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Machines . Chapter 7. 7:1 Goals for Learning:. Learning Targets: 1. To gain a thorough understanding of how machines make work easier. 2. To be able to explain a machine’s mechanical advantage. Success Criteria:
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Machines Chapter 7
7:1 Goals for Learning: • Learning Targets: 1. To gain a thorough understanding of how machines make work easier. 2. To be able to explain a machine’s mechanical advantage. • Success Criteria: • Using the equation for work input/output, I can calculate and explain how machines make work easier for us, citing examples in the process. • I can use the mechanical advantage equation to determine how much a machine will help me.
7-1: Why We Use Machines What Are Simple Machines?: • List some examples of machines that you have encountered today. • Based on this list, can you come up with a definition for the word “machine?” • A machine is a device that makes work easier. • Machines come in all shapes and sizes. • They tend to be named based on how they work. • What is a simple machine? • A simple machine is a device that does work with only one movement. • There are 6 types… • Can you name any?
Advantages of Simple Machines: • A machine makes work easier by changing the force you exert on it in size, direction, or both. • They help you move something that resists being moved. • Ex: Using a screwdriver to lift a paint can lid to overcome friction, or a crowbar to move a large rock, overcoming gravity.
Two forces are involved when a machine is used to do work. • The force you apply to the machine is called the effort force (Fe). • The force applied by the machine on an object to overcome a resistance is called the resistance force (Fr). • Let’s try a few examples… • A crate is lifted by a pulley system, where a rope swings over the pulley and is attached to the crate. You are on the other side of the rope, pulling on it. • Identify Feand Fr. • Can opener
There are two kinds of work to be considered when a machine is used- the work done on the machine and the work done by the machine. *Equations • Work done on the machine is called work input • Win = Fe * de • Work done by the machine is called work output. • Wout = Fr * dr
In an ideal machine there is no loss of energy, so the work input equals work output. *Equation: Win = Wout
Example: • How much force is needed if a screwdriver is moved .007 m to lift a can lid .002 m with 300 N of force?
Solution: Given: Unknown: de= .007 m Fe = ? dr = .002 m Equation: Fr = 300 N Win = Wout Fe * de =Wout = Fr * de Solution: Fe = (Fr * dr)/ de = (300 N * .002 m)/ .007 m = 85.71 N with sig. figs., 90 N
In most cases, a machine requires a greater effort distance to move an object a smaller distance, but multiplies the effort force, which helps in moving an object. • A machine can multiply your effort, but you must move it a greater distance. • Look back at the last problem, for example. • The number of times a machine multiplies the effort force is the mechanical advantage (MA). *Equation* MA = Fr/ Fe *There is no unit for mechanical advantage.
Example: • A worker applies an effort force of 1.0 x 101 N to pry open a window that has a resistance of 5.00 x 102 N. What is the mechanical advantage of the crowbar?
Answer: Given: Unknown: Fe = 10. N MA = ? Fr = 5.00 x 102 N Solution: Equation: MA = 500 N/ 10 N MA = Fr/ Fe = 50
Practice Problem: • Find the effort force needed to lift a 2.00 X 103 N rock, using a jack with a mechanical advantage of 10.?
Answer: Given: Unknown: Fr = 2000 N (3 s.f.) Fe = ? MA = 10 N (2 s.f.) Equation: Solution: MA = Fr / Fe Fe = Fr / MA = 2000/10 = 200 N
There are some machines out there that don’t multiply the effort force. • They may simply change the direction of a force. • Blinds • Since the force isn’t changed in magnitude, what would the mechanical advantage of the machine be? • Looking at the Win = Wout equation, what else could a machine do for us? • Machines could also be used to increase the distance an object moves.
7:2 Goals for Learning: Learning Targets: • To know the 6 types of simple machines. • To understand where the mechanical advantage comes from for different types of simple machines. Success Criteria: • I can identify and describe the 6 types of simple machines, citing examples of each. • I can explain how each simple machine can be used to assist me.
7:2 The Simple Machines Levers • A lever is a bar that is free to pivot, or turn, about a fixed point. • The fixed point is called the fulcrum. • The part of the lever that exerts the resistance force is called the resistance arm. • The part of the lever on which the effort force is applied is called the effort arm. • Can you come up with any examples? • Draw a picture of a seesaw or can opener and label the fulcrum, resistance arm, and effort arm.
The longer the effort arm compared to the resistance arm, the greater the mechanical advantage. • There are 3 types (classes) of levers. • These are based on the positions of the effort force, resistance force, and fulcrum. • Based on the diagrams, can you come up with an example for each?
Pulleys • A pulley is a grooved wheel with a rope or a chain running along the groove. • Pulley’s are similar to levers in that the axle acts as the fulcrum and the rope on either side are the effort and resistance arms. • Where have you seen pulleys being used?
Pulleys can be fixed or movable. • Fixed pulleys do not move and tend to just change the direction of the force. • MA is 1. • Movable pulleys change the force’s direction and magnitude, so the MA is > 1. • At the same time, the effort distance must be increased. • Block and tackle pulleys are combinations of fixed and movable pulleys. • They can have a very large mechanical advantage.
Wheel and Axle • A wheel and axle is a simple machine consisting of 2 wheels of different sizes that rotate together. • It is basically a lever attached to a shaft, with the fulcrum at the center of the axle. • The bigger the wheel, the larger the MA. • Example: Door knob, gears • The largest gear is usually the effort gear.
Inclined Planes: • If your house were raised 6 feet off of the ground, would it be easier to climb straight up a ladder, or is there an easier way? • An inclined plane would make things a lot easier. • An inclined plane is a sloped surface used to raise objects. • For these, you tend to use less force to move an object, but need to cover a greater distance.
Screw • A screw is an inclined plane that is wrapped in a helix (spiral) around a cylindrical post. • The threads form a small ramp, which slide into an object when turned.
Wedge: • A wedge is an inclined plane with 1-2 sloping sides. • Wedges are used to move through materials. • Can you think of any examples?
7:4 Using Machines Learning Targets: • To understand compound machines. • To understand machine efficiency. • Know how power is related to work and time. Success Criteria: • I can recognize the simple machines that make up a compound machine. • I can calculate the efficiency of a machine. • I can describe how work, power, and time are related.
Compound Machines: • Most machines that you use are made of a combination of simple machines. • These are called compound machines. In your groups, pick one of the below common compound machines. Then, identify as many simple machines as you can in it. • Bike • Push Lawn Mower • Pencil Sharpener • Mechanical Watch
Efficiency: • We already learned about ideal machines. • Why is it that in the real world, machines never reach their full potential? • Some of the energy put into a machine is lost as thermal energy, which was caused by friction. • Efficiency is a measure of how much of the work put into a machine is changed to useful work put out by a machine **Equation** Efficiency = (Wout/Win) x 100% • Why is a machine’s efficiency rating always less than 100%? • How can we increase the efficiency of a machine?
Practice Problem: • A sofa weighing 1500. N must be placed in a truck bed 1.0 m off the ground. A worker uses a force of 500. N to push the sofa up an inclined plane that has a slope length of 4.0 m. What is the efficiency of the inclined plane?
Answer Given: Unknown: Fr = 1500. N Efficiency = ? dr = 1.0 m Fe = 500. N de = 4.0 m Equation: Efficiency = (Wout/Win) X 100% Solution: (1500N x 1.0m)/(500N x 4.0m) x 100% = 75%
Power: • Suppose that you and a friend need to carry a stack of books up a staircase to a classroom upstairs (definitely not here in Merrill). Each book stack weighs the same as the other. It takes you 60 seconds to go up the stairs, while it takes your friend only 45 seconds. • Who did more work?
In the previous example, your friend had more power than you. • Power is the rate at which work is done. **Equation** P = W/t Power is measured in watts (W).
Example: • A figure skater lifts his partner, who weighs 450 N, 1.00 m in 3.0 s. How much power is required?
Answer: Given(s): Unknown: -F = 450 N P=? -d = 1.00 m -t = 3.0 s Equation: P = W/t Solution: P = (450*1.00)/3.0 = 150 W