# Homework Help: Flywheel breaking problem

1. Dec 10, 2007

I need a bit of help with a few questions.

In a hybrid vehicle a flywheel is used to capture the energy of the car during braking. If the car has a total mass of 850kg and the flywheel has a mass-moment of inertia equal to 14kgm2 determine the speed of the flywheel when the car stops for traffic lights in an urban area (you may assume 100% efficiency).

and...

A downhill cart race is to start at 10.30 am precisely. The hill has a slope that varies from 1:4 to 1:10 and has a vertical drop of 75m. If the track length is 0.75km (the first 0.5 km being the hill itself) and assuming no energy losses determine
a) the maximum speed attained
b) the time the first cart crosses the finish line

thanks

2. Dec 11, 2007

### Kurdt

Staff Emeritus
Welcome to PF! What have you attempted so far with these questions. You cannot receive any help until you have made some effort yourself.

3. Dec 11, 2007

change in kinetic energy of car to rotational energy of flywheel
so would have 0.5MV^2=0.5Iw^2

=> w^2=(M/I)V^2
=> w=7.79V
and then the drop from 30mph to zero needs to be taken into account somehow.

am i right?

4. Dec 11, 2007

### Astronuc

Staff Emeritus
Yes - the change in kinetic energy based on the velocity decreasing from 30 mph to 0 goes into the flywell, which increases it rotational kinetic energy.

BTW, the 30 mph is not mentioned in the OP. This is necessary peice of information in this problem.

5. Dec 11, 2007

it mentions that it is in an urban area so i assumed that the speed limit would be 30 therefore the speed of the car would be 30?? i dont know though

6. Dec 11, 2007

### Kurdt

Staff Emeritus
Yes, if you're in the Uk then the speed limit will be 30mph for the urban area. You can then work out the change in kinetic energy.

7. Dec 11, 2007

would the maximum speed attained down the hill be 38.36m/s?
using v=sqrt2gh?
or is velocity not the same as maximum speed?

8. Jul 1, 2008

### zeuso

Re: Help

Could anyone confirm if this is correct, including the thread starter?

Thanks