Unit 3 Reflection

This unit in physics I learned about...

Newton’s 3rd Law:
When began our unit by learning about Newton's second law which states that:

For every action there is an equal and opposite reaction

We did a lab where a large truck and a small civic collide and we used Newtons second law, a=fnet/m, to find the force on each of the cars to prove Newton's 3rd law. We found out that the acceleration of the small car was 1.61 m/s^2, and its mass was .488 kg. We then found the large car's acceleration of 1.85, and mass of .991. We used Newton's 2nd law to solve, the small car: 1.61*.488= .861N. We did the same for the large car: 1.85 *.991= .997. The forces are so similar, and in a situation where we have more exact results, the forces would be exact, therefore proving Newton's 3rd law.

Action/Reaction Pairs:
With action and reaction pairs, there are 3 rules:
--> subject stays the same
--> verb is the same
--> directions switch
the example of an apple sitting on a table has an action/reaction pair: 
the apple pulls the earth up
the earth pulls the apple down
the verbs and the subject both stay the same, while the direction switches.

In the horse and buggy problem, the reason the cart moves is because of action/reaction pairs:

1. horse pulls buggy forward
buggy pulls horse backward
2. horse pushes ground backward
ground pushes horse forward
3. buggy pushes ground forward
ground pushes buggy forward

The horse pulls the buggy with an equal force that the buggy pulls back with. We know this because of Newton's 3rd law which states that with every action there is an equal and opposite reaction.
The horse and cart move forward in the horses direction because the horse pushes the ground harder then the buggy pushes the ground.

When a train runs into a parked car, both will exert the same force because of Newton's 3rd law: every action has an equal and opposite reaction. When we think about which one will have the greater acceleration though, it changes. The smaller car has the greater acceleration because the mass is smaller, and the force needs to be equal to the larger, so the mass is small and the acceleration is large, as shown below.

F = Am vs F=AM

Forces in perpendicular directions:

Why does the block slide down the ramp? When solving this problem. begin with drawing in the total gravity (the box being pulled down by the earth) and the total support (both in blue). Then connect them (in green), and draw a diagonal (in orange) and this is the total force on the system, thus the direction the box with slide down. If we were to plug in 100N for both fsupport and fgravity, the Pythagorean theorem equation (a^2 + b^2 = c^2) would be used and result in:

100^2 + 100^2 = c^2

10,000 + 10,000 = c^2

20,000 = c^2

c = 141N

This example is a lot like the example of which side of the rope will be more likely to break, shown below. The first step is to show that the ball has force acting on it and draw in the fweight going down and the fsupport going up. Using this fsupport arrow, draw two parallel lines to the ropes both intersecting at the top of the arrow. Where the line drawn intersects the rope, draw a line from that to the ball on both sides. This represents the ftension. The side with the longest line is the side that will break because it has more tension on it.

Gravity and Tides:

Everything with a mass attracts all other things with mass. Meaning that even out beyond the earth, the universe is being pulled together by its attraction towards each other. Force depends on the mass of an object (f~m) and the distance between the objects (f~1/d). When a person is standing on the top of a mountain, the force will be less compared to sea level becasue the distance is greater from the surface on the mountain, and therefore the force and the weight will be less as well.

The Universal Gravitational Formula:

F = G m1m2

d^2

A person is standing on the face of the earth and we are trying to find the gravity, and in order to do this, we can use the universal gravitational formula and simply plug things in. The G will always be 6.67x10^-11, and is this case, lets say that the mass of the person will be 600N or 6.0x10^1. The mass of the earth = 5.98x11^24kg. And finally the distance = 6.37x10^6m (radius of earth). To begin, you plug everything in, not forgetting that in the denominator, both the 6.37 and the 6 from 10^6 is squared. From here, to make it easier, separate the exponents, making the decimals in one group and the scientific notation numbers in another. In the first group multiply all the numbers on top and then divide by the numerator that has been squared. For the scientific notation numbers, simply add the exponents in the numerator, and square the exponent in the denominator, and then subtract as shown off to the right side of the image. From here, there is a decimal

multiplied by a number with an exponent: 5.90 x 100 is 590N which is compared to 600 N (the person). To find the acceleration, plug in the numbers to a = fnet

m

590/60 = 9.8m/s^2 pulled to the earth

GRAVITY!!

The reasons for tides is all connected to the moon and sun and the force between them. As the sun is significantly larger than the moon and, the moon still causes the tides because of the difference in force is greater from the moon. If the moon on side A of the earth has a force of 15, and a force of 5 on side B, and then sun has a force on earth of 20 on side A and 18 on side B, then the reason that tides is causes by the moon and not the sun is becsue of the difference: 20-18=2, while 15-5=10N. The reason that lakes don't experience tides is because it is not large enough and doesn't have a body of water on the opposite side to effect it and pull towards the moon.

Momentum, and Impulse momentum relationship:

We begin with knowing that:

P = momentum

J = Impulse

We know that momentum = mass x velocity (P=mv)

change in P = Pfinal - Pinitial

The change in P is going to be the same no matter how you stop it, and that is why the impulse = force x time force is applied. J = change in P

So why do airbags keep is safe?

p = mv When someone gets in a car accident, the car is going from moving to not moving

no matter how they are stopped

Δp=  Therefore the change in momentum is the same regardless of how the car is stopped

pfinal-pinitial

ΔP = J Since the change in momentum (P) is the same no matter how the car stops, the

impulse (J) is also the same

J= FΔt The car with airbags will stop over a period of time. This causes the force on the 

J= F Δt person to be same, A small force means less danger so the person will be safer

This explanation can be used for why climbers use nylon rope, why gymnast use bouncy mats or even how to win an egg toss challenge.

We can use these equations to help us solve the problem of: a car with a mass of 100kg starts at a speed of 50m/s and comes to a stop.

Δp = pfinal-pinitial

p = mv

Δmv = mvfinal - mvinitial 

(1000)(0) - (1000)(50)

0 -50000 kgm/s

Δp = -50000 kgm/s

J = 50000 Ns

The reason that bullet proof vests are safe is becasue the bullets stick instead of bounce off. Vests that have bullets bounce off taek 2 forces:when the bullet hits, that is one force, and this is what vests that stick do, but a bullet that bounces must now bounce off creating another force.

Conservation of Momentum:

We know that forces are equal and opposite because of Newton's 3rd law. So if 2 carts collide, one (cart a) being 2kg and going 10m/s to the right, and the other (cart b) weighing 5kg and not moving with a velocity of 0m/s, we can solve to find the velocity of carb b since it will go and cart a will stop. As seen to the left, you simply plug in the numbers and into the equation and solve. This equation is meant for when the 2 objects don't stick together, but for when 2 objects do stick together, the equation is different:

MaVa + MbVb = Ma + b (Vab)

*Something to remember is that when 2 objects are going in opposite directions, one will be positive and the other will be negative. 

When 2 objects hit each other, and go off into different directions, to the right is what happens. The momentum is conserved, having the same x and y in the before and after. it is a simple 3,4,5 triangle and results in the 2 sides adding up to become equal in the end.

Overview of the Lab:

In our lab Cart A (2kg) rolled into Cart B (1kg) that was at rest. The carts stuck together, and with gathered data, we can confirm the Law of Conservation of Momentum.

First, write the equation of the line --> y = 2.933x

Second, fill in the equation with what is on the x and y axis --> ptotal before = 2.33x

Third, ptotal before = ptotal after which is ptotal before = (Ma +b)(Vab) is the equation used

Forth, Figure out part of the equation that represents slope --> Ma + b, 2 +1 =3 compared to 2.933

Fifth, compare the theatrical slope to the actual slope and a close number confirms --> 3 compared to 2.933 = confirmed

In this unit I found that understanding tides and gravity was very difficult for me. I was able to understand them a little better by help from my peers, and watching the video's Mrs. Lawrence made for us. I still have trouble with them because the idea is so large for me to wrap my mind around. I think that this unit I was bale to keep up well, but I still struggled with managing my time and really fully studying for my quizzes. I did my homework, and turned in my blog posts on time. I think this unit I performed weaker than the previous units. I think something I can improve is my persistence, because I think that if I am more persistence in class I will be able to help myself understand the problems and thus help me to understand the unit better. For my unit podcast, I think my group was able to get it done, but we struggled with all working together. My goals for next unit is to get all my homework done on time. I think that if I am able to get my homework in on time, I will be able to study better, and it will help me at the moment.