Chapter 8Work and Machines

Section 1: Objectives• Determine when work is being

done on an object.

• Calculate the amount of work done on an object.

• Explain the difference between work and power.

Work and Power• Work is the transfer of energy to

an object by using a force that causes the object to move in the direction of the force.

• Transfer of Energy: One way you can tell that work is being done is that energy is transferred.

Work and PowerApplying a force doesn’t always

result in work being done.

For work to be done on an object, the object must move in the same direction as the force.◦This is the same principle as forces

being applied to an object: Same direction: Add the 2 forces Different direction: Subtract the 2 forces

Work and Power

Work and Power

.

Work and PowerThe amount of work (W) done in

moving an object can be calculated by multiplying the force (F) applied to the object by the distance (d) through which the force is applied:

W F d

The unit used to express work is the newton-meter (N m), which is more simply called the joule.

Work and PowerPower is the rate at which energy

is transferred. To calculate power (P), you divide the amount of work done (W) by the time (t) it takes to do that work:

P = W / T

The unit used to express power is joules per second (J/s), also called the watt. One watt (W) is equal to 1 J/s.

Work and Power Examples

Example # 1If it takes you 10 seconds to do

150 J of work on a box to move it up a ramp, what is your power output?

P = W / T

Example # 2A light bulb is on for 12 seconds,

and during that time it uses 1,200 J of electrical energy. What is the wattage (power) of the light bulb?

P = W / T

Example # 3An object in motion is

experiencing 23 N of force while moving 15 meters. What is the amount of work being done on the object?

W = F x D

Example # 4A person uses 125 Joules of work

to lift a heavy box off the ground a distance of 1.5 meters. What is the amount of force acting on the object?

W = F x D

Power Output ExampleIt may take you longer to sand a

wooden shelf by hand than by using an electric sander, but the amount of energy needed is the same either way.

The power output is lower when you sand the shelf by hand.

So it will take you longer to sand the shelf by hand than to sand it with the electric sander.

Ch. 8 Sec. 1 Pop Quiz1) List 1 way that you can tell

work is being done.2) What direction must the object

move in order for work to be done on the object?

3) List the units for work and power.

4) List the formulas used to calculate work and power.

5) 1 watt equals ________ J/s.

Section 2: Objectives• Explain how a machine makes work

easier.

• Describe and give examples of the force-distance trade-off that occurs when a machine is used.

• Calculate mechanical advantage.

• Explain why machines are not 100% efficient.

MachinesA machine is a device that makes work

easier by changing the size or direction of a force.

Machines come in many shapes and sizes and accomplish many different tasks.◦Machines can be simple of complex.

The basic goal for a machine is to decrease the work load for a task.

Machines• The work that you do on a

machine is called work input. • The work done by the machine on

an object is called work output.

• Machines allow force to be applied over a greater distance. • This means that less force will be

needed for the same amount of work.

Machines• Machines make work easier by

changing the size or direction of the input force.

• The Force-Distance Trade Off When a machine changes the size of the force, the distance through which the force is exerted must also change.

Machines

MachinesA machine’s mechanical

advantage is the number of times the machine multiplies force.

You can find mechanical advantage by using the following equation:

◦Mechanical advantage (MA) = Output forceInput force

Machines• The less work a machine has to do

to overcome friction, the more efficient the machine is.

• Mechanical efficiency is a comparison of a machine’s work output with the work input.

• Calculating Efficiency A machine’s mechanical efficiency is calculated using the following equation:

MachinesCalculating Efficiency A

machine’s mechanical efficiency is calculated using the following equation:

Mechanical efficiency (ME) = Work outputWork input X 100

Machines• An ideal machine would be a

machine that had 100% mechanical efficiency.

• Ideal machines are impossible to build, because every machine has moving parts.

• Moving parts always use some of the work input to overcome friction.

Section 3: Objectives• Identify and give examples of

the six types of simple machines.

• Analyze the mechanical advantage provided by each simple machine.

• Identify the simple machines that make up a compound machine.

Types of Machines• A lever is a simple machine that

has a bar that pivots at a fixed point, called a fulcrum.• There are 3 types of levers:

• First-Class Levers With a first-class lever, the fulcrum is between the input force and the load.

First Class Levers

Types of MachinesSecond-Class Levers The load

of a second-class lever is between the fulcrum and the input force.

Types of MachinesThird-Class Levers The input

force in a third-class lever is between the fulcrum and the load.

Types of Machines• A pulley is a simple machine that consists of a

wheel over which a rope, chain, or wire passes.• There are 3 types of pulleys:• Fixed Pulleys A fixed pulley is attached to

something that does not move.

• Movable Pulleys Unlike fixed pulleys, movable pulleys are attached to the object being moved.

• Blocks and Tackles When a fixed pulley and a movable pulley are used together, the pulley system is called a block and tackle.

Types of Machines

Types of MachinesWhat Is a Wheel and Axle? A

wheel and axle is a simple machine consisting of two circular objects of different sizes.

Types of MachinesMechanical Advantage of a

Wheel and Axle The mechanical advantage of a wheel and axle can be found by dividing the radius (the distance from the center to the edge) of the wheel by the radius of the axle.

MA = R (of wheel) / R ( of axle)

MA Of Wheel and Axles

Types of Machines• An inclined plane is a simple machine

that is a straight, slanted surface. • Inclined planes are often called ramps.

• The mechanical advantage (MA) of an inclined plane can be calculated by dividing the length of the inclined plane by the height to which the load is lifted.

• MA = L (of inclined plane) / H (load is lifted)

MA Of Inclined Planes

Example # 1A wheel chair ramp is 9 meters

long and 1.5 meters high. What is the mechanical advantage of the ramp?

Remember: MA = L / H

Example # 2As a pyramid is built, a block of

stone is dragged up a ramp that is 120 m long and 20 m high. What is the mechanical advantage of the ramp?

Remember: MA = L / H

Example # 3If an inclined plane were 2

meters long and 8 meters high, what would be its mechanical advantage?

Remember: MA = L / H

Types of Machines• Wedges A wedge is a pair of

inclined planes that move.

• Mechanical Advantage of Wedges can be found by dividing the length of the wedge by its greatest thickness.

• MA = L (of wedge) / Greatest thickness

MA Of Wedges

Types of Machines• Screws A screw is an inclined

plane that is wrapped in a spiral around a cylinder.• This is a type of inclined plane.

• Mechanical Advantage of Screws The longer the spiral on a screw is and the closer together the threads are, the greater the screw’s mechanical advantage is.

Types of Machines• Compound machines are machines that

are made of two or more simple machines.

• Mechanical Efficiency of Compound Machines The mechanical efficiency of most compound machines is low, because compound machines have more moving parts than simple machines do. Thus, there is more friction to overcome.

Concept Map

Concept Map