Unit 3 Summary

1.) Newton's Third Law and Action Reaction Pairs:

 Newton's Third Law: Every action has an equal and opposite reaction.

For instance, a pair of tug of war teams are always pulling on each other with equal and opposite forces. A car and a truck crashing exert equal and opposite forces on each other. You state this pair like this:

Car pushes truck forward
Turck pushes car backward
(be sure to include vectors)

Or a skateboard:

- The effect that makes a difference in the pairs, especially one like the tug of war example, is friction. The greater force of friction allows one team or the other to win. Two things that affect friction are weight and the nature of the surface.

- Many times there are multiple action reaction pairs for every situation. For example, a grapefruit sitting on a table is pairing with both the earth and the table. These pairs are not equal and opposite necessarily, only the individual pairs are equal and opposite.
- Make sure to remember that accelerations in the opposite direction are negative.

This page contains great practice questions for understanding Newton's Third Law.

2.) Vectors 

Vectors are a device that show both direction and magnitude.  
More simply, vectors are arrows. We show magnitude with the length (and possibly opaqueness if you want) of the arrow, and the direction is the direction the arrow is pointing.

Adding vectors is helpful when determining the actual direction something will be traveling and the speed at which it will be traveling, by considering the forces acting upon the object.


A good example is a canoe on a river. If there is a current running south and you are paddling east, using vectors, you can determine where the canoe's path will take.

To add these vectors, draw lines parallel to each of the existing vectors, and then the line of actual direction is from the center to the intersection of the parallels.
Use this method also to help determine tension.

When trying to determine a lesser or greater tension, inverse the parallels so that they intersect the opposite line. The tension is represented by the distance between the center and the points of intersection. Be sure to put direction on the friction lines- I have made this mistake before. 

3.) Tides/Gravitational Force  
The formula for Gravitational Force is F=G m1m2/d^2

This formula explains the gravitational force between masses like planets. 
This equation and the information we can extract from it help explain a lot of things including tides.

Tides are caused by the gravitational force between the moon and the earth. Even though the sun has much (much much much) more mass, the closeness of the moon ultimately makes the force between the moon and earth greater and have a greater effect. 

The equation dictates that distance between the objects and the force are inversely proportionate. Same with mass, so the close distance between the moon and Earth made up for it's small mass.


-The Earth has a tidal bulge around it that is created by the moon's gravitational force and it creates high tides on each side of the moon and low tides on the adjacent sides.  The tides are caused by a difference in force from the moon because the distance is different at the two points relative to the center of the moon. There are six hours between each low and high tide and 12 between high tides and 12 between low tides. So, there are two low tides and two high tides each day. 

- When the moon and sun and earth are in line, the tides are called spring tides which are higher than the normal high, and lower than the normal low.When the sun is not in line it is rather during a quarter phase of orbit, it is called a neap tide. These tides are more lukewarm than the spring tides, in fact they are lower than the normal time tides. 

                                                           

This video is really helpful in understanding tides:

3.) Momentum and the Conservation of it

The momentum formula is p=mv
(the units for p is therefore kgm/s)

p total before= p total after (because of Newton's Third Law)



An important detail about momentum is that the momentum os a system is conserved. That is, 

When we are assessing two objects, like carts, collide, we can calculate the speed after by taking this assumption. Use this equation:

MaVa+MbVb= (Vab)(Ma+b)    - plug in the masses and velocities to find the velocity after the carts collide.

 
3b.) Impulse
Impulse (or J) is the change in momentum.

To change momentum, you need a change in force.
When thinking about changing the Force, two main things matter: 

1. The amount of Force applied  2. Time F is applied

For these things, we explain the equation for Impulse: J=F(change in t)

This equation shows that distance and force react in a certain way to each other. When time increases force decreases and vise versa. 

An Example of this would be landing on a hardwood floor rather than a memory foam mattress. The mattress increases the time of force applied (do not forget that the change in momentum/impulse is the same), which decreases the force. That is why you wouldn't get as hurt on the mattress. 
When answering these problems be sure to mention:

-That the object will go from moving to not moving no matter what
- The relationship between change in momentum and impulse
- The relationship between time and force when talking about impulse

This page helps to connect Newton's Second Law, Newton's Third Law and Change in Momentum in contribution to Impulse. The building blocks are really good to understand so check it out.