A car travels in a straight line with an average velocity of 75 km/h for 2.5 h and then with an average velocity of 35 km/h for 1.5 h.

Question

(a) What is the total displacement for the 4 h trip?
km
(b) What is the average velocity for the total trip?
km/h

Answer 1

displacement in a straight line or distance = velocity x time

 

distance 1 = 75km/h x 2.5h = 187.5 km

 

distance 2 = 35 km/h x 1.5h = 52.5 km

 

total distance = distance 1 + distance 2 = 240 km

 

average velocity = total distance / total time = 240 km / 4h = 60 km/h

 

Notice that average velocity is not the average of 75 km/h and 35 km/h which would be 55 km/h. The reason is more time was spent going the 75 km/h velocity.