Chapter 14

Work, Power, and Simple Machines

Work • Force acting through a distance

• Must be movement

• Work = force x distance

Newton

metersJoule

• If the direction of the movement is Not the same as the force… NO work is done!

Motion

applied force

Motion Applied force

Work

Is Work Done?

SI unit of Work

Joule = Newton x meter

(J) = N x m

A weight lifter lifts a 1600 newton barbell over his

head. The barbell is lifted to a height of 2.0 meters.

Calculate the work done.

Work = Force x Distance

A student rows a boat across a still pond with a force of 72 newtons. The student travels a distance of 13 meters Calculate the

work done.

Work = Force x Distance

Power •Involves time• How fast is work done?• Power = Work / time

Joules seconds

Watts

• Large amounts of power are measured in kilowatts

Doing work faster requires more power.

You can increase the amount of work done in a given time.

ORYou can do a given amount of work in less time.

SI unit of Power

Watt = Joule second(W) = J/s

A truck pulls a trailer at a constant velocity for 100 m

while exerting a force of 480 N for 1 minute (60 s). Calculate the work done

and the power.

Work = Force x DistancePower = Work time

Complete the Math Practice on page 415

Horse-power

• equal to 750 watts

• 1 strong horse can move a 750 N object 1 meter in 1 second

• family car = 100 hp

• 1 hp = small electric motor

• diesel train = 10,000 hp

Energy, Work, Power, and Energy Worksheet Answers

Part 1: Work and Power

1. Amy uses 20-N of force to push a lawn mower 10meters. How much work does she do?

Part 1: Work and Power

2. Frank does 2400-J of work in climbing a set of stairs. If he does the work in 6 seconds, what is his power output?

Part 1: Work and Power

3. A girl weighing 420 Newtons takes 55 seconds to climb a flight of stairs 18 meters high. What is her power output vertically?

Part 1: Work and Power

4. How much work does an elephant do while moving a circus wagon 20 meters with a pulling force of 200-N?

Part 1: Work and Power

5. A 40 N force is used to push a 2.00 kg cart a distance of 5 meters. What is the work done on the cart?

Part 1: Work and Power

6. A 900-N mountain climber scales a 100 meter cliff. How much work is done by the mountain climber?

Part 1: Work and Power

7. A small motor does applies a 200N force over 10m in 20 seconds. What is the power of the motor in watts?

Section 14:2

WORK AND MACHINES

Yes…machines do work

Machine • makes work easier increase force increase distance change the directionWork

Input

Work Output

• Work applied to the machine by you

• Work done by the machine

Work Input = Input Force x Input

Distance

Work Output = Output Force x Output

Distance

14:3 Mechanical Advantage

Number of times a machine multiplies the force applied

Ex. Cracking Pecans

Actual Mechanical Advantage

The mechanical advantage determined by measuring the actual forces acting on a machine.

Actual Mechanical Advantage = Output ForceInput Force

http://www.youtube.com/watch?v=lo_BxRDWyRE

Ideal Mechanical Advantage

The mechanical advantage in the absence of friction.

Ideal Mechanical Advantage = Input DistanceOutput Distance

http://www.youtube.com/watch?v=E59b5DSJRvo

A woman drives her car onto wheel ramps to perform some repairs. If she drives a distance of 1.8 meters along the ramp to raise the car 0.3 meter, what is the ideal mechanical advantage (IMA) of the wheel ramps?

IMA = Input Distance Output Distance

Complete the Math Practice on Page 425.

1. 6 2. 10 3. 2.5 m

Part 2: Machines and Mechanical Advantage

1.

2.

3.

Part 3: Torque

The drawing above represents a wrench. The left end of the wrench is attached to a bolt. Four equal forces of 100N are applied as indicated in the drawing.

A. A 100N force would cause the most torque if it was placed at which letter above? (Why)

B. What is the torque at A

C. What is the torque at C

4. What is the boy weight in order for the see-saw to be in equilibrium?

5. How far is the boy from the fulcrum in the see-saw in equilibrium below?

Efficiency • How much work input is used to create work output

Ex. Gas mileage = Miles driven

Gallons of gas

• Eff. = Work output x 100%

Work input

Complete the Math Practice on Page 426.

14:4Simple Machines

Does work with one movement

Six Types:√ Lever √ Wheel & Axle√ Inclined Plane √ Wedge√ Screw √ Pulley

• bar that is free to pivot around a fixed point called a ________.fulcrum

Lever

•Classified in 3 categories

http://www.neok12.com/video/Simple-Machines/zX7d4d664c41666974425163.htm

1st class: The _______ is located between the ______ and the _________.

fulcrum effort

resistance

Output Force

Fulcrum

Ex. Seesaw, crowbar

Input Force

2nd class: The ________ is located between the ______ and the _______.

resistancefulcrumeffort

Input Force

Output Force

Fulcrum Ex. Wheelbarrow, hole

punch

3rd class: The ________ is located between the ______ and the __________.resistanc

e

fulcrum

effort

Input Force

Output Force

fulcrum

Ex. Baseball bat, rake

Wheel and Axle•_______ sized wheels

rotating together.

• Ex: door knob, tires, can opener

Different

• Sloping surfa

•ce• Ex: ramp, slide

•IMA = length/height

4 45 12

Inclined Planehttp://www.neok12.com/Simple-Machines.htm

Wedge

An inclined plane that moves

Ex. Axe, sledgehammer

Screw

An inclined plane wrapped around a cylinder

Ex. Light bulb, bolts, lids

http://www.neok12.com/video/Simple-Machines/zX4b56517b586556415a5a45.htm

Let’s make our own screw Take out a sheet of

paper. Fold it at an angle. Tear along the crease. Slowly wrap it around

your pen or pencil. Turn your pen/pencil.

•_______ with a rope, chain, or cable.

•Three different types

Cylinder

PULLEYShttp://www.neok12.com/video/Simple-Machines/zX5c757c52524a4e59434f02.htm

Fixed Pulley•A wheel is

attached in a fixed location.

•Rotate in place.

•Ex. flagpole

Movable Pulley

•Attached to the moving

object.

•Ex. Sails

Pulley System

•Combines fixed and movable pulleys

•Ex. Crane

http://www.neok12.com/video/Simple-Machines/zX7a7d457d065b790405707f.htm

Compound MachinesCombination of two

or more simple machines that operate together.

Ex. Car, watch, washing machine