# a question on power -physics

Thread starter 2 years ago
#1
A driver notices that her 1080-kg car, when in neutral, slows down from 95 km/hr to 65 km/hr in about 7.0s on a flat horizontal road. Approximately what power (watts and hp) is needed to keep the car traveling at a constant 80 km/h?

Here's what I did, I calculated the energy I would need to oppose the 95-65 change and then I added the energy needed to move at 80 km/hr then I divided by 7 . I think I have a misconception + a mistake somewhere because the book's answer is different. Can someone tell me where I went wrong ?
0
2 years ago
#2
(Original post by JanaALEVEL)
A driver notices that her 1080-kg car, when in neutral, slows down from 95 km/hr to 65 km/hr in about 7.0s on a flat horizontal road. Approximately what power (watts and hp) is needed to keep the car traveling at a constant 80 km/h?

Here's what I did, I calculated the energy I would need to oppose the 95-65 change and then I added the energy needed to move at 80 km/hr then I divided by 7 . I think I have a misconception + a mistake somewhere because the book's answer is different. Can someone tell me where I went wrong ?

We don’t need to supply a force to cause an object to move at constant velocity, this means that no energy is required, too.

In the presence of frictional force, we would need to counteract the frictional force to cause the object to move at constant velocity.

So you don’t need to add the energy needed to move at 80 km/hr.
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Thread starter 2 years ago
#3
(Original post by Eimmanuel)
We don’t need to supply a force to cause an object to move at constant velocity, this means that no energy is required, too.

In the presence of frictional force, we would need to counteract the frictional force to cause the object to move at constant velocity.

So you don’t need to add the energy needed to move at 80 km/hr.
ohhhh
okay 1 more (dumb) question

If I'm contradicting the frictional force, aren't I returning the velocity V to 95 ? Don't I want it to be 80 ?
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2 years ago
#4
(Original post by JanaALEVEL)
ohhhh
okay 1 more (dumb) question
It is not a dumb question. It was found out by Galileo and Newton.

(Original post by JanaALEVEL)
If I'm contradicting the frictional force, aren't I returning the velocity V to 95 ? Don't I want it to be 80 ?
Not sure what do you mean by "contradicting the frictional force" and also don't really understand what you are asking.
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