Unit 4 blog reflection

Rotational and Tangential Velocity and Inertia

What is rotational Inertia?
-The property of an object which resists changes in motion
-It depends on: mass (more mass=harder to spin) (less mass= easier to spin)

Rotational velocity?
-How fast you rotate

Why do runners bend their legs instead of keeping them straight?
-Because when they bend their knees, they are bringing them closer to the axis of rotation. The closer to the axis of rotation the less inertia and the easier it is to move.



The skater is experiencing the same thing:

In the first picture her mass is spread out far from her axis of rotation so she will have more rotational inertia so she will not go as fast.

In the second picture her mass is brought towards the center, so she will have less inertia thus making her rotational velocity increase.




What is tangential velocity?
-The linear speed of something moving along a circular path

How does a train's wheels stay on the track?

In this picture the inside part of the wheel to the left has a higher tangential velocity than the one to the right because it is larger and covering more distance.

Since the tangential is higher on the inside of one wheel, it causes the train to tilt inward and self correct.

Who has a higher tangential velocity on a carousel? Rotational?

-The person furthest from the center has the greatest tangential velocity

- They have the same rotational because the go around the same amount but the person on the outside is covering more distance than the person on the inside so they have a higher tangential velocity.

In these gears how does the Tangential and Rotational velocities compare? 

The smaller gear in this photo will rotate twice while the bigger one rotates once which means they have different rotational velocities. They have the exact same tangential velocity though because they cover the same amount of distance in the same amount of time.

How does Inertia apply when you are rounding a curve in a car and you push up against the car door?

-Since you are still moving forward when the car is turning, the door is the object that stops you from moving in that direction and causes you to move in the direction of the car again

Conservation of Angular Momentum 

How do you find the angular momentum?

-angular momentum before = angular momentum after

-angular momentum = rotational inertia X rotational velocity 

What does angular momentum rely on?
 -rotational inertia
 - rotational velocity

When someone is running and they bend their knee, what happens to their mass as they bend their knee?
-their mass is lowered and so is their center of gravity which stays within the base of support


Torque 

What is torque?
 -it causes rotation
 -torque= force X lever arm
 -the larger the force to greater the torque
 -the larger the lever arm the greater the torque

What is the lever arm? 

 -the distance from the axis of rotation

For example: 

In this picture more force is required to push the door when you are pushing towards the hinges.

Less force is required when you push the door further from the hinges because the lever arm is larger.

*BIG TORQUE = BIG ROTATION*

If you attached a rope to the end of a wrench would that increase the torque? Why or why not?
 -no because the lever arm actually isn't longer since you are unable to apply a large force on it. You would need to use something stronger like a pipe.

Where is the best place to put a door stop: in the middle of the door or the edge?
 -towards the edge because it would require less force than the middle of the door since the lever arm is longer

Center of mass/ gravity

What is the center of mass?
 -An object's average position of all of the mass

How does an object balance?
 -If the center of mass stays within the object's base of support

Why do you bend your knees in sports?
 -gives us a big support base
 -makes it so that your center of gravity lowers and remains within the base of support

Why is it more beneficial for a tight rope walker to hold a pole that droops?
-because it lowers the center of gravity so you won't have a torque
-so that you can distribute your mass-->balance

Why is it better to hold two buckets rather than one?
-so that the mass is distributed which creates balance

If you draw a picture and draw a straight line coming from the center of gravity down and it stays within the base of support it means it will not fall over. If you draw a different picture and the box is tilting and the center of gravity is outside of the base of support it will fall over. (sorry I can't draw a picture-tech difficulties)

Centripetal and Centrifugal force

What is centripetal force?
-a force that is center seeking
-for example if you are driving around a curve the centripetal force is what causes the car to curve

What happens in a washing machine?

The water is in motion so it will continue to stay in motion unless acted upon by an outside force and in this case the centripetal force is not acting on it so it will continue moving.

CENTRIFUGAL FORCE IS NON EXISTANT 

But some say that centrifugal force is what pushes you against your car door when you are rounding a corner. This is not true. What is happening is due to inertia. An object in motion tends to stay in motion unless acted upon by an outside force. In this case your body is still moving forward and the car door is the object that stops you and makes you move in the direction the car is going again.

This unit went pretty well for me. I enjoyed the concepts that we learned and I overall did well on my quizzes. My quiz grades was something that I wanted to improve and I think that I reached this goal. It took me a little while to fully understand rotational and tangential velocity but once I really sat down and really wrestled with it I finally got it. Next unit I want to focus again on my quiz grades, and I think that if I am able to do well on my quizzes I will do well on my test. Overall I really enjoyed this unit and the labs that we did. I think it went smoothly because I really tried to stay focused in class.

Meter stick Challenge

Challenge: find the mass of a meter stick using only a meter stick and a 100g (.1kg) lead weight.

Finding the Torque of the left side of the meter stick:

First you must balance the stick on a table. My meter stick balanced at 25cm, and the center of gravity would be the middle of the stick which is 50cm. There is then 25 cm between where the stick meets the table and where the stick reaches center gravity.

Next it is important to know that:

            TORQUE= (FORCE) (LEVER ARM)

In this case you know that the force on the left of the ruler (where the weight is) is (.1kg)(9.8). The 9.8 represents gravity. 

             TORQUE= (.1kg)(9.8)(lever arm)

As mentioned before we already know what the lever arm is. The lever arm is the distance between the weight and the table, which is 25cm. 

              TORQUE= (.98N)(25cm)

So...

TORQUE= 24.5Ncm

This picture shows the lever arms, where the center of gravity is, and where the force is.

Finding the Torque of the right side of the stick:

First it is important to know that the torque on the left side is the exact same torque as the right side. 

Once you know that you use the same equation:

       TORQUE= (FORCE)(LEVER ARM)

The torque is 24.5

        24.5= (FORCE)(LEVER ARM)

We also know the lever arm. It is 25 cm, because the distance from the place the stick touches the table and the center of gravity.

         24.5=(FORCE)(25cm)

Then you solve for force

         FORCE=.98

Solving for the mass: 

Now you have the torque (which is the same on both sides of the stick) and you have the force, you must solve for the mass.

To solve for the mass you use the equation:

        w=mg

We know the weight because we used it before. The weight is (gravity)(the weight), so .98

        .98=mg

We also know that gravity is always 9.8

        .98=9.8m

Then you just solve for m

         m=100g

The mass of the meter stick is 100g 

We were able to figure out the mass of the meter stick by calculating the torque and then plugging it in to find the force which we then used to find the mass. The only way that this stick could balance on the table is if the torque's are both equal. We got .1 kg as our calculation which was very close to the actual mass which is .117 kg. 

Torque and The Center of Gravity

This video explains what Torque and the center of gravity is. Torque is a perpendicular force applied over a lever arm (distance from axis) causing rotation. In the video the man talks about how a door will rotate around it's hinges which is the same as saying you have applied a torque. It is impossible to rotate the door if you push it parallel, you must apply the force perpendicular in order to apply a torque (cause the door to rotate). As the video states, you want to push the door as far away from the hinges as you can. This will create a larger lever arm which allows the door to rotate. You can increase the torque by increasing the force, increasing the lever arm, or both.
Center of Gravity is the average placement of all your weight. It does not have to be in the exact center of the object. The video explains that you can find the center by hanging the object from something or you can find the center of gravity by rotating it. If you want something stable you must make sure that the center of gravity does not cross over the base of support. If the center of gravity goes over the base of support it will most likely topple over.
This video really helped to further my knowledge of these two concepts, because it provided good examples that were then thoroughly explained.

Torque and center of gravity video

Angular Momentum

This video provides a great explanation and demonstration of angular momentum. Angular momentum depends on the rotational inertia and the rotational velocity. Angular momentum is how fast an object rotates. In this video he is up and space and he demonstrates angular momentum. He begins to spin and then pulls his arms and legs in close to his body which then causes him to spin even faster. This happens because when you pull your arms in the rotational inertia decreases thus causing to rotational velocity to increase.

Horse and buggy/ tug of war

Unit 3 reflection

In Unit 3, I have learned about action and reaction pairs, Newton's 3rd law, how tug of war works, how a horse is able to pull a buggy, adding forces, vectors at angles, gravity and tides, momentum,  impulse and momentum relationship, and the conservation of momentum.

Action and reaction pairs/ Newton's 3rd law

Newtons third law states: every action has an equal and opposite reaction. That means if you touch someone then they touch you back with the same amount of force. In order to find that force you would use the equation:

F=Ma
Reaction and action pairs, are examples of Newton's 3rd law. Here are some examples:

In this picture the man is pushing the wall with 100N so, because of Newton's third law, the wall will push the man back with the same amount of force that the man pushes on the wall.

Another example would be: rocket pushes fuel backward so fuel pushes rocket forward or hammer pushes nail so nail pushes hammer

No matter how hard you push on something it will push back on you with the exact same force. Forces do not exist without other forces.

Tug of war and horse and buggy

How do you win a tug of war contest? It all depends on your firm stance on the ground, because you push ground forward therefore the ground pushes you backward. If you were to wear socks while playing tug of war you would most likely be part of the losing team. This is because if you do not have good traction with the ground you will just slide forward.
How does a horse pull a buggy forward? First it is important to look at the action and reaction pairs, in order to see how the horse is able to pull the buggy forward.

It is important to remember Newton's third law (every action has an equal and opposite reaction). This would mean that the horse and buggy pull on each other with the same amount of force. 

So why does the horse pull the buggy forward? Because the horse pushes on the ground with more force than the buggy pushes on the ground. When looking at the diagram it is important to notice that the arrows that say horse pushes ground back ward therefor the ground pushes the horse forward are longer than the arrows that say the buggy pushes ground forward therefore the ground pushes the buggy backward.

Adding forces and Vectors with angles 

    Vectors are used in order to find things such as the tensions in a rope like the following:

The more tension something (rope) has....the more likely it is to break.

In this diagram the right side would be more likely to break, because there is more tension.

Why does a box slide down a ramp?

Because the gravity pushes it down and the ramp pushes it up, so when the vectors are added together it shows that the box will slide down the ramp.

Gravity and tides 

The universal gravitational force says that, everything with mass attracts al other things with mass. And the formula for this is F=G(m1m2/d^2). 

What does force depend on?

1.) the mass of objects because force is proportional to mass. The small the objects the smaller the force.

2.) the distance between the objects. The force is proportional to 1/d. The greater the distance the weaker the force and the smaller the distance means that the force is more powerful. 

Is the force of gravity greater at high elevations or at sea level? 

It is greater at sea level, because the distance would be much smaller at the beach then say at Mt. Everest. This is because it is measured from the center of the earth which is closer to sea level than Mt. Everest is. 

      Why are there high tides on both sides of the earth? Why not just the side closest to the moon, because the force is greater with the smaller amount of distance? 

      Because when the earth pulls on the moon, the moon pulls back on the earth so that is why there is a high tide on the opposite side of the earth. 

       When the sun, moon, and the earth are all in line we experience Spring tides. (full/ new moon) This means that the tides are higher than they normally are.

        When we experience a half moon then the tides are called neap tides. This means that the tides are lower than they normally are. 

        Although the sun has a stronger force on the earth than the moon (because it has a greater mass), the difference between the moon and the earth and the sun and the earth is smaller, so it has a stronger force on the earth thus the distance is what causes tides.

Momentum/ Impulse and momentum relationship

     The proper way to answer a question regarding Momentum and Impulse, is:

     Why do climbers prefer stretchy ropes to non-stretchy ropes?

            -Because no matter how the climber is stopped, the climber goes from moving to not moving, therefore the change in momentum is the same no matter how the climber is stopped.

           -p=mv

           -p= pfinal-pinitial 

- Since the change in momentum is the same no matter how the climber is stopped, the impulse will also be the same

-J=p

 

---------------> the more time= the less force

       -The climber prefers a stretchy rope rather than a non-stretchy rope, because it increases the time it                  takes for the climber to come to a stop, thus because the impulse is constant the force will be less. A smaller force means the less injury when the climber comes to a stop. 

Conservation of momentum

     The conservation of momentum means that the momentum before, after, and during the collision is the same no matter what. This means that if a car and a truck collide head on the momentum of the system will always be the same. The momentum of the singular car though can have a change in momentum, just not the whole collision. 

Here is an example:

Cart A and Cart B are moving in opposite directions and they stick together after they collide.

How fast will the carts be moving after the collision and in which direction will the carts be traveling?

      

Conclusion

      The most difficult part of this unit for me was understanding vectors as well as the momentum and impulse relationship. I feel like this blog post has helped me to review those concepts so reviewing my notes and having to explain these things really helped me. Another thing that has helped me is watching the podcasts made by my classmates, as well as making one with my group. 

      I would say that I have put a lot of effort into the class and I really enjoy to structure of the class. I have completed almost every homework assignment and have come in during conference period if I didn't understand something. I also feel that working in small groups has helped me grow in this class. It is nice to be able to discuss certain concepts that I struggle with and ask questions. 

       

Tides resource

This video provides a very clear and engaging explanation of tides and how the moon and the sun have to do with it. The creator of this video made a diagram of where the moon has to be when there is a high tide and low tide. The side closest to the moon will experience high tide because of the gravitational pull and the opposite side will experience the high tide as well because the earth is pulling on the moon as well as the moon is pulling on the earth. I definitely believe that this video helped me to see how the tides are effected by the moon. When the moon is full then the tides will be higher than normal and when it is a half moon the tides are lower than normal.