Speed Physics Example 3

Follow the full solution, then compare it with the other examples linked below.

Example 3

medium
A train travels 120 km120 \text{ km} at 60 km/h60 \text{ km/h} and then 120 km120 \text{ km} at 40 km/h40 \text{ km/h}. What is the average speed for the whole trip?

Solution

  1. 1
    Time for first part: t1=12060=2 ht_1 = \frac{120}{60} = 2 \text{ h}.
  2. 2
    Time for second part: t2=12040=3 ht_2 = \frac{120}{40} = 3 \text{ h}.
  3. 3
    Average speed: savg=total distancetotal time=2405=48 km/hs_{\text{avg}} = \frac{\text{total distance}}{\text{total time}} = \frac{240}{5} = 48 \text{ km/h}

Answer

savg=48 km/hs_{\text{avg}} = 48 \text{ km/h}
Average speed is not simply the arithmetic mean of two speeds. It is total distance divided by total time. Spending more time at the lower speed pulls the average below 50 km/h50 \text{ km/h}.

About Speed

The rate at which an object covers distance over time, calculated as total distance divided by total time, always expressed as a non-negative scalar quantity.

Learn more about Speed →

More Speed Examples

Example 1 easy

A jogger runs [formula] in [formula]. What is the average speed in m/s?

Example 2 easy

Light travels at [formula]. How far does light travel in [formula]?

Example 4 medium

A runner completes the first half of a race ([formula]) at [formula] and the second half at [formula

← All Speed Examples Speed Concept