 # Work, Power, and Simple Machines

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Work, Power, and Simple Machines

What is Work? Work is done when a force is exerted on an object and causes it to move in the same direction the force was exerted. Force – a push or pull

The amount of work you do is not affected by how fast or slow you do the work. If you carry a backpack up a flight of stairs, you are still carrying the same amount of weight over the same amount of distance. Time is not a factor in work.

Calculate Work Work (W) = Force(F) x Distance(D) W = F x D
Force is measured in Newtons (N) Distance is measured in meters (m) So, Work is measured in N*m or Joules (J)

What is a Joule? A joule is the amount of work you do when you exert a force of 1 Newton to move an object a distance of 1 meter.

If a 50N force was exerted lifting a plant and carrying it a distance of 0.5m, how much work was done? W = ? W = F x D F = 50N D = 0.5m

W = F x D W = (50N) (0.5m) W = 25 N*m OR 25 Joules (J)

What is the SI unit for work?
The Joule (J)

What is Power? Power is the RATE at which work is done. Rate = Time

Calculating Power Power(P) = Work(W) / time(t) P = W/t
P = Force x Distance / time Work is measured in Joules (J) Time is measured in seconds (s) Power is measure in J/s J/s = Watts

A tow truck exerts a force of 11,000N to pull a car out of a ditch
A tow truck exerts a force of 11,000N to pull a car out of a ditch. It moves the car a distance of 5 m in 25 seconds. What is the power of the tow truck? P = ? P = F x D / t OR P = W/t F = 11,000 N D = 5 m t = 25 s

P = 11,000 N x 5 m / 25 s P = 55,000Nxm(J) / 25 s P = 2,200 J/s OR watts

A watt is a relatively small unit of power.
One kilowatt (kW) = 1,000 watts (W) One horsepower = 746 watts

How Machines Do Work? What is a Machine?
A machine allows you to do work easier or more effective. HOW?

3 ways in which a machine can make work easier.
A machine may change the amount of force you exert. By taking some of the force away from you. The distance over which you exert your force. Or their direction in which you exert your force.

Remember that in order for work to be done a force must be applied.
You have an input force and an output force

Input force/Output force
The force you exert on the machine – input force. The machine in turn applies the force to the object – Output force The distance the machine moves after the input force is applied - input distance Output distance – the machine exerts a force over another distance, distance object moves

Changing force

Increases Distance

Changes direction

Mechanical Advantage The number of times a machine increases a force exerted on it. Mechanical advantage = output force/ input force MA = Fo / Fi

What is the MA if the input force of 10N is exerted on a hand-held can opener, and the opener exerts an output force of 30N? MA = ? MA = output force / input force Output force = 30N Input force = 10N

MA = 30N / 10N MA = 3 The can opener triples your input force.

Are machines actually efficient?
Efficiency – compares the output work to the input work. Efficiency is expressed as a percent. The higher the percent the more efficient the machine.

Calculating Efficiency
Efficiency = Output Work / Input Work x 100% Eff = Wo / Wi Multiplied by 100% We multiply because we end up with a decimal, so we move out decimal two places to the right.

Example efficiency You do 250,000 J of work to cut a lawn with a hand mower. If the work done by the mower is 200,000 J, what is the efficiency of the lawn mower? Efficiency = ? Eff = output work / input work x 100% Work input = 250,000 J Work output = 200,000 J

Eff = 200,000J / 250,000J x 100% .8 x 100% eff = 80%

What would your ideal efficiency of a machine be?
100% efficient

Simple Machines Inclined plane Screw lever Wedge Pulley Wheel and axle

What is a simple machine?
Remember a machine makes work easier by changing the force, the direction, or the distance the force is applies.

1. Inclined Plane A flat, sloped surface
Allows you to exert your input force over a longer distance. As a result the input force needed is less than the output force Ideal MA = Length of incline / height of incline

2. Wedge A device that is thick at one end and tapers to a thin edge at the other end. Axe Knife Door stop

3. Screw An inclined plane wrapped around a cylinder.
The MA of a screw is the length around the threads divided by the length of the screw.

4. Levers A rigid bar that is free to pivot, or rotate on a fixed point. MA = distance from fulcrum to input force distance from fulcrum to output force

Types of Levers 1st class lever 2nd class lever 3rd class lever

1st class lever

2nd class lever

3rd class lever

Wheel and Axle Made of two circular or cylindrical objects fastened together that rotate about a common axis Screw driver

6. Pulleys Made of a grooved wheel with a rope or cable wrapped around it.

Types of Pulleys Fixed pulley Movable pulley Block and tackle

Fixed pulley

Movable pulley

Block and Tackle

1.  A fork is an example of a: pulley       wedge       wheel and axle   2.  A bicycle is an example of a:  wheel and axle       wedge       lever  3.  A bathtub is an example of a/an:  pulley       screw       inclined plane  4.  A swivel chair is an example of a:  lever       screw       wedge  5.  A teeter totter on the playground is a:  lever       inclined plane       pulley  6.  You would use a pulley to:  cut food       hold pieces of wood together  help open and close venetian blinds

Simple Machines and the Human Body
Elbow Neck Ankle Wedges/mouth

Compound Machines A machine that utilizes two or more simple machines.

Homework You are to make a foldable List all 6 simple machines
For each machine show a picture and give its definition You will need 4 halves sheets of paper

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