Chapter 4 Science .txt
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What is Work?
You do WORK when you exert a force on an object that causes the object to move some distance
Work is done on an object when the object moves in the same direction in which the force is exerted.
Examples of "Work"
- ➡If you push a child on a swing, you are doing the work on the child
- ➡If you pull your books out of your backpack, you do work on the books
- ➡If you lift a bag of groceries out of a shopping cart, you do work on the bag of groceries.
"No Work Without Motion" ➡➡➡➡➡What does this mean?!?!?!?!
To do work on an object, the object must move some distance as a result of your force.
If the object doesn't move, no work is done, no matter how much force is exerted.
"Force in the Same Direction"➡➡➡➡➡What does this mean?!?!?!?
To do work on an object, the force you exert MUST be in the SAME direction as the objects motion.
How do you calculate Work?
Work = Force x Distance
The amount of work you do when you exert a force of 1 newton to move an object a distance of 1 meter
What is Power?
The rate at which work is done
Power = the amount of work done on an object in a unit of time.
How do you calculate power?
Dividing the amount of work done by the amount of time it takes to do the work.
What is a machine?
A device that allows you to do work in a way that is easier.
Ex: Shovels & wheelbarrows
A machine makes work easier by changing at least one of what three factors?
- ➡The amount of FORCE you exert
- ➡The DISTANCE over which you exert your force
- ➡The DIRECTION in which you exert your force
The force you exert on the machine
The force the machine exerts on an object
Input force x input distance
Output force x output distance
What is a machines mechanical advantage?
The number of times a machine increases a force exerted on it
What is the efficiency of a machine?
- ➡The efficiency of a machine compares the output work to the input work
- ➡Efficiency is expressed as a percent; the higher the percent, the more efficient the machine is.
What do you need to know in order to calculate a machines efficiency?
Input work & output work of machine
How do you calculate the efficiency of a machine?
Divide the output work by the input work and multiply the result by 100%
What are the six kinds of simple machines?
- The inclined plane
- The wedge
- The screw
- The lever
- The wheel & axle
- The pulley
What is an Inclined Plane?
A flat, sloped surface
Allows you to exert your input force over a longer distance, resulting in less input force needed than the output force
What is a Wedge?
A device that is thick at one end and tapers to a thin edge at the other end.
When you use a wedge, instead of moving an object along the inclined plane, you move the inclined plane itself.
Ex:Zipper depends on wedges to close and open; Pencil sharpener, cheese grater, and shovel all make use of wedges
What is a screw?!?!
Can be thought of as an inclined plane wrapped around a cylinder. This spiral inclined plane forms the threads of the screw.
When you twist a screw into a piece of wood, you exert an input force on the screw. The threads of a screw act like an inclined plane to increase the distance over which you exert the input force. As the threads of screw turn, they exert an output force on the wood, pulling the screw into the wood. Friction between the screw and wood holds screw in place.
Ex: Bolts, light bulbs, Jar lids
What is a lever?
A rigid bar that is free to pivot, or rotate, on a fixed point.
What is the fixed point that a lever pivots around?
What are the 3 different types of Levers?
- 1st-Class Levers
- 2nd-Class Levers
- 3rd-Class Levers
- Always change the direction of the input force.
- If fulcrum is closer to the output force, these levers also increase force
- If fulcrum is closer to the input force, these levers also increase distance
- EX: Scissors, pliers, & seesaws
- Increase force, but do not change the direction of the input force.
- EX: Doors, nutcrackers, and bottle openers
- Increase distance, but do not change the direction of the input force.
- EX: fishing poles, shovels, and baseball bats
What is the wheel and axle?
- **Simple machine made of two circular or cylindrical objects fastened together that rotate about a common axis.
- **EX: Screwdriver-Handle=Wheel;Shaft=Axle
- >>Doorknob & a car's steering wheel
- **When you use a screwdriver, you apply an input force to turn the handle, or wheel; BC the wheel is larger than the shaft, or axle, the axle rotates and exerts a large output force. The wheel and axle increases your force, but you must exert your force over a long distance.
What is a pulley?
- A simple machine made of a grooved wheel with a rope or cable wrapped around it
- EX: raise a flag on flagpole; open and close window blinds
- You use a pulley by pulling on one end of the rope; input force. At the other end of the rope, the output force pulls up on the object you want to move.
IMA=# of sections of rope that support the object.
What are the 3 types of pulleys?
- Fixed Pulley
- Movable Pulley
- Block & Tackle
- Does not change the amount of force applied
- Does change the direction of the force
- Mechanical advantage = 1
- Decreases the amount of input force needed
- Does not change direction of the force
- MA = 2
Block & Tackle
- Pulley system made up of fixed and movable pulleys
- MA = 3
Lifting Levers in your Body
- Every time you move, you use a muscle
- Your muscles are attached to your bones by connecting structures called tendons
- Tendons and muscles pull on bones, making them work as levers
- The joint, near where the tendon is attached to the bone, acts as the fulcrum
- The muscles produce the input force
- The output force is used for doing work, such as lifting your hand
Working Wedges in your Body
- When you bite into an apple, you use your sharp front teeth, called incisors.
- Your incisors are shaped like wedges to enable you to bite off pieces of food.
- When you bite down on something, the wedge shape of your front teeth produces enough force to break it into pieces, just as an as splits a log.
What is a Compound Machine?
- >>A machine that utilizes two or more simple machines.
- >>The ideal mechanical advantage of a compound machine is the product of the individual ideal mechanical advantages of the simple machines that make it up.
EX: An apple peeler has 4 different machines that hold it up
- 1. The handle is the wheel and axle
- 2. The axle is also a screw that turns the apple
- 3. A wedge peels the apple's skin
- 4. To hold the machine in place, a lever can be switched to engage a suction cup
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