Mechanical Systems
Machines are tools that help humans do work
Machines help people use energy more effectively.
Machine :
A device that helps us to do work.
An example of technology developing is a combine harvester.
Simple Machines – Meet Human Needs
Early machines
1) Were very simple devices
2) Depended on people & animals for their source of energy.
3) Example: Plow, windmill, watermill
How did earlier civilizations get water to their homes?
Roman Aqueducts
Used for transporting water for many kilometres to supply cities.
Aqueduct contain 3 basic parts:
Pump – raise water into reservoirs.
Channels – on a slope to carry the water.
Distribution system – distributes water within a city.
Sakia (Persian wheel)
Series of buckets attached to a long rope, draped over a wheel.
Wheel is turned by animals which raises the buckets of water.
After water is raised it is stored in tanks.
Gravity moves water through pipes and into homes.
Archimedes Screw
A screw picks up the water and carries it up to the top of the tube.
Originally powered by hand, then later by gas or electric motors.
Leonardo da Vinci later used 2 Archimedes screws to increase efficiency
Modern day water pumps use a motor connected to a diaphragm, however some still use the archimedes screw method.
Simple Machines
Simple Machine:
A tool or device made up of one basic machine.
There are 6 simple machines that help us do work.
Each machine has its own advantages and disadvantages.
A simple machine can increase or change the direction of the force that you apply. But, the cost is that the force the user applies must move farther than the load.
1) Lever:
A rigid bar or plank that can rotate around a fixed point called a pivot or fulcrum.
Enables the user to move a larger load than without.
But the user must move a greater distance than the load.
3 types of Levers:
First class lever – fulcrum between the load and the point of effort.
Second class lever – load is between the effort and the fulcrum.
Third class lever – has the effort between the load and the fulcrum.
2) Inclined Plane:
A flat surface that is at an angle to another flat surface, such as the ground.
Enables the user to move a larger load than without.
But the user must move a greater distance than the load.
The ramp cannot be too steep in order to work.
3) Wedge:
Similar to an inclined plane, but is forced into an object.
By pressing on the wide end, the narrow end splits the object.
Can only be used in one direction, to push objects apart.
Enables the user to apply a greater force on an object.
But the user must move a greater distance than the split.
4) Screw:
Consists of a cylinder with a groove cut in a spiral on the outside.
Can penetrate materials using a relatively small force.
Convert rotational motion to linear motion.
Most screws move objects very slowly.
5)Pulley:
Made up of a wire, rope, or cable moving on a grooved wheel.
May be made up of one or many wheels.
Can be fixed in place or movable.
Block and tackle:
A type of rope and pulley that uses blocks with many pulleys for even more lifting strength.
Each time a pulley is added to the block and tackle mechanism the lifting force increases.
Input Force x Number of pulley segments = Output Force
Ex. The block and tackle image shows a load of 100N being lifted by pulling at 25 N. The total number of pulleys is 4. Each pulley segment adds a mechanical advantage of 1. The 4 pulley segments pulling add up to 100 N. Hey, thats the same force as the load.
6) Wheel and Axle:
A machine made up of two wheels of different diameters that turn together. The larger wheel is considered the ‘wheel’ while the smaller wheel is considered the ‘axle’.
When the simple machine turns around once, we call it one revolution
Force is usually linear it’s either a push or pull, however when force follows a circular motion it is called torque
During one revolution the larger wheel produces a longer motion, than the smaller axle which produces a shorter motion.
Increasing distance
If force is applied the smaller wheel torque decreases but rotational speed and distance travelled increases
Ex. Bicycle pedals and wheel.
Increasing force
If force is applied the larger wheel torque increases but rotational speed and distance travelled decreases
Ex. Hex key (Allen Key)
ACTIVITY
For each type of simple machine write down two examples that you could find in your house (hint: kitchens and garages are full of simple machines)
Complex Machines – Simple machines working together
Why complex machines?
As larger communities developed, newer more complicated machines developed.
New larger energy sources like coal, oil, and electricity combined with new technologies, caused an industrial revolution.
This led to an increase in people’s standard of living.
But has also led to people now being dependent on technology.
Complex Machines:
A system in which simple machines all work together.
System:
A group of parts that work together to perform a function.
Ex. Bicycle (which simple machines does a bicycle employ)
Subsystem:
A smaller group of parts in a complex machine with one function.
(Ex) Car - braking and steering
Name all the subsystems of a bike!
Think of a household item that is a complex machine!
Subsystems that Transfer Forces
Linkage:
A belt or chain to transfer energy from a energy source to an object.
(Ex) bicycle chain
Transmission:
A special type of linkage for transferring energy from the engine to the wheel in large vehicles such as cars or trucks.
more useful when larger loads must be moved.
Also known as a gearbox, or gear train
Gears:
A pair of wheels with teeth that interlink; when they rotate together, one gearwheel transfers turning motion and force to the other.
gear wheels work together in gear trains (2 or more gears).
gears can change the speed, force, and direction of motion
gears will alternate the direction of spin, if a driving gear spin clockwise the adjacent driven gear will spin counterclockwise
gears integrate with other gears , while gears that integrate with a linkage (ex. gears on a bike) are called sprockets
DRIVING GEAR:
Gear that has original force applied to it. (Can also be called the ‘driver gear’)
DRIVEN GEAR:
Gear that receives the force after.
How do gears affect speed?
multiplying gears:
When driving gear is larger than the driven gear = The turning speed in the system increases.
The larger driving gear will make one rotation while the smaller driven gear will make many rotations
reducing gears:
When the driving gear is smaller than the driven gear = The turning speed in the system decreases.
The smaller driving gear will make many rotation while the larger driven gear will make one rotation
mechanical advantage
Aka How well a machine increases force
Simple machines can be used to increase force. For example a winding road is actually a series of inclined planes with switchbacks. This allow cars to drive up a steep hill using less force, however the trade-off is that the car must travel a greater distance.
SWITCHBACKS OF LOMBARD STREET SAN FRANCISCO
Mechanical Advantage :
Amount by which a machine can multiply a force. Also called the Force Ratio.
Input Force:
Force applied to the machine.
Output Force:
Force the machine applies to the object.
Force is measured in Newtons (N).
Mechanical Advantage (MA) = Force Output (N) / Force Input (N)
EX 1
To pull a weed out of a garden, you can apply a force of 50 N to the shovel. The shovel applies a force of 600 N to the weed. What is the mechanical advantage of the shovel?
EX 2
To pry open a soda can lid, you can apply a force of 50 N to a car key. The car key applies a force of 390 N to the lid. What is the mechanical advantage of the car key?
EX 3
To pry a wooden board off of a treehouse, you can apply a force of 50 N to a lever. The lever applies a force of 750 N to the board. What is the mechanical advantage of the lever?
A Mechanical Advantage Less Than 1
Useful for tasks that do not require a large output force. Or for increasing speed.
(Ex) Bicycle – the output force is used for speed.
(Ex) Larger gear driving a smaller gear
Example Answer Key
Example 1 - Mechanical Advantage of 12
Example 2 - Mechanical Advantage of 7.8
Example 3 - Mechanical Advantage of 15
Speed Ratio (Gear ratio)
Aka How well a machine increases speed
5 OUT OF 5 WOULD EAT HERE.
Speed:
Measures the distance an object travels in a given amount of time.
Speed = Distance (m) / Time (s)
Speed Ratio:
A measure of how the speed of the object is affected by a machine.
Certain machines increase speeds (bicycle) but reduce force
Certain machines decrease speeds (pulley) but increase force
When a device or machine uses gears we can can calculate a speed ratio for gear, it is called a gear ratio
Speed ratio describes how much faster the user is moving than the load is moving.
Speed Ratio (SR) = Input Distance (m) / Output Distance (m)
Gear Ratio (GR) = Teeth Amount Driven Gear / Teeth Amount Driving Gear
Ex 1
A gear mechanism of two gears is in motion. The driving gear moves a total of 12m, while the driven gear moves a total of 3m. What is the speed ratio of this mechanism?
EX 2
A pulley system is pulled a distance of 10 m, this moves a box that is attached to the pulley 1.5m. What is the speed ratio of the pulley system?
** A machine can increase or change the direction of the force that you apply. But, the cost is that the force the user applies must move farther than the load.
Example Answer Key
Example 1 - Speed Ratio of 4 (this means the driving gear moves 4 times faster than the driven gear)
Example 2 - Speed Ratio of 6.67 (this means the pulley system is being pulled 6.67 times faster than the box moves)
The Effect of Friction
FRICTION CAUSED BY A BRAKE PAD RELEASES ENERGY AS HEAT
Often in physics mechanical advantage and speed ratio are calculated in real world situations, however the math doesn’t always add up perfectly? Some forces are not being transferred by the machine, the machine loses a fraction of energy somehow?
Friction:
Force that opposes motion.
caused by the roughness of surfaces, as roughness of a surface increases so does the effect of friction.
friction creates HEAT, and some of the input force is converted to heat in machines
friction causes inefficiency in machines, however energy must be released to protect the system
How does engine oil increase efficiency of an engine?
Efficiency
Efficiency:
Measurement of how well a machine or device uses energy.
Efficiency = (Mechanical Advantage / Speed Ratio) x 100
Efficiency of a system is negatively affected by friction.
Most energy is lost and unusable (Ex. Lost as heat)
Most complex machines are very inefficient: waste energy.
Ex. A car is only 15% efficient, where does the rest of the energy go?
The Science of Work
Work:
Done when a force acts on an object to make the object move.
Movement is needed before one can say that work has been done.
Work can be referred to as energy, work is measured in Joules (J) or Newton-meters (N x m)
Amount of work done depends on 2 things:
Amount of force exerted on the object
Distance the object moved in the direction of the applied force.
Work (J) = Force (N) x Distance (m)
The joule is used in calculating work and energy!
EX 1
You apply 300 N of force in 15 m. How much work did you do?
EX 2
A forklift lifts a box 2 m by applying 1000 N of force. How much work did the forklift do?
Ex 3
You have used 1000 J of energy to apply a force of 200 N to an object. How far did that object move?
Fun fact ! 1Kg = 9.81N , I wonder how many 10 Kgs is equal to?
Power:
How much work is achieved in a certain amount of time, measured in Watts
Power (W) = Work (J) / Time (s)
EX 1
You apply 200 J to an object in 15 s. How much power did you use?
EX 2
An engine provides 5000 J of work to an axle over the period of 10 seconds. How much power does the engine have?
Ex 3
A horse pulls a wagon by working 4500 J over a time of 6 s. How much power does the horse produce?
Energy and Work
Energy and work are closely related, can not have one without the other.
Ex. Car – needs energy (gasoline) in order to work (move)
Work and Machines
Using a machine does not decrease the amount of work, it decreases the force needed.
Work output = Work input
This equation is affected by friction (just like mechanical advantage)
Calculating efficiency
Efficiency can be calculated without using Mechanical Advantage nor Speed Ratio
Efficiency = (Work Output/ Work Input) x 100
EX 1
A construction worker puts 20 J of energy in to one strike of his hammer on the head of a nail. The energy transferred to driving the nail in to the wood is 8.0 J. What is the efficiency of the construction worker's hammering?
EX 2
Mr. K spent 25 J of energy to spike a volleyball over the net. A player received that volleyball and determined that the energy transferred to the ball was 20 J of energy. What is the efficiency of Mr. K's spike?
EX 3
A particular chemical process has an energy efficiency of only 3.00%. To complete this large-scale chemical process, 140,000 J of energy is input. What is the energy output of this process?
Simple Machines Calculation Assignment
For this assignment you will be drawing out four different situations and calculating the following for each situation:
Draw the simple machine
Label the force & the direction of force
Calculate work input and work output
Calculate mechanical advantage
Calculate efficiency
Draw and calculate the following:
Robin is using a pulley to lift a box, she pulls with a force of 140 N for 1.8 m. The box weighs 20 Kg moves up a distance of 0.7 m.
Josh is using an axe to chop some wood, with a force of 60 N he drops the axe head 0.5 m onto a piece of wood. The wood splits 0.012 m apart with a force of 1015 N.
Helen is using a screw to fasten two pieces of wood together. She rotates the screwdriver once with 100 N of force, the screwdriver rotates 0.025 m. The screw bites downward into the wood with a force of 350 N and moves approximately 0.01 m into the wood.
Mark uses a second class lever to lift a load that weighs 10.5 Kg up by 35 cm. He applied a force of 50 N to the lever which moved a total of 85 cm.
Ajuante pushes down on his bicycle pedal which is connected to a large gear, the gear moves a total distance of 1m creating 250 J of work. Via a linkage the force is transmitted to a smaller gear connected to the bicycle’s back wheel. The back wheel moves a distance of 0.5 m with a force (aka torque) of 400 N.
A crane uses a steel cable with block and tackle mechanism to lift a shipping crate 8 m into the air, the crate weighs 500 kg. The force exerted by the crane on to the steel cables is 3500 N over a distance of 12m. The block and tackle mechanism contains 4 pulleys.