A woman drives a car from one city to another with different constant speeds along the trip. She drives at a speed of 70.0 km/h for 25.0 min, 75.0 km/h for 20.0 min, makes a stop for 45.0 min, then continues at 45.0 km/h for 35.0 min, at which point she reaches her destination.
(a) What is the total distance between her starting point and
destination (in km)?
km
(b) What is the average speed for the entire trip (in units of
km/h)?
km/h
a)
v1 = 70.0 km/h
t1 = 25.0 min = 25/60 h
Use:
d1 = v1*t1
= (70.0 km/h) * (25/60 h)
= 29.2 km
v2 = 75.0 km/h
t2 = 20.0 min = 20/60 h
Use:
d2 = v2*t2
= (75.0 km/h) * (20/60 h)
= 25.0 km
v3 = 45.0 km/h
t3 = 35.0 min = 35.0/60 h
Use:
d3 = v3*t3
= (45.0 km/h) * (35.0/60 h)
= 26.3 km
Total distance,
d = d1 + d2 + d3
= 29.2 km + 25.0 km + 26.3 km
= 80.5 km
Answer: 80.5 km
b)
total time = 25.0 min + 20.0 min + 45.0 min + 35.0 min
= 125 min
= 125/60 h
Use:
average speed = total distance / total time
= 80.5 km / (125/60 h)
= 80.5*60/125 km/h
= 38.6 km/h
Answer: 38.6 km/h
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