# Homework Help: Max speed of an ev at give HP

1. Aug 20, 2007

### ruteckyb

Problem is to solve for max speed of an electric vehicle, given is the HP of motor (3 total), and the wind+rolling resistance of the vehicle at 60mph. Wind resistance should be regarded as a constant for this problem. Also given is EV weight with cargo and rider

Motor HP = 5, 10 , 20
Resistance is constant @ 165lbs.
EV weight = 2250lbs.

Solve for the given 3 hp varables.

2. Aug 20, 2007

### mgb_phys

You really have to at least try and solve the question - see the sticky at the top of the thread,

To get you started.
Energy is force * distance
Power is energy / time
Speed is distance / time
At constant speed you only have to overcome resistance

3. Aug 20, 2007

### ruteckyb

I can solve for horsepower needed at a given speed. For example: with the above constants it is 22.3hp to maintain 60mph. However I can not take a power amount and result with the max speed. I could use trial and error perhaps, until I get results with the required hp ratings but this is a very poor way to answer the question.

4. Aug 20, 2007

### ruteckyb

PS this is not homework I am building this EV, I need to know what size engine.

5. Aug 20, 2007

### mgb_phys

Ok then, 5hp is 3728W 165lbs force = 734 Newtons.
Power = force * distance in 1 sec
speed = distance/s = 3728/734 = 5 m/s = 11.2 MPH
Because you are going at a constant speed on the flat the mass is ignored.

ps above about 10mph air resistance is proportional to velocity squared, so you constant resistance isn't a good model.

6. Aug 21, 2007

### ruteckyb

I see, the problem I was having was I neglected to ignore the mass. Thank you.

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