Physics avg speed question?

Question

A car travels up a hill at a constant speed of 40km/h and returns down the hill at a constant speed of 60km/h. Calculate the average speed for the round trip.

When I look at this I just think to add both 40 and 60 and divide by 2. Which gives me an average speed of 50 km/h.

Is this how to solve this problem with physics mentality?

Please explain.

Answer 1

Yeah u took it right.

Avg speed = (initial speed + final speed)/2

Ans = 50km/h

Answer 2

If s is the distance to the top of the hill, then the whole distance traveled is 2s.

The time needed to travel the distance of 2s is:

t=s/40 + s/60 ==>

The average speed:

v=2s/t=2s/(s/40 + s/60)

v=2*40*60/(40+60)

v=48 km/h