Average speed for a round trip at different speeds

[Tom McCurdy]

You drive a car at a speed of 40 km/hr to a place and then 60 km/hr back... what is the average speed of the car.

I feel really stupid for asking this but its really making me mad..

 

[Tom McCurdy]

hahaha as soon as i posted it it was so obvious...

Set distance equal to one

t1=1/40 t2=1/60

$$\frac{\Delta d}{\Delta t}=avg vel$$

$$\frac{2}{\frac{1}{40}+\frac{1}{60}}$$

 

[Tom McCurdy]

Which produces 48 km/hr as answer

 

[TD]

You are correct. Often people make the obvious mistake to take the arithmetic mean, being (40+60)/2 = 50 but in this case, you need the harmonic mean

 

[ivybond]

$$\frac{2}{\frac{1}{40}+\frac{1}{60}}$$

harmonic mean could be calculated slightly easier:
$$\frac{v_1 v_2}{\frac{v_1 + v_2}{2}}$$