Chapter Eight: Work

8.1 Work

8.2 Efficiency and Power

8.1 Work In science, work is a

form of energy you either use or get when a force is applied over a distance.

You do 1 joule of work if you push with a force of 1 newton for a distance of 1 meter.

8.1 Work When thinking about work, remember

that work is done by forces that cause movement.

If nothing moves (distance is zero), then no work is done.

8.1 Work

W = F x dDistance (m)

Force (N)

Work (joules)

8.1 Work and energyDoing work always means transferring

energy.

The energy may be transferred to the object you apply the force to, or it may go somewhere else.

8.1 Work and energy

You can do work to increase an object’s potential energy.

Then the potential energy can be converted to kinetic energy.

8.1 Work A raised object’s potential

energy equals the amount of work it can do as it moves down.

The amount of kinetic energy an object has equals the amount of work the object can do by exerting force as it stops.

8.1 Work If force is equivalent to the

weight of the object in newtons, and

height (h) is equivalent to distance (d),

Then multiplying the weight by height gives you the amount of work the object can accomplish as it moves down (as well as its potential energy).

8.1 Work Force A does no work

because it does not cause the block to move.

Force B is applied at an angle to the direction of motion, so only part of force B does work.

The most effective force to move the block is force C.

How much work is done by a person who pulls a cart with a force of 50 newtons if the cart moves 20 meters in the direction of the force?

Solving Problems

8.2 PowerThe rate at which work is done is called power.

It makes a difference how fast you do work.

8.2 Power

Michael and Jim do the same amount of work.

Jim’s power is greater because he gets the work done in less time.

8.2 PowerPower is calculated in watts.

One watt (W) is equal to 1 joule of work per second.

James Watt, a Scottish engineer, invented the steam engine.

Jame Watt explained power as the number of horses his engine could replace.

One horsepower still equals 746 watts.

8.2 Power

P = W t Time (s)

Work (joules)

Power (watts)

Allen lifts his weight (500 newtons) up a staircase that is 5 meters high in 30 seconds.

How much power does he use?

How does his power compare with a 100-watt light bulb?

Solving Problems

Chapter Nine: Simple Machines

9.1 Types of Simple Machines

9.2 Mechanical Advantage

9.3 Levers in the Human Body

9.1 Using Machines

A machine is a device with moving parts that work together to accomplish a task.

A bicycle is a good example.

9.1 Using Machines The input includes everything you do to

make the machine accomplish a task, like pushing on the bicycle pedals.

The output is what the machine does for you, like going fast or climbing a steep hill.

9.1 Forces in Machines

A simple machine is an unpowered mechanical device, such as a lever.

9.1 Review of work Every process that is done by machines

can be simplified in terms of work:

1. work input: the work or energy supplied to the process (or machine).

2. work output: the work or energy that comes out of the process (or machine).

9.1 Review of energyA rope and pulley

machine illustrates a rule that is true for all processes that transform energy.

The total energy or work output can never be greater than the total energy or work input.

9.2 Mechanical advantageMechanical advantage is the ratio of output force to input force.

9.2 Mechanical advantageMachines multiply forces.

One person could lift an elephant—quite a heavy load—with a properly designed system of ropes and pulleys!

9.1 Mechanical Advantage

MA = Fo

Fi

Output force (N)

Input force (N)

mechanicaladvantage

9.2 The LeverA lever includes a stiff structure (the lever) that rotates around a fixed point called the fulcrum.

9.2 The LeverLevers are useful because you can arrange the fulcrum and the input arm and output arm to adjust the mechanical advantage of the lever.

9.2 The LeverEach class of levers is defined by the location of the input and output forces relative to the fulcrum.

9.2 Gears

Many machines require that rotating motion be transmitted from one place to another.

Gears change force and speed.

9.2 Designing Gear MachinesThe gear ratio is the ratio of output

turns to input turns.You can predict how force and speed are

affected when gears turn by knowing the number of teeth for each gear.

9.2 Gear Ratios

To = Ni

Ti No

Number of teethon input gearTurns of output gear

Turns of input gearNumber of teeth

on input gear

9.2 TensionRopes and strings carry tension forces

along their length.

If the rope is not moving, its tension is equal tothe force pulling on each end.

9.2 RampsA ramp is a simple machine that

allows you to raise a heavy object with less force than you would need to lift it straight up.

The mechanical advantage of a ramp is the ramp length divided by the height of the ramp.

9.2 ScrewsA screw is a rotating ramp.

You find the mechanical advantage of a screw by dividing its circumference by the lead.

9.2 ScrewsA wedge is like a ramp that

can work while in motion (a ramp is always stationary).

A wedge has a side that slopes down to a thin edge.

The mechanical advantage for a wedge is inversely related to the size of the wedge angle.

9.2 Wheel and axleA wheel rotates around a

rod called an axle.

The mechanical advantage is the ratio of the radius of the wheel to the radius of the axle.

The wheel and axle move together to move or lift loads.

Solving Problems

A crowbar is a type of lever that you use to pull a nail out of a piece of wood.

If the handle of a crowbar is 40 centimeters and the foot is 2 centimeters, what is its mechanical advantage?

9.3 Levers in the human bodyThe human body is

a complex machine that includes a number of simple machines—levers.

Your arms and legs, for example, work as levers to move and lift objects.

9.3 Levers in the human body A classic example of

a third class lever is a broom.

A broom does not multiply force, but it does multiply speed.

Since your limbs are third class levers, they multiply speed to do tasks quickly.

9.3 Levers in the human bodyHuman arms and legs

are examples of third class levers because the input forces are between a fulcrum and the output force.

The output force is what you accomplish with your hands and feet.

Where is the input force and the fulcrum?

9.3 Levers in the human bodyIn the human body, all bones act

as levers and each joint can serve as a fulcrum. When lifting your head, your neck works

as a first-class lever. When you stand on your toes, the feet act

as second-class levers. When biting, your jaw works as a third-

class lever.