How to get the resistive force

• sanisydap
In summary: An equation expressing the proportional relation between force and velocity can be found in your textbook or in class notes.
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

Last edited:
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.

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:
sanisydap said:
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
Try looking in your textbook and class notes and see what equation would relate power, force and velocity.

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

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.

Redbelly98 said:
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?

1. How does an object experience resistive force?

An object experiences resistive force when it moves through a medium, such as air or water, which causes resistance to its motion.

2. What factors affect the amount of resistive force experienced by an object?

The amount of resistive force experienced by an object depends on its speed, size, and shape, as well as the properties of the medium it is moving through.

3. What is the formula for calculating resistive force?

The formula for resistive force is F = -bv, where F is the resistive force, b is the drag coefficient, and v is the velocity of the object.

4. How can an object reduce the amount of resistive force it experiences?

An object can reduce the amount of resistive force it experiences by changing its shape to be more streamlined, decreasing its speed, or by using a lubricant or other substance to reduce friction with the medium.

5. How does resistive force affect the motion and velocity of an object?

Resistive force acts in the opposite direction of an object's motion, so it can slow down an object's velocity and change its direction of motion. It can also cause an object to reach a terminal velocity, where the resistive force is equal to the force propelling the object, resulting in a constant velocity.

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