Final answer:
The speed of the truck is calculated using kinematics concepts, considering the distances and speeds involved. The bus's constant speed and the time it takes to overtake are used to find the relative speed and then determine the truck's speed. The calculated speed of the truck is approximately 56.12 km/h.
Step-by-step explanation:
The question involves a bus overtaking a trailer and requires us to calculate the speed of the truck. To solve this, we will employ basic concepts from kinematics. First, we need to understand the scenario: the bus is travelling at a constant speed of 72 km/h (which we convert to 20 m/s), and it takes 10.2 seconds to overtake the trailer completely. The total distance the bus needs to cover to overtake the trailer is the length of the bus, the length of the trailer, plus the distance between them initially, which sums up to 10 m + 15 m + 20 m = 45 meters.
Since the bus is moving at a constant speed, the distance covered during the overtaking process (which is 45 meters) divided by the time taken to overtake (which is 10.2 seconds) gives us the relative speed. The relative speed is thus 45 m / 10.2 s = 4.41 m/s. The bus's speed is 20 m/s, so to find the trailer's speed, we subtract the relative speed from the bus's speed, getting 20 m/s - 4.41 m/s = 15.59 m/s. Converting this back to km/h, we multiply by 3.6 to get the trailer's speed, approximately 56.12 km/h.