# Homework Help: Flywheel calculations

1. Aug 6, 2015

### Katy96

1. The problem statement, all variables and given/known data

A flywheel made of glass fibre has been developed by BP to store the kinetic energy of buses when they come to a bus stop. The maximum energy stored in the flywheel is equivalent to the energy lost by a 16,000 kg bus braking from 48 km when it comes to rest. The bulk of the flywheel’s mass is in its rim which moves at 1300 km when the flywheel rotates at the maximum angular velocity of 16,000 rev . After a typical stop of two minutes during which time the flywheel slows down, four fifths of its energy is still available to accelerate the vehicle.

From this data show that the flywheel has the following properties:

1. Maximum kinetic energy = 1.42 MJ (2 marks)

2. Moment of Inertia = 1.01 kg (3 marks)

3. Radius = 0.215 m (2 marks)

4. Minimum mass = 21.9 kg (3 marks)

(Each section must be answered in the order given. You may not assume data from the following sections).

e) Determine also the

5. Determine also the mean frictional torque causing the slowing down in the two minute stop. (6 marks)

2. Relevant equations
not really sure!!
3. The attempt at a solution
I cant figure out how to do this as I don't know how to do it In this order?

2. Aug 6, 2015

3. Aug 6, 2015

### Katy96

I tried using and then I didn't know which other equation I could use but I don't know the radius or anything

4. Aug 6, 2015

### PeroK

Where does the KE come from?

5. Aug 6, 2015

### Katy96

The kinetic energy of an object is the energy it possesses because of its motion?

6. Aug 6, 2015

### PeroK

Okay, I'll get you started. The energy comes from the bus. It tells you that in the question.

7. Aug 6, 2015

### Katy96

well yes I understand that part, I just don't understand how I use what ive been given to work it out as im used to using

8. Aug 6, 2015

### PeroK

How else could you work out the energy of the flywheel? Hint: think about the energy of the bus.

9. Aug 6, 2015

### Katy96

E = ½mv2?

10. Aug 6, 2015

### PeroK

If you mean the energy of the flywheel is equal to the energy of the bus, and you're given the mass and velocity of the bus, then yes!

11. Aug 6, 2015

### Katy96

yeah that's what I meant, so I use the mass of the bus and the speed from which it is braking from?

12. Aug 6, 2015

### PeroK

Yes, that's what the question implies.

13. Aug 6, 2015

### Katy96

but if I do that I get that E=18432000?

14. Aug 6, 2015

### PeroK

What units are you using? I assume the speed is 48 km/h? What if the speed of the bus was given in mph (miles per hour)? Or cm/s?

15. Aug 6, 2015

### Katy96

yes its 48km/h so what I did was (0.5)(16000)(48)^2

16. Aug 6, 2015

### PeroK

So, why is that wrong? Note that 48 km/h is about 30 mph. So, why not (0.5)(16000)(30)^2?

17. Aug 6, 2015

### Katy96

the answer, Maximum kinetic energy = 1.42 MJ, is given in the question, when 30mph is used you get 7200000 which is also the wrong answer

18. Aug 6, 2015

### PeroK

So, if you take the speed in different units, you get different numerical answers. There is only one set of units in the SI:

Kilograms, Metres, Seconds and Joules

You must use mass in kilograms and speed in metres per second to get KE in Joules.

19. Aug 6, 2015

### Katy96

so how do I change 48km/h into m/s? never mind I got it now!!

Last edited: Aug 6, 2015
20. Aug 6, 2015

### PeroK

Are you really saying that if a bus is going at 48 km/h, you don't know how far it travels each second?

21. Aug 6, 2015

### Katy96

ive done it, and done part a, im just getting confused on which equations I need to use for which parts.

22. Aug 6, 2015

### Katy96

so in part b I know I need to use

23. Aug 6, 2015

### PeroK

Yes, you've got the KE now and you're given $\omega$ (although you need to watch the units again), so you should be able to calculate $I$.

24. Aug 6, 2015

### Katy96

which is ω?

25. Aug 6, 2015