# Calculate power to have the car at a certain speed

1. May 5, 2015

### SelenaT

1. The problem statement, all variables and given/known data
**Rollercoaster is being built and the values are the specifications

Determine the power needed to raise the cars to the top of the hill and have them travelling at the wanted speed.

Height of hill: 65m
Speed at top of hill: 2.5m/s
Time to raise cars: 40 seconds
Total mass of cars: 1250kg

State all assumptions.

2. Relevant equations

P = mgh/t
Eg = mgh
Ek = 1/2mv^2

3. The attempt at a solution

P = mgh / t
= 1250(9.8)(65) / 40
= 19906.3W

Assuming constant speed and no friction.

I dont know how to finish the question.

Last edited: May 5, 2015
2. May 5, 2015

### BvU

Well, if the question is

and your answer is 20 kW (*), what more is there to finish ?

You could add a statement of the assumption that the cars start off at 2.5 m/s, so your lifting device doesn't have to provide the corresponding kinetic energy... but in fact you already did: "assuming constant speed"

(*) your given data is in two digits of precision, so it's reasonable to provide the answer in two digits, too. 19906.3 W suggests an accuracy that isn't justified.

3. May 5, 2015

### SelenaT

But the speed isn't incorporated in the answer. The question is asking for the power at a certain speed. I believe do something to 20kW to get the answer

4. May 5, 2015

### haruspex

Then assume the cars start at rest.

5. Jun 15, 2015

### dean barry

You might be better off guessing a motor size, and using the motor performance specs to calculate the performance of your car, if its not what you need then adjust the size of the motor and recalculate.