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Sci 8- Topics 1-2-3 Mechanical Systems
Transcript of Sci 8- Topics 1-2-3 Mechanical Systems
Levers & Inclined Planes
Lever –-is a rigid bar or plank that can rotate around a fixed point
Levers are used to reduce the force need to do a particular task.
You can move a very large load, but you must move a greater distance than the load moves.
What is Work?
work is done when a force acts on an object to make that object move.
In order to say that work is being done, there must be movement.
The formula looks like this:
W = F x d
Force is measured in Newtons and distance is measured in meters.
The resulting work unit is called a joule
named after the English scientist James Joule.
Work and Machines
There are different types of simple machines that can help us do work.
Work input is the work needed to use, or operate the machine
Work output is the work done by the machine.
youtube- how levers work 1:42
youtube- Eureka! episode 11 - inclined plane 5 mins
youtube- Eureka! episode 12 - the Lever 5 mins
Work and Friction
Friction is the reason that work input does not equal work output in real situations. Friction affects the machine’s efficiency.
Mechanical Advantage is the comparison of the force produced by a machine to the force applied to the machine.
the size of the load vs the size of the force needed to move the load
youtube- Eureka! episode 13 - MA & Friction 5 mins
Or ramp, makes it easier to move a load higher than it is, but, it has to be moved over a much longer distance.
(examples: loading ramp, wheelchair access ramp)
What is 'ergonomics'
The Wheel and Axle, Gears and Pulleys
youtube- Eureka! Episode 14 The Screw & Wheel 5mins
youtube- Eureka! Episode 15 The Pulley 5mins
A Lever That keeps on Lifting
A winch consists of a small cylinder that has a crank or handle.
The axle of the winch acts like the fulcrum, the handle is the effort arm.
Because the handle is longer than the radius of the wheel, the effort force is smaller than the load – making it act like a small lever over and over again.
The Wheel and Axle
combination of two wheels of different diameters that turn together
A longer motion on the wheel produces a more powerful motion on the axle, thus giving it a mechanical advantage
(steering wheel in a car).
Several simple machines all working together in a system are called complex machines.
A system is a group of parts that work together to perform a function.
The bicycle is a good example of a complex machine because it is a system for moving a person from one place to another.
How many simple Machines?
In the bicycle, the chain is the linkage.
The linkage is the part of the subsystem that transfers your energy from the pedals to the back wheel.
– one that drives the motion and the other will follow in the same direction.
Chains or belts form a direct link between two wheels
Machines that are more complex than a bicycle move much larger loads.
A transmission contains a number of different gears.
READ PAGE 285
READ page 286
They consist of a pair of wheels
that have teeth that interlink.
Gears are essential components of most mechanical systems
Read page 287
When they rotate together, one gearwheel transfers turning motion
and force to the other.
what happens here?
Gears transfer energy in a mechanical system.
Gear wheels – which are wheels with precisely
manufactured, identical teeth around its edge
The gear that supplies the energy is called the driving gear. The gear to which the force is directed is called the driven gear.
It can be calculated by dividing the number of driver gear teeth by the number of follower gear teeth.
One or more combinations of wheels and ropes can be
fixed in place or moveable.
Depending on the number of pulleys used,
A Pulley consists of a wire, rope, or cable
moving on a grooved wheel.
single fixed pulley!
single movable pulley!
Pulleys help you lift larger loads.
A very complex combination of pulleys, including fixed and movable is called a....
block and tackle
a block and tackle can have a large mechanical advantage.
reading page 289
292-293 in textbook for more information
Energy, Friction, and Efficiency
Work and Energy
Machines help people do things that they normally couldn’t do on their own
Work is a transfer of energy.
what type of energy is moving the windmills?
what type of energy is being created?
Examples of energy:
In the example of the bicycle – your energy (chemical energy from your food) is transferred to the pedals giving them kinetic energy, or energy of motion.
Stored energy is called potential energy.
Much of the energy for machines is stored as chemical potential energy.
Gravitational potential energy is transferred to kinetic energy in machines in a dam to generate electrical energy.
Energy cannot be created nor destroyed – It is only transformed or transferred!
let's make some energy transmittions
No Machine is 100% Efficient
An ideal machine would transfer all the energy it received to a load or to another machine. However there are no ideal machines.
Real machines lose energy. The work output of a real
machine is always less than the work input.
read pages 296
read pages 298-299
The reason that machines are never 100% efficient is because of the energy that is lost by a machine to friction.
The higher the efficiency, the better the machine is transferring energy.
Eureka! Episode 8 - Work- 5mins
Eureka! Episode 9 - Kinetic Energy
Eureka! Episode 10 - Potential Energy
Since some of the effort force put into a machine is used to overcome the frictional force of the
reducing the surface area interaction where parts rub together and good maintenance of the machine
will help to reduce friction and increase efficiency.
There are reasons why we need friction in a machine for it to perform properly.
Slipping and sliding would occur and proper gripping would be impossible without friction.
There are many places where friction is useful, including:
tires create friction with the road surface to give you grip
use rosin to provide grip on slippery metal surfaces (like rings or bars)
sweep the ice in front of the rock to decrease friction with the ice surface
A speedy review
1. The load in a wheelbarrow would be easier to lift if the handles on the wheelbarrow were shorter.
2. A teeter-totter is a Class 1 lever.
3. Every time you move one of your fingers, arms, or toes, you are using a lever.
4. When you slide a box up a ramp onto a stage, you do less work than if you were to lift the box onto the stage.
5. If you lift a box weighing 100 N a height of 5 meter into the back of a van, you have
done 500 J of work.
6. If, when using a lever, you exert an effort force of 5 N to lift a load of 10 N, your lever has a mechanical advantage of 2.
7. Friction, noise, and heat have no effect on the efficiency of machines.
8. In what unit is work measured?
9. The Canadarm functions much like a human arm. Which type of lever does it use most?
A) Class 1 lever
B) Class 2 lever
C) Class 3 lever
D) the mobile servicing system
10. Two simple machines that are part of a bicycle are _______________________ .
A) a gear and a wheel and axle
B) an inclined plane and a lever
C) an inclined plane and a wedge
D) a screw and an inclined plane