# Homework Help: Car engine and constant power

1. Apr 1, 2014

### jim_ringo

1. The problem statement, all variables and given/known data
a cars engine produces constant power therefore the cars speed will increase at a constant acceleration.

is this true or false

2. Relevant equations
Power=w/t
work=fd

3. The attempt at a solution

so if p is constant it appears a must be constant. Therefore the statement is true

When I logically think about this situation I think it is false. I imagine myself driving down the highway in my stang turning 2500 rpm at 100 mph. Assume I stay in the same gear. Unless I raise the revs by giving it more gas I will not accelerate. When raising the engine rpm my engine makes more power....so how can I accelerate if my engine maintains a constant rpm(same power output)?
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

2. Apr 1, 2014

### BOYLANATOR

Your equations are correct and so is your logic. However, can you think of any additional forces which may act against the acceleration of the car?

3. Apr 1, 2014

### jim_ringo

Ah, wind resistance and friction between the road and tires...and then theres also the some power loss due to the weight of the drive train components.

4. Apr 1, 2014

### BOYLANATOR

Yeah. I think it's funny to think about a bike. The wheels are so thin, but that little surface area combined with the air stops you from going as fast as you want.

5. Apr 1, 2014

### jim_ringo

so just that little bit of resistance from the tires and the air hitting the body of the car prevents the car from accelerating,correct?

6. Apr 1, 2014

### jim_ringo

so the answer would be true?

7. Apr 1, 2014

### BOYLANATOR

Yes, so it's not that little really.

8. Apr 1, 2014

### BOYLANATOR

Well in real life the answer is false for a car. A rocket in deep space could accelerate indefinitely.

9. Apr 2, 2014

### Staff: Mentor

Whoa! I'm not convinced that you have demonstrated it to be true. All I see you having shown here is that
the factor (ad/t) is constant. I think you are asked to determine whether or not a is constant---am I correct in believing that is what the problem is asking?

10. Apr 2, 2014

### jim_ringo

Yes that is what the problem is asking