This unit talked a lot about Newton's third law! Newton's third law states that for every action there is an equal and opposite reaction. An example of Newton's third law is a car crash. You would imagine that if a monster truck and a tiny car crashed the force exerted by the truck would be much greater than the force exerted by the car, right? Wrong! According to Newton's third law the forces are equal and opposite even if the size or mass of one is an infinite number larger than the other. Watch the video bellow to observe some more examples of Newton's third law:
In order to understand Newton's third law better, we learned about action and reaction pairs. Action and reaction pairs act accordingly to Newton's third law because they are forces opposite and equal. To write out an action and reaction pair you should follow the following format:
Object A pulls object B up
Object B pulls object A down
Notice that the verb remains the same while the directions oppose each other and the objects switch place. An example of an action and reaction pair is an apple falling from a tree, the action-reaction pair would be written like this:
Earth pulls apple down
Apple pulls earth up
If you still don't believe in Newton's third law...
Rope Tension:
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http://00.edu-cdn.com/files/static/mcgrawhillprof/9780071623209/TENSION_2_03.GIF McGraw-Hill Companies |
To figure it out what the rope tension on each of the ropes of the figure above you just need to follow a few simple steps:
1) Draw a vector pointing down, that will represent the force of the weight of the box;
2) Draw a vector pointing up that is the same size as the vector representing the weight. This vector will represent the net force up.
3) Draw two parallel lines to the ropes from the top of the arrow of the vector representing the net force up.
4) Draw the vector on each side from the hanging point to where the parallel line intersects the rope. Those two vectors are the vectors which represent the tension on each side of the rope.
After drawing the tension vectors you will find out that the rope with 45 degrees has more tension, and therefore is more likely to break.
Law of Universal Gravitation:
The only equation for the unit resumes that every body attracts every other body with a force that, for any two bodies, is directly proportional to the masses times each other and inversely proportional to the distance between those two objects, squared. The basic equation is:
F ~ m1m2/ d^2
To use it to figure out the forces of attraction we use
F = Gm1m2/ d^2
where G is the universal gravitational constant
Tides:
The last thing we learned in this unit was about how tides work! We found out that tides are caused by difference in force on opposite sides of the earth. That was one of the difficulties this unit, remembering that tides are not only caused by differences in force but difference in force in opposite sides of the earth. That statement explains why the sun, which exerts a greater force on the earth, does not have a larger effect on tides as the moon does. We learned about the different types of tides like the spring tide and neap tide.
Spring Tide: Happens when the moon, earth and sun are on a line. Spring tides cause higher than normal high tides, and lower than normal low tides.
Neap Tide: Happens when moon, earth and sun form a 90 degree angle. Neap tides cause lower than normal high tides, and higher than normal low tides.
Reflection...
The hardest part of this unit was to explain why a horse can pull on a buggy if their forces are equal and opposite. I understood this concept better after we went to the hallway with two wheel chairs and a rope and tested pulling on each other without our feet on the ground, and then with one of us with our feet on the ground. Seeing Newton's third law in practice helped me understand better the concept.
