Unit 4 mainly talked about momentum and impulse. We stablished that momentum is inertia in motion and is equal to the mass times the velocity of the body (P = mv). The product of the force acting on an object and the time which acts its called impulse (J = f Δt). Impulse is also the result of change in momentum (J = ΔP). Momentum and impulse can explain a lot of everyday life situation. For example: why is it better for a karate fighter to exert a force for a short period of time when breaking a brick? The answer to that questions seems like it would be prety easy, but... in order two variables in an equation, the other variable needs to be proven constant. In the equation J = f Δt, in order to prove that time determines the force necessary to achieve a certain impulse, the impulse needs to be proven constant. To prove impulse is constant you need to state that regardless of the time, (continuing with the karate example) the hand of the fighter is moving and comes to a stop therefore since the mass is obviously the same and P = mv, the change in impulse is the same (ΔP = Pfinal - Pinitial). We stated before that J = ΔP therefore J is constant! To continue to answer the question, you should now state that J is also J = fΔt. Because of the last equation is possible to afirm that the smallest the time of contact the greater the force will be in order to achieve the constant J. Since to break a brick you need the greatest force possible, the time of contact should be as small as possible.
Still don't understand? Try watching this explanation
After learning about the relationship of momentum and impulse we talked about bouncing.
To explain bouncing, my class watched a demonstration of two identical balls in direction of a wood block each. One of the ball sticks and the other one bounces. The question was which one of the balls will knock the wood block over? The answer to that question was the bouncing ball! The reason why the bouncing ball knocks the block over is because there is not only one force acting on that block. Because of Newton's third law, we know that for every action there is an equal and opposite direction therefore ball pushes block, block pushes ball in both cases. The bouncing ball not only causes that force, the ball will bounce on the block adding another force taht causes the block to fall over.
After talking about bouncing, we talked about concervation of momentum. Conservation of momentum is apliable when there is a system. Conservation of momentum is represented by the equation:
P before = P after . When dealing with collisions conservation of momentum is verry helpful. We learned that ther are two different types of collisions:
Elastic collision:
A collision in which colliding objects rebound wothout lasting deformation or the generation of heat.
Inelastic Collisions:
A collision in wich colliding objects become distorted, generate heat, and possibly stick together.
We learned that because of the concept of collision and conservation of momentum is possible to find out many things about the colliding objects. If for example you know the masses and the velocity of the objects before the collision and they stick together, or one object is at rest, it only takes a few steps to find out the velocity of those objects after the collision by using the conservation of law affirmation.
Reflection:
The hardest part of this unit was writing out a complete answer for a problem like the karate one. It became easier to remember to include all the information neeeded after we answered homework questions on the board and got a chance to learn with each others mistake.
Tuesday, December 6, 2011
Monday, November 14, 2011
Unit 3 Reflection!
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:
 |
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.
Saturday, October 22, 2011
Unit 2 reflection
We started this unit learning about Newton's 2nd Law! Newton's second Law states that acceleration is directly proportional to the Net Force and inversely proportional to mass. The statement is simplified in the equation:
a = Fnet/m
After learning a little bit about Newton's second law we learned the differences between mass and weight. Mass is a measure of inertia and is represented in Kilograms. Mass never changes from one place to another! Weight is a measure of force always represented in Newtons! Weight does change from one place to another because w = mg and the gravitational force is different in different places. If you are curious to find out how much you weight on the moon or other planets check out the link bellow!
http://www.exploratorium.edu/ronh/weight/
After making it clear the differences between mass and weight we started to talk about objects in free fall. An object in free fall is falling only under the influence of gravity. We found out that the acceleration of an object in free fall is always gravity. That is explain by the fact that Newton's second law says a = Fnet/m. Knowing that the Fnet is the weight of the object and that w = mg rewriting the equation we now have a = mg/m. The masses will cancel out leaving a = g! Now that we established that acceleration of free falling objects is gravity we can take the 'how far' and 'how fast' equations and substitute a for g!
How fast: v = gt
How far: d = 1/2 gt2
Let's say that you want to know how tall a building is and you don't have any type of measuring instrument. What now? It's easy! Go to the top of the building drop a ball from the top of it and record how long it took to reach the ground. Do this a few times plug in the average time for t in the how far equation and 9.8 for g! Do the math... and now you know how tall the building is!
Now we moved on to talk about projectiles. Projectiles are objects that move through air or space under the influence of gravity. We learned that when an object is not only dropped straight down it has a vertical velocity and a horizontal velocity. The two are independent of each other! Horizontal velocity will remain constant and vertical velocity will accelerate 10 m/s each second due to gravity. One of the hardest things for me to understand this unit was when to use the vertical velocity and when to use the horizontal velocity when answering questions. I learned that when you want to know how long an object stayed in the air distance from the ground is the only thing that matters! The "object falling from a plane" problems also gave me some trouble but after going over some quizzes I understand them a lot better. We learned how to find out the actual velocity of a projectile by using triangles and the two velocities.
Now that we know how objects behave falling only due to gravity we can talk about objects falling with air resistance! A term that is really important to understand is terminal velocity. Terminal velocity happens when the F air = F weight, the acceleration at terminal velocity is 0 and the velocity is constant! It is also important to know that there are two ways to increase air resistance:
*Increasing surface area
*Increasing the velocity
Have you ever wondered why a crumpled piece of paper falls faster than a flat one? The reason that happens is because when objects are falling with air resistance they want to reach terminal velocity that is they want their weight to equal the Fair. The papers have the same weight so they need the same Fair. The difference between the two is that one of them has a lot more surface area than the other. Because surface area increases air resistance the flat paper can move slower and still reach terminal velocity. The crumpled paper needs to increase velocity in order to increase air resistance to reach terminal velocity. The same concept of surface area applies to skydiving! If you open your arms and therefore increase your surface area you will slow down because the air resistance will be greater! When you open a parachute the concept is the same again, the parachute will increase your surface area considerably so you will slow down considerably!
The content of this unit can be observed everywhere! Now you know why some things fall faster than others, why you can slow down and speed up while skydiving and even how a parachute works!
a = Fnet/m
After learning a little bit about Newton's second law we learned the differences between mass and weight. Mass is a measure of inertia and is represented in Kilograms. Mass never changes from one place to another! Weight is a measure of force always represented in Newtons! Weight does change from one place to another because w = mg and the gravitational force is different in different places. If you are curious to find out how much you weight on the moon or other planets check out the link bellow!
http://www.exploratorium.edu/ronh/weight/
After making it clear the differences between mass and weight we started to talk about objects in free fall. An object in free fall is falling only under the influence of gravity. We found out that the acceleration of an object in free fall is always gravity. That is explain by the fact that Newton's second law says a = Fnet/m. Knowing that the Fnet is the weight of the object and that w = mg rewriting the equation we now have a = mg/m. The masses will cancel out leaving a = g! Now that we established that acceleration of free falling objects is gravity we can take the 'how far' and 'how fast' equations and substitute a for g!
How fast: v = gt
How far: d = 1/2 gt2
Let's say that you want to know how tall a building is and you don't have any type of measuring instrument. What now? It's easy! Go to the top of the building drop a ball from the top of it and record how long it took to reach the ground. Do this a few times plug in the average time for t in the how far equation and 9.8 for g! Do the math... and now you know how tall the building is!
Now we moved on to talk about projectiles. Projectiles are objects that move through air or space under the influence of gravity. We learned that when an object is not only dropped straight down it has a vertical velocity and a horizontal velocity. The two are independent of each other! Horizontal velocity will remain constant and vertical velocity will accelerate 10 m/s each second due to gravity. One of the hardest things for me to understand this unit was when to use the vertical velocity and when to use the horizontal velocity when answering questions. I learned that when you want to know how long an object stayed in the air distance from the ground is the only thing that matters! The "object falling from a plane" problems also gave me some trouble but after going over some quizzes I understand them a lot better. We learned how to find out the actual velocity of a projectile by using triangles and the two velocities.
Now that we know how objects behave falling only due to gravity we can talk about objects falling with air resistance! A term that is really important to understand is terminal velocity. Terminal velocity happens when the F air = F weight, the acceleration at terminal velocity is 0 and the velocity is constant! It is also important to know that there are two ways to increase air resistance:
*Increasing surface area
*Increasing the velocity
Have you ever wondered why a crumpled piece of paper falls faster than a flat one? The reason that happens is because when objects are falling with air resistance they want to reach terminal velocity that is they want their weight to equal the Fair. The papers have the same weight so they need the same Fair. The difference between the two is that one of them has a lot more surface area than the other. Because surface area increases air resistance the flat paper can move slower and still reach terminal velocity. The crumpled paper needs to increase velocity in order to increase air resistance to reach terminal velocity. The same concept of surface area applies to skydiving! If you open your arms and therefore increase your surface area you will slow down because the air resistance will be greater! When you open a parachute the concept is the same again, the parachute will increase your surface area considerably so you will slow down considerably!
The content of this unit can be observed everywhere! Now you know why some things fall faster than others, why you can slow down and speed up while skydiving and even how a parachute works!
Friday, September 23, 2011
Unit 1 Reflection
Inertia!
On the first unit of my physics class we learned a lot about Newton's first law. Newton's first law
states that inertia is the name of the property that's states that an object in motion wants to stay in motion and an object in rest wants to stay at rest unless a net force acts upon it. It's important to remember that inertia is not the reason why objects want to keep doing what they are already doing, the reason for that is unknown and inertia is only the name of this property.
The video above is an example of inertia. When the tablecloth is pulled from under the dishes, they don't fall on the floor because they were at rest and for this reason they wanted to stay at rest, because there was no outside force acting on them they stayed where they were.
While talking about inertia we approach other new terms such as equilibrium and net force. Net force is when more than one force acts on an object. When the net forces are balanced and for this reason they add up to 0 Newton (measure of force) the object is said to be in equilibrium. When a body is in equilibrium it can be moving at constant speed or it can be at rest.
Speed, Velocity and Acceleration:
After talking about inertia, we moved on to talk about speed, velocity and acceleration. We learned that speed is a measure of the magnitude of distance covered in a time period. Velocity is distance in a determined time and also takes in consideration the direction of the movement, therefore even if a car maintain the same speed in a curvy road, the velocity will not be constant. Acceleration is the rate of change in velocity over a period of time. The equations that represent those terms are:
s = d/t
v = Dd/t
a = Dv/t
Constant acceleration:
How fast v = at
How far: d = 1/2 at2
This equations bring me to what was the hardest part of this unit. Even though the equations are straight forward, learning which one to use in each specific situation was not easy. On some of my quizzes. After some time my problem solving skills got a lot stronger because I started to understand how to use the formulas correctly. I understood the concepts of inertia and equilibrium pretty easily probably because of the hovercraft lab we did on one of our first classes. The lab helped me understand that objects in motion really do stay in motion unless there is a force acting upon it.
Conection to Real life:
This unit is extremely connected to our everyday life. Who never forgot something on top of a car and took of? Have you ever stopped to observed that the object is at the same place it was before but on the ground? This concept is explained by Newton's first law! The object was at rest and therefore wanted to stay at rest, there was no outside force to stop it from doing so therefore the object continued in the same place but on the floor. This is just one example of many situations that can be explained by inertia!
On the first unit of my physics class we learned a lot about Newton's first law. Newton's first law
states that inertia is the name of the property that's states that an object in motion wants to stay in motion and an object in rest wants to stay at rest unless a net force acts upon it. It's important to remember that inertia is not the reason why objects want to keep doing what they are already doing, the reason for that is unknown and inertia is only the name of this property.
Speed, Velocity and Acceleration:
After talking about inertia, we moved on to talk about speed, velocity and acceleration. We learned that speed is a measure of the magnitude of distance covered in a time period. Velocity is distance in a determined time and also takes in consideration the direction of the movement, therefore even if a car maintain the same speed in a curvy road, the velocity will not be constant. Acceleration is the rate of change in velocity over a period of time. The equations that represent those terms are:
s = d/t
v = Dd/t
a = Dv/t
Constant acceleration:
How fast v = at
How far: d = 1/2 at2
This equations bring me to what was the hardest part of this unit. Even though the equations are straight forward, learning which one to use in each specific situation was not easy. On some of my quizzes. After some time my problem solving skills got a lot stronger because I started to understand how to use the formulas correctly. I understood the concepts of inertia and equilibrium pretty easily probably because of the hovercraft lab we did on one of our first classes. The lab helped me understand that objects in motion really do stay in motion unless there is a force acting upon it.
Conection to Real life:
This unit is extremely connected to our everyday life. Who never forgot something on top of a car and took of? Have you ever stopped to observed that the object is at the same place it was before but on the ground? This concept is explained by Newton's first law! The object was at rest and therefore wanted to stay at rest, there was no outside force to stop it from doing so therefore the object continued in the same place but on the floor. This is just one example of many situations that can be explained by inertia!
Subscribe to:
Posts (Atom)