Unit 5 Summery

In this unit we discussed work, power, kinetic energy, the conservation of energy, and machines.

Work

·      Work= Force x distance

·      It transfers energy

·      Work is measured in Joules

Work can only be done when the force and distance are parallel. 

However, work cannot be done when the force and distance are perpendicular. For example, if you carry a box weighing 20N up the same flight of stairs- the force and distance are not parallel and therefor no work is done.

If you exert a force onto a wall and the wall does not move, no work is done. This is because you did not move any distance though you exerted the force.

Power

·      Power= work/ time (the amount of work done per unit of time)

·      Work is responsible for power

·      Power is measured in watts (J/s)

·      746 watts= 1 Horsepower

So, you weigh 200N and go up the stairs to third Lawrence, which is 10m and it takes you 40 seconds to do this. How much work was done? How much power was generated?

Work= F x d                 

Work= 200 x 10

Work=2000J

Power=work/time

Power=2000/ 40

Power= 50 watts

Kinetic Energy

·      It is defined as the measurement of movement.

·      There is no KE when something is at rest.

·      KE= ½ mv²

·      Change in KE= work

·      Change in KE= KE final- KE initial

In terms of energy, why do airbags keep us safe?

           -When you crash, you go from moving to not moving regardless of what stopped you. If KE doesn’t change neither does work. The airbag increases the distance to stop and since work is conserved and remains the same the F is compensated and decreased.

No airbag= FORCE x distance

Airbag= force= DISTANCE

Potential Energy

·      PE= mgh

·      PE= (mass) (gravity) (height)

·      Defined as the energy of a certain position.

·      Unlike KE, this can occur whether or not an object is moving.

·      PE when falling will convert to and from KE

·      (for more info watch the link)

Conservation of Energy

     

No energy can be created nor destroyed- only converted.

·      Work requires energy!

Machines

·      Machines increase the amount of distance and decrease the amount of force needed (this could be an inclined plane).

 Because work remains constant and the distance is increased, the force must compensates and decreases.

For example: If Janice and Stitch were just you know, chillin’, and they needed to move their grandma’s 300N piano into her new home, it would be easier for them to use a ramp than to lift straight up 2m. Because then it would take…

work= F x d

Work= 300N (2m)

Work= 600J

But if they used a 4m ramp then…

Work= F x d

600= F x 4

F=150N

Thank you!

            

Unit 4 Summer Blog Post

Unit 4 Summer Blog Post

In this unit we learned a lot about rotational and tangential velocity, Rotational Inertia and Angular Momentum, Torque, and centripetal force. 

Rotational and Tangential Velocity

Tangential Speed- this is the speed of an object moving in along circular path. (radial distance x rotational spin) or (v~rw)

Rotational Speed- (A.K.A angular speed) involves the amount of rotations per unit of time.

In the photograph above we see two little girls on a carousel. A carousel is the perfect demonstration to compare tangential and rotational speed. In this picture, you will recognize that these girls are going to rotate the same number of times.   

However, the little nugget on the left has a greater tangential velocity. This is because she is located further from the axis of rotation and will have to cover the same amount of distance than the girl on the right in the same amount of time.

How are train wheel designed to keep them on the track?

Train wheels are designed tapered with the fat/ wider part in the middle and the more narrow part on the outside. All parts of the wheel have the same rotational speed but the wider part has a greater tangential speed. This difference causes the wheels to curve when the wider part in the middle causes the train to always move towards the middle of the track.

This is probably wordy and might make no sense, so for more information watch this video we made!

Rotational Inertia and Angular Momentum

In previous blog posts, we have discussed inertia, which is the property of an object to resist changes in motion.

However, Rotational Inertia is the property of an object to resist the spin.

In rotational inertia it is important to notice mass distribution. Where the mass is located has a huge affect on the speed.

In the image above, you can see that the ice skater with her mass distributed closer to her body, increases her speed.

We know that angular momentum before=angular momentum after.

When she is in a tight ball, her mass is closer to her axis of rotation. When she spreads out, her mass is farther from the axis. In the ball she has a very fast ROTATIONAL VELOCITY and a small rotational inertia. But the one with her arms out has a small rotational velocity and a big ROTATIONAL INERTIA. Thus, ROTATIONAL VELOCITY x rotational inertia= rotational velocity x ROTATIONAL INERTIA.  Because her arms are closer to her body, her momentum is conserved, when she extends her body and moves her mass further from the axis of rotation, it increases her rotational inertia but her rotational velocity will decrease.

Torque

Torque causes something to rotate.

Torque= (Force) (lever arm)

A lever arm is the distance away from the axis of rotation.

Below are pictures of wrenches. The longest one or the one with the bigger lever arm will have more torque.

Now, because torque causes rotation, how do we stay balanced?

For starts, our torque has to stay balanced. We must have equal torques.

Another concept that is important in understanding torque is the center of mass (an objects average position of mass). You also have a base of support. If the center of mass is with in the base of support, it will not tip over. This is one of the reasons football players bend their legs apart (bigger base of support) and bend their knees (lower center of gravity).

In my previous blog we discussed how we balanced the meter stick. I went pretty in depth explaining it so I won't go into full detail. However, I will highlight the following equations.

• w=mg

• Torque= F x lever arm

F x lever arm = F x lever arm. 

1 (2) = 2(F)

2= 2F

1=F

1N 

This is an example equation of solving for the weight of a side of the meter stick. 

Centripetal Force

A centripetal force is a center seeking force. It keeps you going on a curve. You can find the centripetal force by adding the Fweight and Fsupport.  A centrifugal force is a center fleeing force that doesn’t exist.

Why do clothes get drier in the spin cycle of the washing machine? Because the centripetal force of the wall keeps the clothes in while it moves at a certain velocity that keeps the clothes spinning. This is also the reason the cup stays in its curved path. 

Thank you for reading!