1.) Work and Power
2.) Work and Kinetic Energy Relationship
3.) Conservation of Energy
4.) Machines
Unlike our past units, I did not learned an "expectional" amount of formulas, information, and applications to everyday life. Don't get me wrong, we still did learn some, just less than usual. However, as usual I will pour in some questions, use a dash of example problems, and sprinkle it all with my "goofballness" (according to my mother). Bon Appetite!
Part one: Work and Power
1.) What is the formula for work?
The formula is Work= Force x Distance.
2.) What is work responsible for?
Work is responsible for power.
3.) What is work measure in?
Work is measured in Joules.
4.) What relationship must the force and distance have to have in order for a force to do work on an object.
The force and the distance must be parallel for the force to do work on an object.
5.) If no distance is covered when force is applied, is there work being done?
No, there is no work being done. The force must cause a certain distance to be covered by the object.
6.) What is the formula for power?
The formula is Power = Work/ Time.
7.) What is power measured in?
Power is measured in watts.
8.) How many watts equals one horsepower?
746 watts equals one horsepower.
Examples and Practice Problems
1.) Your friend just had surgery, but they just bought a new apartment. They need to pack up their stuff and move it across the hall to the new apartment. You pack up a box and push with 200N for 5m. How much work was done?
Here is the picture to use to solve the problem....
Answer......
2.) Oh no! You forgot to do leg day at the gym! So you decide to lift the box using 200N of force. You carry the box using one hand for 10m. How much work was done on the box? (remember that when lifting the box there is work done, but as soon as you start walking...see answer below)
Here is the picture to refer to, to answer the question.....
Answer.....
No work is done on the box! This is because the force and distance aren't parallel. For work to be done, the force and distance must be parallel. They are perpendicular here as he is mixing with the box.
3.) Is there work done here? If so, explain why?
Here is a picture to refer to for the answer....
Answer.....
No, there is no work done here. While the force and distance are parallel, there is no distance covered. There must be a distance covered for work to be done.
4.) Finally, language class has ended. Mr. Alexander finished grading your test, so you walk upstairs slowly afraid to see your grade. You apply 600N of force going up the stairs which are 4 meters tall. How much work was done here?
Here is the picture to refer to to help you answer the question....
Answer.... (Keep in mind the time)
4.5) How much power was there? Use the formula you learned about in the questions above to answer the question.
Answer....
5.) Oh no! You spend to long doing a celebration dance, happy you got a 100 on the test and you are late for history on the fourth floor. You grab your text books, which you apply 20 more newtons of force on the stairs. How much work is down now?
Here is the picture to refer to for the answer....
Answer.....
5.5.) How much work was done now? Keep in mind the change in work and time
Answer....
Part two: Work and Kinetic Energy Relationship
1.) What is the formula for Kinetic energy?
2.) What is the relationship between work and energy?
3.) Where does the word energy come from? How is this related to work?
The work energy comes from the word "energy" or energy. It is the ability to do work.
4.) Where does the word Kinetic come from?
It comes from the greek word "Kinema" which means movement or kinetic energy.
Examples and Practice Problems
1.) A car is going 20m/s when the driver slams on he breaks. The car skids 6m to a stop. A while later the same car is going 40m/s on the same pavement and the driver slams on his breaks. How far does the car skid before stopping?
2.) You are lost in the woods! All if silent, then all of a sudden with the glow of the moon shining down on a 20kg wild bear. He is accelerating toward you going from 10m/s to 20m/s in 10s over a course of 100 meters. What is the change in Kinetic energy the crazed bear experiences? How much work was done? How much power does the fiercely fabulous bear do? How much was the force that caused the bear to accelerate?
Change in KE.....
Answer...
Work....
Power....
Force....
3.) Why do airbags keep you safe?
4.) Two people each exert 600N of force went up to the top of the Eiffel Tower, which is approximately 100m. One took the stairs and the other took the elevator. Which person did gravity do more work on. (Show work or explain)
Explanation....
They did the same amount of work because work= force X distance. They both exerted the same force over the same distance. Since the force and distance are the same and parallel, gravity did the same amount of work on each of them.
Work....
Part 3: Conservation of Energy
1.) What is the formula for PE (Potential Energy)?
The formula is PE = mgh (Mass X Gravity X Height).
2.) What is the relationship between PE and KE?
PE= KE at its lowest point.
3.) What is PE measured in?
It is measured in Joules.
4.) Can something be moving and have Potential Energy? If so, provide an example and your reasoning?
Yes, something can be moving and have potential energy. This is because it is the energy of position and at any position it has height.
5.) Can something be at rest and have Kinetic Energy? If so, provide an example and your reasoning?
No, it cannot be at rest and have Kinetic energy. This is because Kinetic energy depends on velocity and when at rest the velocity is zero.
6.) What does the law of the conservation of energy imply?
It implies that energy is conserved. Energy can neither be created nor destroyed, but can be changed into another form.
Examples and Practice Problems
1.) You and a friend decide to drop a 20kg watermelon from a 4m ladder. How much potential energy does the watermelon have? What will the Kinetic Energy be just before it hits the ground? What is the speed that the ball will have just before it hits the ground?
Potential Energy....
Kinetic Energy just before it hits the ground....
Speed.....
2.) You decide to ride the Hulk at Universal in Orlando, Florida. A chain takes you to the top of the hill, but once released you move only due to the laws of physics. Why are you all able to complete the ride on the roller coaster?
Here is what the roller coaster looks like....
Here is what the coaster looks like with PE and KE labels....
Answer: According to the law of the conservation of physics, energy can neither be created nor destroyed. Instead, it is converted into another form. In this situation, the PE is being converted into KE, but it is always conserved. It is for this reason why we continue to go, whether or not the next hill is taller or smaller than the previous. As long as it is smaller than the initial, we should be able to go over it.
3.) A ball pendulum has a potential energy of 40J and is 20cm taller than its lowest point. What will the KE be at the bottom of the swing? How do you know? What will the PE be at position 3? How do you know? How high from the lowest position will the ball be at position 3? How do you know?
Here is the pendulum...
The KE at the bottom will be 40J because PE= KE at it's lowest point. The Pe at the third position will be 40J because it is at the same height at when it began and PE depends on height. The ball will be 20cm from the lowest point from position 3 because you get so much energy out as you put in, therefore the height you begin at is the height you will end at.
4.) With what force does a rock that weighs 10N strike the ground if dropped from a rest position of 10m high? The question cannot be answered unless you know more. Why?
What we do know is that the work the rock does on the ground is equal to its PE before being dropped. What we don't know is the distance the ground that the rock penetrates into the ground. If we do not know the distance we cannot calculate the force. If we knew the time then using the impulse - momentum relationship relationship then we could, but we don't.
5.) Listed below is the mass and speed of three different vehicles, A, B, and C. Rank them from greatest to least.
a. Momentum (p= mv)
b. Kinetic Energy
c. Work done to bring them up to their respective speeds from rest
Here is the algebraic work.... (answers posted below this picture)
Answers...
A.) BAC
B.) CBA
C.) CBA
Part four: Machines
1.) What does a machine do? Does it decrease the amount of work put in or the amount of force used?
A machine decreases the amount of force by increasing the distance.
2.) Label where the work in and the work out is occurring on the ramp.
Answer...
3.) What is the relationship between the work in and the work out?
The relationship is work in equals work out.
4.) What is the efficiency formula?
5.) What is the ratio of work out to work in when a machine reaches maximum efficiency?
It is a 1:1 ratio.
6.) When maximum efficiency is reached, there is an absence what three things?
There is an absence of heat, light, or sound.
Examples and Practice Problems
1.) Your cat suddenly became a youtube sensation like grumpy cat. Your cat decided he would like to move across town to live in the high end kitty condos. You are pushing his 200N box up the ramp. The ramp is 10m long and goes to a height of 5 meters. How would you know the potential energy of your box and your cat's box compare? How do you know? When did you do more work, going up the ramp or lifting the box into the truck? How do you know? What is the amount of work done?
The potential energy would be the same because the change in kinetic energy is the same. I know this because work and the change in kinetic energy is the same. I know that work is the same because machines don't decrease the amount of energy needed, they reduce the amount of force applied by increasing the distance.
Diagram for the amount of work done...
Answer....
2.) On the pulley system below label where the work in and out are.
Answer...
3.) Scissors for cutting paper have long blades and short handles, whereas metal-cutting shears have long handles and short blades. Bolt cutters have very long handles and very short blades. Why is this so?
Scissors and shears are levers. The applied force is usually exerted over a short distance for scissors so that the output force is exerted over a relatively long distance. With metal cutting shears, the handles are long so that a relatively small input force is erected over a long distance to produce a large output force over a short distance.
4.) If a Ferrari were to have a 100% efficient engine, transferring all of the fuel's energy to work, would the engine be warm to the touch? Would its exhaust heat the surrounding air? Would it make any noise? Would it vibrate? Would any of its fuel go unused?
The engine wouldn't be warm to the touch. The exhaust heat wouldn't heat the surrounding air. The car wouldn't make any noise. The car wouldn't vibrate. None of the fuel would go unused. All of this is because when a machine is 100% efficient, none of the energy is lost to light, heat, or sound.
5.) Why is a 1:1 pulley system not a simple machine?
It doesn't reduce anything, the force and distance are the same.
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