work, power, and simple machines. what is work? work is done when a force is exerted on an object...
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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 DF = 50ND = 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. 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 sP = 55,000Nxm(J) / 25 sP = 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.
1. A machine may change the amount of force you exert. By taking some of the force away from you.
2. The distance over which you exert your force.
3. 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
Calculating Mechanical Advantage
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 / 10NMA = 3The 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
WedgePulleyWheel 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 lever2nd class lever3rd 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 pulleyMovable pulleyBlock 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 foldableList all 6 simple machinesFor each machine show a
picture and give its definition
You will need 4 halves sheets of paper