Calculate the AMA and IMA of the system.

• What is the efficiency of the pulley system?

To the lab!

• Exploring levers…

December15

More Machines

So back to simple machines…

Work in is what you put into the system. (Force and Distance)

Work out is the affect on the object. (force (weight) and distance)

Machines are used to multiply force or change direction of the

force.

Example Problem:

• You use a pulley to lift a 20 Newton mass 15 centimeters. You pull down 30 centimeters on the rope. With how much force do you pull down?

Example Problem:

• You push a box up an incline. You push the box a distance of 50 centimeters with a force of 90 Newtons. The box is actually 120 Newtons. How far are you actually raising the box?

Today’s Plan:

Review of Mechanical Advantage and Efficiency

Incline planes, wheel and axle, wedge

More Machines Lab

Review

• What does it mean when a machine has a high MA?

• How do you calculate efficiency?

• If you calculate your IMA– What should you anticipate about your AMA?

Example Problem

• The distance from the rock to the fulcrum is 30 centimeters and the distance from my hand to the fulcrum is 40 centimeters. What is the IMA?

Output distance

Input distance

I

Example Problem

• I hike a hill that has a vertical elevation of 3000 ft.

• The trail wraps around the mountain and I actually have to hike 12000 ft.

• What is the IMA of the trail?

Example Problem

• I have the pulley system to the right – how far does the monkey have to pull to raise the elephant 10 centimeters?

Example problem

• To push the 30 Newton box up the ramp to the truck, I have to use 10 Newtons of force. What is the AMA of the ramp?

Example Problem

• I use a lever to dislodge a 30 Newton boulder and I have to put 10 Newtons of force on the far side of the lever. What is the AMA of the lever?

Example Problem:

• You use a pulley to lift a 2000 Newton tarp full of leaves 10 feet.

• You only have to use a force of 800 Newtons but you have to pull the rope 30 feet.

• Calculate the AMA and IMA of the system.

• What is the efficiency of the pulley system?

Example Problem:

• A pulley system has 3 supporting ropes, but you determine that the actual mechanical advantage is 2.5. What is the efficiency of the pulley system?

Pulleys

• Number of support ropes can be used to determine IMA.

• Can use din/dout for IMA

• Use Fout/Fin for AMA

Inclined Plane

• Use din/dout for IMA

• Use Fout/Fin for AMA45 Newton box

Lever

• Can use din/dout for IMA

• Use Fout/Fin for AMA

Wheel and Axle

• IMA = Radius of wheel / radius of axle

• (R/r)

• Use Fout/Fin for AMA

Wedge• IMA = Length of wedge / thickness of wedge

• Use Fout/Fin for AMA

The screw

• What machine does a screw remind you of?

What happens

• When you combine machines?

Your Machine

• Groups of 2 or 3

• Design must include 3 machines with different MA

• Total IMA of the machines must be between 15 and 30

Today’s Plan:

Quiz

Review Notes over compound machines

Lab: Design, Construct, and Measure Complex Machines

Compound Machines

• The mechanical advantage of a compound machine is the product of the individual mechanical advantages.

Why are bolt cutters so effective?

Today in lab, I want:

• A good diagram of your machine – everyone must turn in a separate diagram.

• Listed neatly next to each machine, its IMA.– And the dimensions I need to verify your calculation

• Measure the AMA of the total machine. • Determine the IMA of total machine.• Determine efficiency of the machine….• Turn in lab sheet with questions and diagram

Review: Energy, Work & Machines

After winter break…

Energy, Power, Work?

• Your mass is 65 kg

• Calculate your weight

• If you climb the stairs– 6 meters…– How much energy did you use?

• What is your PE at this point?

Work II

• You push your bike to school– Applying a force of 12 N for 5 meters

• How much work do you do?

• If you start at rest, what is your kinetic energy at the end?

• How fast are you going?

Conservation of energy

• You throw a 10 kg turkey out of the second story of your house (12 m).

• What is the PE at the top?

• What is the PE and KE at 4 meters?

Power

• You accomplish 3,000 J of work in 15 seconds. What is your power use?

Machines

• An incline plane has a height of 2 meters and is 12 meters long.– What is the IMA?

• If you push a 400 N barrel up the ramp what is the minimum force you can apply?

• If you have to apply 100 N– Determine the IMA, AMA and efficiency

Draw the following machines

• Label the critical parts and distances:

• Pulley with a 4:1 IMA

• Wheel and axle with a 2:1 IMA

• Incline plane with a 5:1 IMA

• Class II lever with a 3:1 IMA