# Homework Help: Conservation of Energy, Car Driving with Drag Force

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1. Nov 9, 2014

### David Mordigal

1. The problem statement, all variables and given/known data
A 710kg car drives at a constant speed of 23m/s . It is subject to a drag force of 500 N. What power is required from the car's engine to drive the car (a) on level ground? (b) up a hill with a slope of 2.0o? Express your answer to two significant figures.

2. Relevant equations
p = mv
P = ΔW/Δt (but no time information given)

3. The attempt at a solution
I am not sure where to begin. I understand that power is the change in work with respect to time, but there is no time information given in the problem, so I am a little lost.

2. Nov 9, 2014

### SteamKing

Staff Emeritus
The car is moving on level ground and up the hill at constant speed. Speed has got time in it. Why can't you use that?

3. Dec 20, 2014

### dean barry

Since constant speed is the situation, the for and against forces are equal.
On level ground the mass of the car doesnt come into it.
Then :
Power consumption ( Watts ) = force overcome (N) * speed (m/s)
(applies to constant or instantaneous speed only)
In the uphill problem, calculate the additional force acting against the car and down the slope due to gravity ( m * g * sine (2 ° ) ) and add to the 500 Newtons of drag, then calculate the power consumption.

4. Dec 20, 2014

### SteamKing

Staff Emeritus
This question is probably moot by now, given that it was first posted in Nov. 2014 and the OP never replied.

5. Dec 20, 2014

OK, thanks