Unit 3

In this unit I learned about…

            -Newton’s Third Law (Every action has an equal and opposite reaction) along with action reaction pairs.

            -How people win tug of wars in relation to physics and why a horse is able to pull a buggy (both problems that are in correlation to Newton’s Third Law).

            -Forces in perpendicular directions

            -Gravity and Tides

            -Momentum and Impulse Momentum

Newton’s Third Law

Newton’s Third Law states that every action has an equal and opposite reaction. For example: truck hits car/ car hits truck. This is an example of an action reaction pair. Action reaction pairs are two equal and opposite forces of which include equal sizes and opposite directions.

Here are some important example problems:

            How does a horse pull a buggy? Label the action reaction pairs, and explain your reasoning.

In this diagram we can pull out three action reaction pairs. These are commonly shown through vectors. It is important to note that the strength of the pull does not matter. The horse will pull the buggy with the same force the buggy pulls the horse with. We know this from Newton’s Third Law that states, “every action has an equal and opposite reaction”. The reason the horse is able to pull the buggy is because the horse is pushing the ground harder than the buggy does. The horse and buggy will move in the direction of the horse.

  The first pair is between the two opposing forces.

The top vector will be labeled “Horse pulls buggy”.

Where as, the bottom one will note, “Buggy pulls horse”.

The second one is that the “buggy pushes ground” and “Ground pushes buggy”. The third being “horse pushes  

ground” and “ground pushes horse”. Make sure the horse’s vectors are longer because it shows that the horse is pushing the ground harder than the buggy.

Forces in different Directions

When we add vectors there are two special coincidences. If one is when the vertical and horizontal are the same number, the diagonal is 1.41. The other is when 5, 4, and 3 all go together. This way we can see easily how to figure out what direction the object will take.

For example: If a 100m/s canoe is paddling down a river with a 100m/s current, what path will the canoe take?

            It will go in between the two at 14.1 m/s.

Gravity and Tides

            If you look out at the ocean you will notice when the waves are closer, and when they are further. These are tides. There are high tides (when it comes further up on the beach) and low tides (when its lower on the beach).

These switch back and forth. There are two high tides and two low tides every 24 hours. It takes about 6 hours for tides to switch from high to low, meaning it takes 12 hours to go from low tide to low tide or from high tide to high tide. The reason for tides is the difference in force felt by opposite sides of the earth. This is just another example of an equal and opposite force, as the moon pulls the earth while the earth pulls the moon.

Sydney Sloan did the diagram above.

This shows the moon’s affect on the earth’s tides. Side B has a greater distance and a smaller force, while side A is the opposite.  The distance has a great affect on the force.

The Universal Gravitational Law is F=Gm1m2/d². This problem essentially demonstrates the relationship between force and distance².

The dotted line around the earth is known as a tidal bulge. The tidal bulge is created by the difference in between the forces between A and B.

For example:

If side B had a force of 2N and side A had 6N, the middle force would be 4N. If we solve for B, we would subtract 6 from 4 and get -2N. And side A would be 6-4 and be 2N.

There are two types of tides: a Neap Tide and a Spring Tide. This has to do with the positioning of the sun, the moon, and the earth. A spring tide is when all three align, proving higher tides. This normally occurs during a full moon. A neap tide is when the moon is perpendicular to the sun in relation to the moon.

Momentum and Impulse Momentum

Momentum (p) is the product of an object’s mass and velocity. Furthermore, p=mv.

The change in momentum is formatted as ∆p=pfinal-pinitial.

We also know that Impulse (Ft) is the force multiplied by the time it is applied. In other words, J=F∆t. Along with this, Ft is equivalent to ∆p.

For example:

1.) What is the momentum of a 10kg carton that slides with a 3m/s?

p=mv                          p=(10)(3)                  p=30kgm/s

2.) In Frozen, why did Anna and Sven not get hurt when they fell off the mountain and landed in snow after being chased by the snow monster?

                                    p=mv                          ∆p=pfinal-pinitial

Anna will go from moving to not moving regardless of the surface she hits. This means that the change in p will stay the same regardless of the surface. If this is so, we know that J will remain the same as well because J=∆p. Another important aspect in answering this problem is that J=F∆t. Because snow increased the time of Anna’s force it decreased the force. Less force simply means less injury.

J=F∆t  and  J=F∆p

3.) Why don’t cars have rubber bumpers anymore?

            The cars will bounce causing two changes in p’s and two J’s. Furthermore, there would be double the force and more injury.