First slide

Average Speed

Question

A car moves from A to B with a speed of $30 {kmh}^{- 1}$ and from B to A with a speed of $20 {kmh}^{- 1}$ . What is the average speed of the car?

Moderate

A

$25 {kmh}^{- 1}$
B

$24 {kmh}^{- 1}$
C

$50 {kmh}^{- 1}$
D

$10 {kmh}^{- 1}$

Solution

v_AB = Speed of car from A to B
v_AB = $30 {kmh}^{- 1}$
v_BA = Speed of car from B to A
v_BA = $20 {kmh}^{- 1}$
$v_{av} = \frac{2 v_{A B} v_{B A}}{v_{A B} + v_{B A}} = 24 {kmh}^{- 1}$

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