A bird flies at 10ms$^{-1}$ for 3s, 15ms$^{-1}$ for 3s and 20 ms$^{-1}$ for 4s. Calculate the bird's average speed

Analyze the question: The bird flies at three different speeds for three different time intervals. Average speed = total distance ÷ total time.

Concept: When the time spent at each speed is different, you can't just average the speeds (10 + 15 + 20)/3. You must find the distance for each stage first (distance = speed × time).

Solution:

• Stage 1: 10 × 3 = 30 m
• Stage 2: 15 × 3 = 45 m
• Stage 3: 20 × 4 = 80 m

Total distance = 30 + 45 + 80 = 155 m

Total time = 3 + 3 + 4 = 10 s

Average speed = 155 ÷ 10 = 15.5 m/s

Tip: Average speed is always total distance ÷ total time. Never average the speeds directly unless the times are equal. Here the simple average (15 m/s) is a trap (option B) because the bird spent more time at the highest speed of 20 m/s, so the true average is a bit higher.