1. ## Physics - Power

Given:

1hp = 746W

1 m/s = 2.237 mi/h.

When an automobile moves with constant velocity, the power developed in a certain engine is 66 hp. What total frictional force acts on the car at v = 50.8 mi/h? Answer in units of N.

Any hints on where to get started at least? This homework is due and our teacher hasn't gone over this yet so I'm kind of in the dark :\

2. Originally Posted by vesperka
Given:

1hp = 746W

1 m/s = 2.237 mi/h.

When an automobile moves with constant velocity, the power developed in a certain engine is 66 hp. What total frictional force acts on the car at v = 50.8 mi/h? Answer in units of N.

Any hints on where to get started at least? This homework is due and our teacher hasn't gone over this yet so I'm kind of in the dark :\
$\displaystyle P = \frac{W}{t}$

If the force is constant then
$\displaystyle P = \frac{Fx}{t}$

$\displaystyle P = F \cdot \frac{x}{t}$

$\displaystyle P = Fv$
where v is the average speed.

So to solve your problem, you know that the car is moving at a constant speed of 50.8 mph and the engine is putting out a power of 66 hp. The friction force may be assumed to be constant and is countering the any force the engine is providing, so friction is providing as much stopping power as the engine is putting out. So convert the numbers to the appropriate units and plug them into P = Fv, where F is the friction.

-Dan

3. Sweet man got it, thanks