# Rotational energy and flywheels

## Homework Statement

Hey guys I have two questions. The first one i'm not sure, while the second one, I have some idea but don't know how to proceed with answering the question.

1. A rapidly spinning flywheel has been suggested as an energy storage mechanism for cars. Let's consider a 300kg uniform disk flywheel of diameter of 0.5m spinning at 100 rotations per second. Find the energy stored in the spinning flywheel. How higha hill could the 1000kg car (including the flywheel) climb with this energy?

2. Calculate the force on a 1kg chunk of metal near the edge of the disk.

## Homework Equations

ω=2∏/T
Erot=0.5Iω2=0.25MR2ω2
Erot=Ep

## The Attempt at a Solution

1.

The total mass of the car is 1000kg which includes the flywheels. In total there are 4 flywheels on the car. So the mass of the car is distributed among the 4 wheels.

M=1000/4=250kg (per wheel)

Using the third equation,
Erot=0.25*250*0.252*6282=1540562.5J
This is the energy for one wheel. Therefore, the total energy of the 4 wheel is 4*1540562.5=6162250J.

Using conservation of energy (last equation) 6162250=250*9.8*h.
Solving for h: h=6162250/(250*9.8)=2515m

2.

For this question, I know that I have to find the tangential force, but, how would i start? Is the tangential force a component of centripetal force???

Many thanks :D

## Answers and Replies

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CWatters
Homework Helper
Gold Member
1. You have this wrong. The mass of the flywheel is stated in the problem as 300kg. It only mentions one flywheel so I've no idea why you are assuming there are four 250kg flywheels?

Use your equation to work out the energy stored in that one 300kg flywheel.

The total car weight is given as 1000kg. Write an equation for the energy required to raise that much mass by height h. Assume all the energy stored in that one 300kg flywheel is converted into raising the car and solve for h.

2. You are making it more complicated than it is. The answer is just the centripetal force. Imagine you have a 1kg mass on a string of length =0.5m rotating at ω. What is the centripetal force (aka tension in the string).

haruspex
Homework Helper
Gold Member
2020 Award
1. You have this wrong. The mass of the flywheel is stated in the problem as 300kg. It only mentions one flywheel so I've no idea why you are assuming there are four 250kg flywheels?
Seems to be a confusion between flywheels and roadwheels.

1. You have this wrong. The mass of the flywheel is stated in the problem as 300kg. It only mentions one flywheel so I've no idea why you are assuming there are four 250kg flywheels?

Use your equation to work out the energy stored in that one 300kg flywheel.

The total car weight is given as 1000kg. Write an equation for the energy required to raise that much mass by height h. Assume all the energy stored in that one 300kg flywheel is converted into raising the car and solve for h.

2. You are making it more complicated than it is. The answer is just the centripetal force. Imagine you have a 1kg mass on a string of length =0.5m rotating at ω. What is the centripetal force (aka tension in the string).
Sorry for the late reply, many thanks for your input!!
Ahhh yes, googled "flywheels", turned out to be some wheel under a car storing energy. Got confused with flywheels being the tyres haha :D

CWatters