# How to get the resistive force

#### sanisydap

1. The engine of an automobile requires 45hp to maintain a constant speed of 80 km/h

a. What is the resistive force against the automobile?
The answer given here is 1510.80N but don't know how they get it.

b. If the resistive force is proportional to the velocity, what must the engine power be to drive at constant speed of 60km/h?

2. I have no idea

3. at letter a. i tried to convert 45hp to 33556.49J/s and converted also 80km/h to 22.22 m/s, after that I divided 33556.49J/s to 22.22 m/s and got an answer 1510.19 N. The letter b. I have no idea

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#### Redbelly98

Staff Emeritus
Homework Helper
Welcome to Physics Forums I see you figured out (a) based on looking at units. There is also an equation that relates power, force and velocity that should be in your textbook.

For (b), here are 2 facts known to you:

1. Force is proportional to velocity.
2. When the velocity is 80 km/hr, the force is 1510 N.

Use those 2 facts can be used to determine the relationship between force and velocity for this automobile.

#### sanisydap

so i will be finding what? I think its power.. i am confused to the equation i will be using.. i kinda do trial and error xD

Last edited:

#### Redbelly98

Staff Emeritus
Homework Helper
so i will be finding what? I think its power..
Re-read what question (a) is asking. (Hint: not power)
i am confused to the equation i will be using.. i kinda do trial and error xD
Try looking in your text book and class notes and see what equation would relate power, force and velocity.

#### sanisydap

I think i got it now..so question b . is asking about the engine power at 60km/h .. then i got a force of 1510.19N, convert 60 km/h to 16.67m/s and multiplied them, then the answer would be 25,174.87 J/s.. i think hehehe

#### Redbelly98

Staff Emeritus
Homework Helper
No, the force is 1510 N when the speed is 80 km/h, but now (in question b) it is 60 km/h. So the force will be less than 1510 N.

For (b), here are 2 facts known to you:

1. Force is proportional to velocity.
2. When the velocity is 80 km/hr, the force is 1510 N.
Can you set up an equation expressing the proportional relation between force and velocity?

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