Simple Machines

Work and Simple Machines

What is a Simple Machine? A simple machine

has few or no moving parts.

Simple machines make work easier

• A machine is a tool used to make work easier.• Simple machines are simple tools used to make

work easier.• Compound machines have two or more simplemachines working together to make work easier.

What is work?In science, the

word work has a different meaning than you may be familiar with.

The scientific definition of work is: using a force to move an object a distance (when both the force and the motion of the object are in the same direction.)

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Work or Not? According to the

scientific definition, what is work and what is not? a teacher lecturing

to her class a mouse pushing a

piece of cheese with its nose across the floor

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What’s work?A scientist delivers a speech to an

audience of his peers. A body builder lifts 350 pounds

above his head. A mother carries her baby from

room to room. A father pushes a baby in a carriage.A woman carries a 20 kg grocery

bag to her car?

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What’s work?What’s work?A scientist delivers a speech to an

audience of his peers. NoNoA body builder lifts 350 pounds

above his head. YesYesA mother carries her baby from

room to room. NoNo A father pushes a baby in a carriage.

YesYesA woman carries a 20 km grocery

bag to her car? NoNo

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Formula for work

Work = Force x Distance

The unit of force is newtonsThe unit of distance is metersThe unit of work is newton-metersOne newton-meter is equal to one jouleSo, the unit of work is a joule

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W=FDWork = Force x

Distance

Calculate: If a man pushes a concrete block 10 meters with a force of 20 N, how much work has he done?

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W=FDWork = Force x

Distance

Calculate: If a man pushes a concrete block 10 meters with a force of 20 N, how much work has he done? 200 joules200 joules(W = 20N x 10m)(W = 20N x 10m)

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PowerPower is the rate at which work is done.

Power = Work**/Time **(force x distance)The unit of power is the watt.

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Check for UnderstandingCheck for Understanding1.1.Two physics students, Ben and Bonnie, are Two physics students, Ben and Bonnie, are in the weightlifting room. Bonnie uses 50 N of in the weightlifting room. Bonnie uses 50 N of force moving a barbell over her head force moving a barbell over her head (approximately .60 m) 10 times in one (approximately .60 m) 10 times in one minute; Ben uses 50 N of force moving a minute; Ben uses 50 N of force moving a barbell the same distance over his head 10 barbell the same distance over his head 10 times in 10 seconds. times in 10 seconds.

Which student does the most work? Which student does the most work?

Which student delivers the most Which student delivers the most power?power?

Explain your answers. Explain your answers.

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Ben and Bonnie Ben and Bonnie do the same amount do the same amount of work; they apply of work; they apply the same force to lift the same force to lift the same barbell the the same barbell the same distance above same distance above their heads. their heads.

Yet, Ben is the Yet, Ben is the most powerful since most powerful since he does the same he does the same work in less time. work in less time.

Power and time Power and time are inversely are inversely proportional.proportional.

  

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2. How much power will it take to 2. How much power will it take to move a 10 kg mass at an acceleration move a 10 kg mass at an acceleration of 2 m/s/s a distance of 10 meters in of 2 m/s/s a distance of 10 meters in 5 seconds? This problem requires you 5 seconds? This problem requires you to use the formulas for force, work, to use the formulas for force, work, and power all in the correct order.and power all in the correct order.

Force=Mass x Acceleration Force=Mass x Acceleration

Work=Force x DistanceWork=Force x Distance

Power = Work/TimePower = Work/Time

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2. How much power will it take to move a 10 kg 2. How much power will it take to move a 10 kg mass at an acceleration of 2 m/s/s a distance of mass at an acceleration of 2 m/s/s a distance of 10 meters in 5 seconds? This problem requires 10 meters in 5 seconds? This problem requires you to use the formulas for force, work, and you to use the formulas for force, work, and power all in the correct order.power all in the correct order.

Force=Mass x Acceleration Force=Mass x Acceleration Force=10 x 2Force=10 x 2Force=20 NForce=20 N

Work=Force x DistanceWork=Force x DistanceWork = 20 x 10Work = 20 x 10

Work = 200 JoulesWork = 200 Joules

Power = Work/TimePower = Work/TimePower = 200/5Power = 200/5

Power = 40 wattsPower = 40 watts

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History of Work

Before engines and motors were invented, people had to do things like lifting or pushing heavy loads by hand. Using an animal could help, but what they really needed were some clever ways to either make work easier or faster.

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Simple MachinesAncient people invented simple

machines that would help them overcome resistive forces and allow them to do the desired work against those forces.

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Simple Machines The six simple machines are:

Lever Wheel and Axle Pulley Inclined Plane Wedge Screw

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Simple MachinesA machine is a device that helps

make work easier to perform by accomplishing one or more of the following functions: Increase the size of the force; OR Change the direction of the force;

OR Change the distance over which

the force is exerted.

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Mechanical Advantage It is useful to think about a machine in

terms of the input force (the force you apply to the machine) and the output force (force the machine applies to the task).

When a machine takes a small input force and increases the magnitude of the output force, a mechanical advantage has been produced.

Output force (machine) MA = Input force (you)

MA = 1 or Greater

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Mechanical Advantage Mechanical advantage is the ratio of output

force divided by input force. If the output force is bigger than the input force, a machine has a mechanical advantage greater than one.

If a machine increases an input force of 10 pounds to an output force of 100 pounds, the machine has a mechanical advantage (MA) of 10.

In machines that increase distance instead of force, the MA is the ratio of the output distance and input distance.

MA = output/input

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Remember….Remember….

No machine can increase No machine can increase bothboth the magnitude and the magnitude and the distance of a force at the distance of a force at the same time.the same time.

Machine EfficiencyNo machine can ever be 100%

efficient. This would mean that 100% of the energy put into the machine would be used by the machine to do the work.

Some of the energy put into the machine has to go into overcoming friction.

The older the machine, the more friction that has to be overcome.

Calculating MA and ME Wo (machine produces) = 40 N x 2 m = 80J (work) Wi (put into the machine) 20 N x 5 m 100J (work)

The mechanical advantage of the machine is 2, because you only look at the amount of forces. Even though the force increased, the output work is less than then input work.. Remember, you can’t get 100%, because some of the Wi goes to overcome friction. So, is the machine efficient? Let’s find out:

Mechanical Efficiency= .8 (the difference in work) x 100%

ME = 80% This means that 80% of the work put into the machine was used by the machine to do work. So, how much of the work put into the machine was used to overcome friction?

How do we know this machine is a good machine? Even though it only used 80% of the work, look at what it did to the initial force!

The Six Different Types of Simple Machines

Wheels and Axles The wheel and axle

are a simple machine

The axle is a rod that goes through the wheel which allows the wheel to turn.

The spokes are incline planes that connect the wheels.

Pulleys Pulley are wheels

and axles with a groove around the outside

A pulley needs a rope, chain or belt around the groove to make it do work

Types of PulleysA fixed pulley only changes the

direction of the force (you pull down, object goes up)

A moveable pulley increases the force.

A fixed and moveable pulley together is a compound pulley, known as a block and tackle.

Inclined Planes An inclined plane is

a flat surface that is higher on one end

Inclined planes make the work of moving things easier

Wedges Two inclined

planes joined back to back.

Wedges are used to split things.

Screws A screw is an

inclined plane wrapped around a shaft or cylinder.

The inclined plane allows the screw to move itself when rotated.

MA of an screw can be calculated by dividing the number of turns per inch.

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The Lever

A lever is a rigid bar that rotates around a fixed point called the fulcrum.

The bar may be either straight or curved.

In use, a lever has both an effort (or applied) force and a load (resistant force).

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The 3 Classes of Levers The class of a lever

is determined by the location of the effort force and the load relative to the fulcrum.

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Types of LeversTypes of Levers

To find the MA of a lever, divide the input force by the output force, or divide the length of the resistance arm by the length of the effort arm.

Levers-First Class In a first class lever

the fulcrum is in the middle and the load and effort is on either side

Think of a see-saw- Changes the direction of the force.

Levers-Second Class In a second class

lever the fulcrum is at the end, with the load in the middle

Think of a wheelbarrow. 2nd class lever increases the force.

Levers-Third Class In a third class lever

the fulcrum is again at the end, but the effort is in the middle

Think of a pair of tweezers, or a shovel or rake. The force will increase with distance.

Compound Machines Simple Machines

can be put together in different ways to make complex machinery

A compound machine is made up of 2 or more simple machines.