UNIT 5 SUMMARY

A) In this unit I learned about...

• Work and Power
• The Relationship between Work and Kinetic Energy
• Conservation of Energy
• Machines

Work & Power

Work is something that occurs every time there is a certain Force and Distance that are parallel to each other. We can express this in the formula, Work = F • d. An example of this would be walking up the stairs. Your force, lets say 600N, multiplied by the height of the stairs, maybe 10m, would give you the amount of work you have done. 600N * 10m = 6000J. The J stands for Joules (Work is measured in Joules).

WORK IS NOT DONE IF..... 

1) THE FORCES ARE NOT PARALLEL 

2) NO DISTANCE IS COVERED

Work is responsible for power. If I wanted to know how much power it took for me to run up the stairs there are two things we need to know. To find power, we need to know our work and the time it took us to do that work. The formula for power is, Power = Work / Time. So, if we think back to someone walking up the stairs, we can guess that it took that person about 10s to get to the top of the staircase. Our equation would come out like this..... 6000J / 10s = 10 watts. (Power is measured in watts). 

FUN FACT: There are about 746 watts in 1 horsepower. The average truck engine has a horsepower of roughly 320..... (thats a lot!)

The Relationship between Work and Kinetic Energy

The formula for Kinetic Energy is KE = 1/2 m (v) ². The word 'energy' comes from the Greek word 'energon'. It translates literally to = 'I have work in me.' But, if we put this is simpler terms, it is the ability to do work. Kinetic also comes from the Greeks. 'Kinema' means = movement. So, if we combine the two, we get 'the energy of movement.' Energy is also measured in Joules. 

You might see a question about the relationship between Work and Kinetic Energy formulated like this....

1) A 20kg car accelerated from 20m/s to 30m/s over a time span of 5 seconds. In that time, it traveled 100 meters. 

a) What was the change in energy that the car experienced?

KE = 1/2 m (v) ² 

∆KE = Kfinal - Kinitial

KE initial = 1/2 (20)(20)²

= 4000 J

KE final = 1/2 (20)(30)²

= 9000 J

9000 - 4000

= 5000J

b) How much work was done?

Work = ∆KE 

Work = 5000J

c) Calculate the force the car used that caused the car to accelerate?

Work = F • d 

5000J / 100 = F • (100) / 100

F = 50N

d) What was the power during the acceleration?

Power = Work / Time

Power = 5000 / 5

= 1000 watts

Then, of course, we have our HUGE point question. The question is.... 

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

KE = 1/2 m (v) ² : You are going to go from moving to not moving regardless of what stops you.

∆KE = Kfinal - Kinitial : Your ∆KE is going to be the same regardless of what surface you hit.

Work = F • d : With an airbag, you are increasing your distance and therefore lessening your force. This is because the force is being applied in smaller increments over a longer period of time. 

So..... 

WITH AIRBAG = F • d

WITHOUT AIRBAG = F • d

Conservation of Energy

Conservation of Energy incorporates Potential Energy into the equation. The formula for Potential Energy is Mass • Gravity • Height. (PE = mgh) Potential energy is also measured in Joules. The relationship between potential energy and kinetic energy is that potential energy = the kinetic energy at its lowest point (and vice-versa). While something CAN be moving and have potential energy, it is IMPOSSIBLE for something to be at rest and have kinetic energy. (Kinetic energy relies on movement and it cannot be present if the velocity is zero!!) The law of conservation of energy tells us that energy cannot be created nor destroyed. It can only be transformed into different forms. 

So, if you weigh 50kg and you jump off of a swing-set that was 5 meters high, your potential energy would be....

PE = mgh

(50) (10) (5)

= (500) (5)

= 2,500 J

This means that at the top of the swingset, my potential energy is 2,500 J and my kinetic energy is 0 J. I can bet that you already know that just before I hit the ground my potential energy will be 0 J and my kinetic energy will be 2,500 J. 

Another way to remember this is with a roller coaster. Have you ever wondered why you keep going forward, up and down the huge hills? Well, as long as all of the hills are smaller than the initial one, you should be able to get over them. We can prove this because we know that energy cannot be created or destroyed, only transformed.

Machines

The job of a machine is to decrease the amount of force you have to use (all at once) by increasing the distance. In this subject we focus on work-in and work-out. Work-in = Work-out. An example of a simple machine is a ramp. We put 'work-in' on the ramp, and get 'work-out' at the top. Another formula presents itself at this time. When talking about work, we want to know the efficiency we are completing this work at. Efficiency = workin / workout. There is hardly any machine in the world that has an efficiency rate of 100%. This is due to lost energy in 3 different forms. These are heat, light, and sound. 

If you are trying to push a heavy box (500N) up a ramp and into a truck, how much force are you actually putting in, with the help of your machine? (The ramp is 10m high and 10m long)

Workin = Workout

F-in • d-in = F-out • d-out

F • (10) / 10 = 500 • 10 /10

F = 500N

THANKS FOR READING!

I HOPE YOU LEARNED A LOT!