DO WORK

Welcome to Unit 6, our last unit of the semester! We have done so much work in this semester that it seems right to end off the year with learning about

Work, Energy, and Power!

Work is defined as any change in energy (or the little triangle and a big E). To find variables while working with work, we use:

Work = Force x Distance

or W = F(d)

The units of work are Newton meters (Nm), otherwise known as Joules (J).

We also learned about the Law of Conservation of Energy which states that in an isolated system energy cannot be created nor destroyed, it just changes forms. Therefore, change(in) = change(out).

There are two kinds of energies:

Gravitational Potential Energy (PEg) - Energy of Position

Gravitational potential energy = mass x gravity x height

PEg = mgh

and...

Kinetic Energy (KE) - Energy of Motion

Kinetic energy = 1/2 x mass x velocity squared

KE = 1/2mv^2

The next thing we talked about was POWER. Power is the rate at which work is being done.

Power = change in energy/time

Power = work/time

The units of power are Joules/seconds, otherwise known as Watts.

When analyzing a force x distance graph, you should always remember: The area under the curve of a force x distance graph is WORK.

The last aspect of this unit that we covered so far was springs and elastic potential. The equation when dealing with springs is:

PEs = 1/2kd^2

where PEs is the elastic potential, k is the spring constant, and d is the distance stretched.

The bigger the spring constant, the harder it is to pull or stretch the spring. Therefore, with a larger spring constant, you need to apply more force.

When I was studying at Coffee Bean and Tea Leaf this afternoon, I accidentally dropped my pencil. I noticed that this was exactly like one of the problems that we discussed in class!

Let's say that my pencil with a mass of .4 kg fell to the floor from a table that was 2 m high. Can we find it's velocity?

Of course we can!

We first use the gravitational potential energy equation to find the amount of energy.

PEg = mgh

PEg = (0.2 kg)(10 m/s^2)(2 m)

PEg = 4 J

Then we use the kinetic energy equation to find the velocity.

KE = 1/2mv^2

4 J = 1/2 (0.5 kg)(v^2)

2 = (0.5 kg)(v^2)

1 = v^2

v = 1 m/s

Yay! We just did work!

Beyond Thankful!

It's the most wonderful time of the year! The holidays give us a chance to appreciate all that we have and give thanks to those that deserve it. So for one, thank you Mr. Coach Chris for giving us this wonderful assignment to do a blog post over the break! This four-day break gave me an opportunity to reflect on what I am thankful for relating to physics! It wasn't really that hard to think of what I am thankful for relating to physics because physics is all around us!! I am thankful for physics because it plays such a large role in our everyday lives that without it, we would probably have a hard time living.

What am I thankful for in physics?

Acceleration

Acceleration (the rate at which the velocity of a body changes with time) plays a huge role in my life and without it, it be stuck! Literally! If my car couldn't accelerate, I'd be stuck at home (too lazy to walk), not able to get to the Thanksgiving party where there is the loads of food! I also wouldn't have been able to get to Ala Moana to do damage on Black Friday!

Relative Motion

I am also very thankful for relative motion because without it, I wouldn't be able to talk to my friends while walking in the halls or say hi to my friend in a car that is going at the same velocity as my car. If I am walking at 2 m/s and my friend is walking at 2 m/s too, relative to each other, we are moving at 0 m/s, allowing us to hold a conversation!

Friction

Friction is so important! It helps us walk, run, stop, and our overall movement because it is the opposing force that acts when objects move against other objects. 

Gravity

I am so thankful for gravity!! Without gravity (9.8 m/s^2), I wouldn't be able to sit down with my family and eat food that didn't float around.

Here is a plate of my amazing food (round 1)!! I wouldn't have been able to enjoy my Thanksgiving break without physics!

There is so much to be thankful for, not only during the holidays, but every day! 

Momentum!!

Our new unit that we are concentrating on focuses on...

Momentum!

Momentum is abbreviated with the letter P (I know, m was already taken). Momentum is the quantity of motion of a moving body, measured as a product of its mass and velocity.The law of conservation of matter states that in a isolated system, momentum will be conserved. Impulse is the change in momentum. We also learned a couple of equations...

Momentum(in) = Momentum(out)

Momentum = mass(kg) x velocity(m/s)

(Therefore, momentum's units are kg(m/s))

Impulse = P(final) - P(initial)

(Therefore, impulse is the change in momentum)

Impulse = F (average force) x t (time)

One of the most important concepts that we learned about had to deal with the last equation: Impulse = (avg force)(time). This equation explains the relationship between impulse, force, and time. When impulse increases, force and time also increase (direct relationship). Force and time have an indirect relationship that means that as force increases, time decreases and vice versa.

My clumsy sister always drops her phone! She drops it outside, inside, off the table and anywhere imaginable. Today, she dropped her phone twice: once on the carpet in my living room, and once on the tile floor in the kitchen. When she dropped it on the carpet, it was fine and she picked it up. But when she dropped it on the tile floor, it cracked open and fell apart. This is a great example of the relationship of force and time! (But not so great for her phone) Remember: impulses are the same because the change in momentum are relatively the same. When the phone dropped on the hard tile floor, the time the phone had to reach velocity zero was very small, making the force a lot bigger. In comparison, the phone that dropped on the carpet had cushion, which helped the phone not to break due to the increase in contact time. An increase in contact time helps to deplete force!

Therefore, drop your phone on surfaces that allow for more contact time!!

Forces that accelerate!!

Forces are fun!! When an outside, unbalanced force acts upon an object, the object accelerates. (When balanced forces act upon an object like rubbing your temples, the force stays at rest.) The inertia of an object causes it to continue until it is acted upon by another force.

This week we focused on forces that accelerate, which relates to Newton's second law: the acceleration of an object is directly proportional to the net force on an object; the acceleration of an object is inversely proportional to the objects mass. This means that when the force increases, the object's will acceleration will increase. As the mass increases, the acceleration will decrease, and as the mass decreases, acceleration is greater.

Let's take a look at an example!! After eating a tasty lunch with my sister, I got thirsty so she rolled me one of the drinks she bought. Knowing acceleration and the mass of the can we can find the force that my sister used to get it to me.

To find this, we would first make a free body diagram and then solve it using the only formula we know.... Fnet=ma!!

Yay forces!

Newton's First Law and Inertia!!

Unit 4 is all about forces! To understand forces better, we learned about the foundation of forces in motion: Newton's three laws. These laws were created by Sir Isaac Newton, a physicist born on January 4, 1643 in Woolsthorpe, England. His ideas became the basis of forces!

Let's focus in on the first of his laws, otherwise known as the Law of Inertia. Newton's Law of Inertia states that "objects in motion will tend to stay in motion unless acted upon by an outside, unbalanced force." This law could also be flipped by saying that "objects at rest will tend to stay at rest unless acted upon by an outside, unbalanced force. You might say, "What does that have to do with that thingy... inertia?" Well, in class we learned that inertia is defined as an object's capability to continue in the state it is in. We also learned that an object's inertia is directly proportional to an object's mass.

Okay, so what does this all mean? The typical thing for objects to do is to stay in the state that they were previously in. If no outside, unbalanced force interrupts this object, it will most likely stay in the state it was in.

Here is an example! (Sorry, it would be much better if I could upload videos but it doesn't let me. :( )

As the water bottle was being acted upon by an outside, unbalanced force (me pushing it), it broke its state of rest and began rolling (state of motion). When it hit the wall, it went from a state of motion to a state of rest. Why? Same thing! The wall is another outside, unbalanced force that changed the water bottle's state of motion to a state of rest. If the wall wasn't there, the water bottle would have kept on rolling along.

Newton's First Law helps us understand more about forces and how they work!!

More on Projectile Motion!!

While studying in Coffee Bean & Tea Leaf this fine Sunday afternoon, I noticed some physics going on! I payed for my drink and cornbread muffin and then went to sit down to study. When I put my wallet down, a quarter fell out of the pocket and fell to the ground. This reminded me of projectile motion!!





Now remember, there is one important rule to follow while using axes: the Vegas rule. In a previous blog post, I explained that the Vegas rule states that "what ever happens on the x axis, stays on the x axis" and "what ever happens on the y axis stays on the y axis". The only thing that is constant when trying to find either position, time, or range is that time is constant in both axes. If time is 3 seconds in the x, time is 3 seconds in the y.



My quarter is a wonderful example of projectile motion. The table that my quarter fell from was 2m from the ground. It fell at an initial velocity of 1m/s. We need to find where it's going to land to pick it up. Lets list our givens!!

With this information, we can use "dat" equation to find time (using info from the y axis)

d = 1/2at^2 + Vot

2m = 1/2(9.8m/s^2)t^2

2m = 4.9 m/s^2 (t^2)

0.408 = t^2

t = 0.64s

Fill in our givens table!

With this new info, we can find the distance it goes in the x axis!!

Use "dat" equation again!

d = 1/2at^2 + Vot

d = (1 m/s)(0.64s)

d = 0.64 m

If I wanted to catch my quarter off of the table, I would put my hands at 0.64 m away from the edge of the table. Great job!! Projectile motion is fun!

Relative Motion Once Again!!

Its been a while since we have talked about relative motion, so lets take it back a few steps! The most important thing to remember when dealing with motion is that it is always relative. Now you probably recall the upcoming question... "RELATIVE TO WHAT!" That's where things can go in all different directions!

For example, my two friends at the volleyball banquet tonight were walking together towards the food line. I snapped this first picture of them (lets just say they're walking at 5 m/s... that seems really unrealistic but I'm telling you, they were hungry!) They were able to carry a conversation because relative to each other, they were moving at 0 m/s. My friend in the blue dress started walking 3 m/s faster (8 m/s) to get to the line faster. To the girl in the blue dress, it seems as though the other is moving backwards (negative) when really she, herself, was just walking faster. Then, the same girl in the blue forgot her ticket! Oh no! She quickly turns around and walks quickly to her table at  5 m/s passing her friend (who is walking at the same speed). When they pass each other, it seems as though they are moving faster because all they can the blue girl can say is "WAIT FOR ME". She could only fit in 3 words because relative to her friend, she was moving at double the speed (10 m/s). When moving in opposite directions at the same speed, an object's speed seems to double!

Much more to learn in physics!!