Journal Entry 2/1

1. You are seated on a bench. The earth exerts a force on you, and the bench exerts a force on you. Do these two forces represent a Newton’s Third Law force pair? Explain.

The earth is exerting a downward force on me, trying to pull me down. The bench on the other hand is exerting an upward force on me, pushing me up. I am not moving which means that there is no unbalanced force on me. Because these forces are opposite and equal, they represent a Newton's Third Law force pair.

2. A pen sits on a desk. The desk exerts a force on the pen, and the pen exerts a force on the desk. Do these two forces represent a Newton’s Third Law force pair? Explain.

The desk is exerting an upward force on the pen, pushing it up. The pen is exerting a downward force on the desk, pushing it down. Again, nothing is moving so there is no unbalanced force in this scenario. Therefore the forces are opposite and equal and represent a Newton's Third force pair.

3. You pull on a rope. The rope pulls on a wagon. Do these two forces represent a Newton’s Third Law force pair? Explain.

If my pull on the rope is to the left, I am exerting a force to the left. Because of me, when the rope pulls on the wagon it will also be exerting a force to the left. Although the forces may be equal, they are not in opposite directions. They cannot be a force pair for Newton's Third Law.

4. A book slides across a tabletop. The book exerts a downward force on the table, and the table exerts a frictional force on the book opposite the direction of its motion. Do these two forces represent a Newton’s Third Law force pair? Explain.

Although there are two forces here, the book's downward force on the table and the frictional force of the table on the book, they do not correlate because one is a vertical force and the other is a horizontal force. Also, because the book is sliding across the tabletop, there is an unbalanced force. Therefore, this is not a force pair of Newton's Third Law.

Journal Entry 1/24

Why is there a debate about whether or not Pluto is a planet?

There is debate about whether or not Pluto is a planet because objects have been discovered beyond it that are much larger and also orbiting the sun. This orbit makes these objects a part of our solar system, and since there are many varied definitions for what a planet actually is, they may all be worthy of the title: "Planet".

How is Pluto similar to the inner planets? How is it different?

Pluto is similar to the inner planets in that it is circular (which means that there is a gravitational force pulling all matter towards the center) and that it orbits the sun. Its orbit is questionable though, because it is the only object that crosses orbits with another (Pluto's "planetary orbit" crosses paths with Neptune). Within the eight inner planets, there are inner and outer planets. The inner planets are: Mercury, Venus, Earth and Mars. The outer planets are: Jupiter, Saturn, Uranus, Neptune. All of the outer planets are larger than the inner ones. Pluto does not fit this mold, for it would be considered an outer planet, but is smaller than the other outers. It is also different in that it does not have the hydrogen and helium atmosphere of the other outer planets.

What is the name of the region in the outer solar system, beyond Pluto, which contains many recently-discovered large objects?

The region beyond Pluto is called the "Kuiper Belt". It contains many recently-discovered large objects.

What do you think of the controversy about whether or not Pluto is a planet? How should we decide who is correct?

In a way it is entertaining that there is such a controversy surrounding Pluto. I think that because this celestial body is so distant and so mysterious, it appeals to the public and there is some kind of instinctual need to protect it. As far as determining whether or not Pluto is a planet, I don't think it should be considered one. Objects that are much larger have been discovered and they have not been classified as planets. The sense of obligation to keep Pluto as a planet is just a product of the human tendency to reject change. Just because Pluto has been considered a planet for 60 years, that does not necessarily mean that it is one. A universal definition of a planet is necessary to decide whether or not Pluto qualifies as one. If this definition is created, the future of planetary science will be a lot more manageable.

Journal Entry 1/11

1. What is the difference between a "force" and a "net force"?

"Force" is the result on an interaction between objects. When a ball is resting on the ground there are two forces interacting with it. The Earth is a force on the ball, pulling in down. The ground is also a force on the object, pushing it up, preventing it from being drawn into the core of the Earth. There is also force on an object when that object is pushed, or pulled, or lifted up. The "net force" is another term for an unbalanced force. In the previous example, the force of the Earth on the ball was balanced by the force of the ground on the ball. If that same ball was rolling along the ground and then a meter stick was pressed against the side of the ball, that meter stick would represent an unbalanced force. In this scenario, the forces of the Earth and the ground are still balanced, but the meter stick is unbalanced.

2. How do you determine the direction of the net force"

The direction of the net force of an object can be determined by using the arrows in a force diagram or by using simple addition and subtraction. Let's say that there is a large block sitting on the sidewalk, and Jen and Dan both stand on opposite sides of the block. Dan pulls the block 50 units to the right and Jen moves the block 35 units to the left. If we were representing this situation with arrows, the forces of the earth on the block and the ground on the block would be balanced, so their vertical arrows would be the same length. Their horizontal arrows would not be the same length however. Because Dan is exerting a greater force on the box (50> 35), the arrow to the right would be longer than the arrow to the left. From the diagram, we would be able to see that the direction of the net force would be to the right. If we were to use basic arithmetic to represent the same situation, we would represent 50 units to the right as (+50) and 35 units to the left as (-35). We would then add these numbers together, to get (+25). Because our answer is positive, we know that Dan is exerting a greater force and therefore the direction of the net force is to the right.

3. What is the connection between the net force exerted on on object by other objects, and the motion of the object?

Often times, the net force represents the direction in which the object moves. In the last example, the box moved to the right because Dan was exerting a greater force than Jen. The net force was going towards the right, so the box was moving to the right. The idea that an object always moves in the direction of the unbalanced force, however, is false. In the first example, when a meter stick is pushed against a ball that is rolling along the ground, the meter stick represents the net force. If the ball was rolling right and the meter stick was pressed against it, the ball would not suddenly start to move left. The ball continues to move to the right. This proves that objects don't always move in the direction of the net force.

Journal Entry 1/4

(a) You do not need to know how big the plane is to calculate the time it takes to get from New York to Los Angeles. you are given the average speed which makes the length of the trip directly discernible. Because the speed is given the size is unnecessary.

(b) When parking the plane, its size is necessary however. the distance between the two gates is directly dependent on the size of the plane. if the size was not specifically given and an estimate was used to construct the gates, there would be a serious problem upon landing.

(c) The size of the plane is not necessary if we are told that the plane taxied along the runway for 100 m. Regardless of the size of the plane, it moved for 100 m. You would consider the change in its distance from the nose of the plane. It does not matter what length is behind the nose because once the distance the nose has gone has reached 100 m, the movement stops.

(d) When a plane moves left its size is necessary because the shift can be depicted with a right triangle. if the plane taxied for 100 m, then turned left, it is 32 m left of that 100 m mark. To find how much the passenger moved you would construct a right triangle with legs 100 and 32 and you would calculate the hypotenuse. The hypotenuse would equal the distance the passenger moved with respect to the ground.