Physics Question: Average Speed?

Question

A car is driven for one hundred miles to a house and back.

As it goes towards the house, it travels at 48mph for half the distance and 79 for the other half.

On the way back, it travels at 48mph for half the time and 79mph for the other half.

What is the average speed to the house?

What is the average speed on the way back?

Answer 1

there: (48+79)/2 = 127/2 = 63.5mph

back: same

Answer 2

To do an average speed, you CANNOT add them and divide by 2.

Towards

first half takes 50 mi / 48 mi/hr = 1.04 hr

sec half takes 50 mi/ 79 mi/hr = 0.633 hr

total = 1.673 hr

Av speed = 100 mi / 1.673 hr = 59.8 mi/hr

back

first half t1 = d1 / 48 mi/hr

sec half t2 = d2 / 79 mi/hr

t's are equal

d1 / 48 = d2 / 79

79d1 = 48d2

d1+d2 = 100

d1 = 100-d2

97(100-d2) = 48d2

9700 - 97d2 = 48d2

145d2 = 9700

d2 = 66.9 mi

d1 = 33.1 mi

t1 = t2 = 33.1/48 = 0.69 hr

total time = 1.38 hr

av speed = 100 mi / 1.38 hr = 72.5 mph

,

Answer 3

you have to be specific by what you mean by time. in this case time can mean 1. hours minutes seconds or 2. half the distance