In this unit I learned about:
- · Newton’s third law of motion
- Action reaction pairs
- Adding non-parallel vectors
- Universal gravitational force
- Tides
- Momentum and impulse relationship
- Conservation of momentum
Newton’s third law of
motion states that whenever one object exerts a force on a second object,
the second object exerts an equal and opposite force on the first. Take a car for
example; the tires of a car push against the road while the road pushes back on
the tires, the road interact with an equal and opposite force.
One of the harder questions that helped me understand newtons third law and action reaction pairs was why a horse pulls a buggy.
Buggy pushes earth forward. Earth pushes buggy backward.
Horse pushes ground backward. Ground pushes horse forward.
Horse pulls buggy forward. Buggy pulls horse backward.
The horse pulls the buggy with the same force as the buggy pulls on the horse. This is because of newtons 3rd law that states with every action there is an equal and opposite reaction. The reason why the horse is able to pull the buggy is because it is able to push on the ground harder than the buggy.
Adding non-parallel vectors
The F net up and F weight down are equal and opposite (action reaction pair).
Universal gravitational force:
Everything with mass attracts all other things with mass.
Force depends on
- Mass of objects F ~ m
- Distance between the objects
- farther apart = less force on the 2 objects, F~1/d (inversely proportional)
Inverse square law:
farther away you get it will be one over the distance squared. F~ 1/d^2 (force is inversely proportional to the distance squared). To see this in an real life example, a man standing on a mountain is farther away from the center of the universe than a man standing on sea level, therefore the man on the mountain is experiencing less of a gravitational pull towards the earth than that of the other man.
Universal gravitational formula:
The purpose of G is to make the force into newtons. G = 7 * 10^-11 Nm^2/kg^2
An example problem for using this equation is; find the force between the moon and the earth. The earths mass is 6 *10^24 and the moons mass is 7 * 10^22, the distance is 4 * 10^8. The first step is the separate the equation like this.
After solving the equation the answer should be 18.4 * 10^19 N.
Tides was the topic that I found most difficult in this unit, because the idea that the forces between the moon and earth were equal on opposite sides of the earth was confusing. I overcame this difficulty by watching video's on youtube and searching on google for different explanations on the topic. What eventually made the lightbulb click was the understanding that tides are not caused by just the force between the moon and earth, but that they are caused by the difference in force on each side of the earth.
There are two types of tides, and they are spring tides and neap tides. This is what they look like:
Tides are caused by the difference in force felt by opposite sides of the earth. Tides change every 6 hours, therefore in a day there are 2 high tides and 2 low tides.
Momentum and impulse relationship
Equation for momentum: p=mv (mass times velocity)
Change in momentum equals the final momentum minus the initial momentum: ∆p= p final - p initial
- The change in momentum is the same regardless of if you stop quickly or slowly
Impulse is the force upon something multiplied by the time that the force is applied: J=F∆t
Impulse is equal to the change in momentum: J=∆p
A question that helped me understand this concept was; why do bun-jee jumpers use elastic cords rather than metal?
In answering this question first establish that the jumper will go from moving to non moving no matter how they are stopped therefore, the change in momentum (∆p) is the same
p=mv
∆p=p final - p initial
Since the change in momentum is the same regardless of how the egg is stopped, the impulse is also going to be the same regardless of how quickly it is stopped
∆p=J
Bun-jee jumpers use elastic cords because it stops them over a longer period of time, since the impulse is constant the force on them is small. Smaller force on the jumper means lower chances of injury.
J= F∆t (metal cord) J= F∆t (elastic cord)
Conservation of momentum;
Because of newtons third law which states that forces are equal and opposite we can prove that the two balls rolling towards each other regardless of their mass and speed will experience the same amount of force. Moreover their will be no net change in momentum after the collision.
FA=-FB
FA∆t = -FB∆B
JA = -JB
∆pA = -∆pB
The momentum of each ball changes in a way in which the sum momentum of each ball is zero. The momentum of the balls before they hit is p total before = p total after.
The two types of collisions are elastic collisions (when the objects bounce off of one another) and an inelastic collisions (when objects collide and don't bounce away).
Elastic collision:
p total before = p total after
mAvA + mBvB = mAvA + mBvB
Inelastic collision:
p total before = p total after
mAvA + mBvB = m A+B(vAB)
But what if ball A hits ball B not straight on? why would both of the balls move?
They both move because momentum in the y-axis is consered. Ball A does not stop because there needs to be a zero net momentum to cancel out the y momentum of the second ball
My effort was mainly focussed towards class and homework problems because it is where I can actively learn by asking questions, and solve problems. My confidence in physics is slowly growing, and the more we learn in class the more I am able to relate class discussions to my everyday life. My approach to solving difficult problems is to first try and solve it by myself using my notes, then if that doesn't help I usually ask my teacher or a classmate. Collaboration with group members is definitely an aspect of class that I need to work on, usually it is hard to find time for all of us to work together and create an equal balance of work on group projects. My goal for next unit is to not get behind on my studies, when taking a quiz on a new topic I don't want to be questioning myself on wether or not I have fully understood the previous topic we had learned.
A connection that I made between what we have studied to everyday life is that while go carting with my brothers, when one of them hits me we both experience the same force. Therefore whenever my brother crashes into me i'm also technically crashing into him.
Here is my podcast video on the horse and buggy question stated earlier, hope you enjoy!
