A1.) What is Work?
A2.) What is Power?
A3.) Kinetic Energy/more work
A4.) Potential Energy
A5.) PE and KE relationship
A6.) Machines
A7.) Extras
A1.) What is Work?
"Work is the effort exerted on something that will change its energy" (Hewitt 102).
- Work = force x distance
- The work will be doubled if one or both of these is doubled. They are directly proportional to work.
- Work has two categories: work done against another force; work done to change the speed of an object.
- Work cannot be done if the force and the distance are not parallel. They must be parallel.
The unit of measurement for work is JOULES, or (newtons)(meters).
A2.) What is Power?
"Power is equal to the amount of work done per time it takes to do it" (Hewitt 103).
- Power = work/time
- Work is directly proportional to power, so if the work is greater, the power is greater.
- Time is indirectly proportional to power, so if the time interval is smaller, the power is more; if the time is bigger, the power is less.
A3.) Kinetic Energy/more work
"Kinetic Energy of an object depends on the mass of the object as well as its speed. It has energy of motion" (Hewitt 106).
- KE = 1/2(m)(v)^2
- Mass and velocity are both directly proportional to KE.
- An object cannot be at rest and have KE, as we know that KE is energy in motion.
- Work = ΔKE
- Example: when an object goes from not-moving to moving, its KE is changed...and work was done on it.
Wyatt throws the snowball with a certain force, which gives the ball velocity and goes a certain distance. The ball gains KE.
A4.) Potential Energy
"Potential energy is...energy that is stored" (Hewitt 105).
- PE = mgh
- PE = (mass)(gravity)(height)
- PE = (weight)(height)
- These are all variations of the formula. Potential energy only exists if the object has mass and is at a certain height.
- An object can have PE if it is moving; (in mid-fall).
A5.) PE and KE relationship
This picture shows how PE transfers into KE once the man falls (loses height, gains velocity).
**Let's fill in the blanks: both the kinetic and potential energies must add to equal 10,000 joules.
10,000 joules PE
0 joules KE
7,500 joules PE
2,500 joules KE
5,000 joules PE
5,000 joules KE
2,500 joules PE
7,500 joules KE
0 joules PE
10,000 joules KE
A6.) Machines
"A machine is a device for multiplying force or simply changing the direction of force" (Hewitt 110).
- Work input = Work output
- (force)(distance)in = (force)(distance)out
- F x d = F x d
~ Conservation of energy for machines: The work output cannot exceed the work input. Energy cannot be created nor destroyed. Both the work in and work out must be the same!
An ideal machine would have 100% of the work input appear as the work output.
But...whenever some energy is transformed from one form to another, (into heat, sound, vibrations, etc.), we say that the machine is inefficient.
A7.) Extras
You carried a computer down the street for 20m. How much work did you do on the computer?
- None. The force and distance aren't parallel! Gravity is the only force on the computer, and it is pulling downward.
- Work = force x distance
- Work = 20N x 12m
- Work = 240 joules
- Power = work/time
- Power = 240 joules/10s = 24 watts
- Power = 240 joules/20s = 12 watts
- Less time is more power!
~ If you use the ramp the 1st time, show how it is easier.
~ If you don't use the ramp the 2nd time, show how it is harder.
- (force in)(distance in) = (force out)(distance out)
- (10N)(3m), with ramp = (30N)(1m), no ramp
- The force input is the push/pull up the ramp; the distance input is the distance win which you pushed/pulled the box; the force output is the weight of the box; the distance output is the height of the truck.
A car is moving at 20m/s speed and requires 10m to stop. How many meters will it take to stop if the speed of the car is 40m/s?
- KE = 1/2(m)(v)^2
- 1/2(m)(20)^2 = 10m (given)
- 20^2 = 400
- 1/2(m)(40)^2 = ?
- 40^2 = 1,600
- Because 1,600 is four-times the velocity, because it is squared, we can use the formula:
- Work = ΔKE
- (force, constant)(distance) = 4x the ΔKE
- (force)(4x the distance) = 4x the ΔKE
- So...the distance required is 40m....which we found my multiplying 10m x 4.
A roller coaster can go up a large hill, small hill, and a medium hill in that order. Once released, the coaster only moves due to the law of physics. Why are the passengers able to complete the ride?
- Because energy cannot be created nor destroyed, the PE of each hill transfers into KE once it falls. When it begins to climb a hill again, the KE transfers to PE, and vice-versa, allowing for the coaster to continue traveling once it goes over each hill individually.
PART B: REFLECTION AND GOALS
I overcame this difficulty by using my quizzes and notes to re-write the answer and descriptions of each term and each problem we did together. It helped be see physics not just on paper, but also in my head.
My effort in this class has been right on target. (I wish my class was more focused though). I typically do my homework and participate every day, because the material is interesting and my classmates are fantastic! The formula/problem-solving questions are fine for me; I do each step carefully and end up with the right answer (unless I did math wrong or didn't have persistence!)
My goal for the next unit is to be better organized with my quizzes and other scattered papers. Right now, I have everything in my spiral notebook and the inside cover of my binder. Also, I want to start doing more things with other people in my class, that I don't typically talk to every day. I am excited for the next chapter!
~Connections:
- The string that pulls my window blinds up and down sometimes sways and I think about the pendulum bob.
- Walking/running up the stairs makes me think of doing extra work :(
- When I carry my blue Star Wars water bottle around-- I'm not doing work! Not parallel!
CHEERS.
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