Question

alana is skateboarding at 19 km/h and throws a tennis ball at 11 km/h to her friend oliver who is behind her leaning against a wall. what is the speed of the tennis ball if someone was watching the exchange on the sidewalk?
○ 8 km/hr
○ 11 km/hr
○ 15 km/hr
○ 30 km/hr

Explanation:

Step1: Analyze relative speed

Alana's speed is \( 19 \, \text{km/h} \), and she throws the ball backward (toward Oliver) at \( 11 \, \text{km/h} \). To find the ball's speed relative to the sidewalk, we subtract the ball's speed relative to Alana from her speed (since the ball is thrown opposite to her direction of motion).

$$ 19 - 11 = 8 $$

Answer:

8 km/hr