Unit 3

Newtons 3rd Law + Action and Reaction Pairs
   Newtons 3rd Law states that every action has an equal and opposite reaction. For instance if a book was sitting on a table, the book would be pushing down on the table and the table would be pushing up on the book. These are called action and reaction pairs. Another example is walking. As a person pushes the ground backwards with their foot, the earth pushes foreword.
Tug of War
   We can use our knowledge of Newtons 3rd Law and Action and Reaction pairs to win a game of tug of war. In tug of war, the amount each team pulls actually doesn't make a difference because of Newtons 3rd Law. What does matter is how hard each team pushes on the earth. If one team pushes back the earth with a force of 5 making the earth push back with an equal force, and the other team pushes the earth with a force of 10, the second team would win.
   We can also use our knowledge of Newtons 3rd Law and Action and Reaction pairs to understand how a horse and buggy move. Similar to the tug of war situation, whatever force the horse pulls foreword, the buggy will pull back in the opposite direction because of Newtons 3rd Law so what makes the buggy move is the horse pushing the ground harder than the buggy pushes the ground.
Forces in Perpendicular Directions
   A person is in a sled on a snowy hill. They slide down the hill. This happens because of forces that occur in perpendicular directions. The weight of the person on the sled is called F gravity. The perpendicular force to this is called F support. If we add up the vectors, we get a force that would bring the sled down the hill. If the F friction is greater than the force that makes the sled move, the sled would stay still. We can use these same ideas in relation to sail boats, currents, and other movements.
   A heavy box is being suspended by a rope which is attached to the ceiling however, one side is longer and has a smaller angle. When drawing in support vectors in either direction we can draw tension vectors. The longer the vector, the more tension there is.  
Momentum and Impulse
   Momentum, or p, is mass, or m, times velocity, or v. The equation for momentum is p=mv. So if a box was being pushed at a velocity of 10 and it had a mass of 5 its momentum would be 50. Impulse, or J, is how much force, or F, occurs over an amount of time, or t. The equation for impulse is J=Ft. We use impulse when talking about something changing momentum like stopping: going from moving to not moving. Continuing off of the box example, if the box stopped, it would go from 50 to 0 no matter how it stopped. This equation is J=change in momentum. If the box was stopped on a concrete wall versus a sponge wall, there would be very different outcomes. To deal with this type of situation we use the J=Ft equation. In this equation, Force is inversely proportional to time meaning if time is small, force is big and vice versa.  If the box stopped on a concrete wall it would be a very abrupt halt = a small amount of time = big force. If the box stopped on a sponge wall it would be a much gentler halt = long time = small force = less damage. This is the reason we have air bags in vehicles.  
Gravity and Tides
   The reason that tides occur is because of the moons pull on the earth. Tides on opposite sides of the earth are always the same. If you picture the globe, the left and right sides would have high tides and the top and bottom would have low tides or vice versa. This happens because of the difference in force felt by each side. For example, one side of the earth, lets call it side A, is adjacent to the moon and the other side of the earth, lets call it side B, is opposite to the first side. Side A would be a shorter distance away from the moon and because of the formula F=m1m2/d^2, it would have a large net force. Therefore, side B would be a larger distance away from the moon and have a smaller net force. The moon would also have a pull on the center of the earth which would be less than the force of side A, but greater than the force of side B. For example, the force of side A is 15, side B is 5 and the middle is 10. As stated earlier, the difference in force felt by each side is what makes opposing sides of the earth have equal tides. To find this difference, we subtract 10, the middle number from 15 and 5, each side. When doing this, we get 5 and -5. This means that we have a force of 5 pulling to the right and to the left. This creates a tidal bulge. Without a difference in force, we would get a net force of 0, creating no tides at all.  High and low tides alternate and occur about every 6 hours with each occurring 2 times a day.  High and low tides occur every 6 hours because of the time it takes the moon to orbit earth. There are also tides called spring tides and neap tides. Spring tides occur when the sun, earth, and moon are lined up either sun, moon, earth or sun, earth, moon. When this happens, there is either a full moon or a new moon and the tides are unusually high and unusually low. Neap tides occur when the sun, moon, and earth do not line up either sun, earth, and moon above or below the globe. When this happens there is a half moon and the difference between the tides are unusually low.
Conservation of Momentum
   Because of what we know from Newtons 3rd Law, momentum is always conserved. When playing pool, one ball hits the other resulting in the ball that was moving to stop and the ball that was still to move with the same speed as the first one originally moved. A ball could also crash into another cart resulting in them both moving together.