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.