Unit Five

During Unit five we learned many things such as:

Work

Work is responsible for power.
Work is measured in Joules.
To find the amount of work done you must know the force and distance the object travelled, or
Work=Fxd

In order for work to be done the force and distance must be parallel NOT perpendicular. So for example work cannot be done when a person is pushing against a wall because the wall does not move therefore the distance is 0 so when we put that in the equation work=0J.

Here is an example of when work is done, Jack pushes a box with 50N for 10m, work=(50)(10) so work=500J.

Power 

Power is how quickly work is done therefore power=work/time

However power is measured in watts, and there are 746 watts in 1 horsepower.

Here is an example of power and work!
Mac (600N) walks up the stairs (10m) in 30 seconds. Calculate his work done and the power created.

Work=Fxd
Work=(600N)(10)
Work=6000J
Power=Work/Time
Power=6000/30
Power=200watts

Now Mac runs up the stairs in 10 seconds, calculate work and power.

Work=Fxd
Work=(600N)(10)
Work=6000J
As you can see work stayed the same this is because time does not effect work.
Power=Work/Time
Power=6000/10
Power=600watts

Kinetic Energy

In order for there to be energy movement must be incorporated, if not it does not have energy.

Energy, like work, is measure in joules.

Work= change in KE
and in order to find change in KE you must subtract Kinetic Energy final-Kinetic Energy initial.
So change in KE=Kinetic Energy final-Kinetic Energy initial.

To find the kinetic energy of of something you use the equation KE=1/2mv^2.

So why do airbags keep us safe?

( KE=1/2mv^2. ) When you crash you go from moving to not moving regardless of how you stop. (change in KE=Kinetic Energy final-Kinetic Energy initial) If the KE does not change neither does the work. (Work=Fxd) The airbag increases the distance to stop and since work is the same, force is compensated and decrease.
Without airbag--> Work=(big)Fx(small)d
With--> Work=small(F)big(d)

If we can find the kinetic energy and work we can then find distance, power, and force.

Potential Energy

Potential Energy is the energy of a position (height)

To find potential energy use, PE=(mass)(gravity)(height).

In this photo, the ball that is lifted has a certain amount of PE and once it hits the other balls it will convert into KE, and then back into PE once the ball on the other end goes up, and so on.

The conservation of energy means that energy of an object will always remain constant, because energy cannot be treated nor destroyed.  

Raising an object twice as high gives the object twice the potential energy.

Machines

Machines increase the distance to do something in order to lower the force needed. So an example of a machine is a ramp. 

The greater the distance the smaller the force.

So the ramp has something call fin/din and fin/dout which is force in/distance in and force out/distance out. These correlate with the ramp fin and din go on the diagonal and the fout and dout go on the horizontal part of the ramp.

Workin=workout

 fin(din)=fin(dout)

An example of this is, Bobby wants to lift an object and put it into the back of his truck, he constructs a ramp 1m high and 5m long, he pushes with 200N to get it in to the truck. So what was the difference between using the ramp to not using it?

Work=Fxd

(with ramp) 200=F(5) --> 200=5F --> F=40N 

(without ramp) 200=F(1) --> F=200N

So Bobby uses much less force with the ramp but both do the same amount of work. 

Efficience=workout/workin (cannot be more than 100% efficient because not possible)

Unit Four Blog Post

There were many things that we learned during unit four, they were, rotational inertia, rotational and tangential velocity, conservative of angular momentum, torque, and centripetal/centrifugal force.

Rotational Inertia
Rotational inertia is the property to resist changes in motion (spin). Rotational inertia is what helps determine how many flips you can do on a trampoline, or how many spins an ice skater can prefer. If your arms are close to your body (axis of rotation) the more spins you will be able to preform.

In this diagram, the two meter sticks have two different rotational inertias. The meter stick with the weights next to the end will have a higher rotational inertia than the meter stick with the weights near the middle will have a lower rotational inertia because most of the mass is near the axis of rotation, therefore easier to spin.

Rotational and Tangential Velocities

Tangential speed is the speed of something moving along a circular path. For example on a merry go around, the person closest to the axis of rotation will have a lower tangential speed than the person on the far outside of the merry go around, however they will both have the same rotational velocity because they both will have gone around a complete time at the same time.

 By looking at these two diagrams you can see that the tangential speed increases as you move farther from the axis of rotation however the rotational speed stays the same the whole time. 

Okay so in this example the outside must travel the same distance as the inner merry go around in order to keep the same rotational velocity. 

 Train wheel are designed with tapered wheels wider on the inside and narrower on the outside. Thus 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 is on the track. When the wider part is on the track it causes the train to self correct its self and get back in the middle!

However gears do not work like train wheels in fact they are the opposite, in most cases the rotational speed is different while the tangental speed is the same.

Conservation of Angular Momentum

The conservation of momentum/angular momentum is equal to the total momentum before to the total momentum after. 

Angular momentum is based off two things,

• Rotational inertia and
• Rotational velocity

Angular momentum before=Angular momentum after

Rotational inertia(Rotational velocity)BEFORE=Rotational inertia(Rotational velocity)AFTER

 

When you go from curled into a ball from stretched your rotational inertia will become higher than before and therefore decreasing rotational velocity.

Rotational inertia and Rotational velocity are directly proportional.

Torque

Torque causes rotation and torque=force(lever arm) a lever arm is the distance from the axis of rotation to the point of contact.  The larger the lever arm the less amount of force there is required and vise versa. AND force(lever arm)=force(lever arm)

As you can see the wrench with a larger lever arm will have a smaller force required than the wrench with the smaller lever arm.

All objects have an average position of their mass, and that is know as their center of mass. Center of gravity must be within base of support or it will fall over.

In this diagram we can see that the people have to adjust their base of support in order they don't fall over.

Counter clockwise torque=clockwise torque 

BALANCE 

In order for something to be balanced, the torques must be equal and opposite,  the center of gravity must be in between the base of support, and the larger the base of support the easier it is for something to become balanced.

Centripetal and Centrifugal Force

Centripetal force is a center seeking force, centrifugal force does not exist it is the concept of a center fleeing force BUT doesn't exist.

The reason we don't fall out of a roller coast when it goes upside down is because the centripetal force pulling towards the center is not a strong as the force pulling the people forward or green arrow.