Relative velocity between a Bus and a Car

[buckybarnes]

Homework Statement

A car travels 9m/s east and a bus travels 13m/s east. The bus observes the car as being 9000m infront of it.

1. How long will it take the bus to reach the initial observation point of the car?
2. How long will it take the bus to reach the car?

Relevant Equations

would you use t=d/s?

for part 1: t= d/s = 9000/13 = 692.31s
for part 2: What i am unsure about is wether or not this is after the initial observation or exactly what they are asking honestly. so i found the relative velocity of the bus to the car and vice vera and came up with: t=d/s = 9000/4 = 2250s however i don't think that this is correct.

 

[FactChecker]

Part 1: Whose's speed should you be using?
Part 2: Which vehicle is moving faster? What can you conclude from that?

 

[Ray Vickson]

for part 1: t= d/s = 9000/13 = 692.31s
for part 2: What i am unsure about is wether or not this is after the initial observation or exactly what they are asking honestly. so i found the relative velocity of the bus to the car and vice vera and came up with: t=d/s = 9000/4 = 2250s however i don't think that this is correct.

In part 1: why do you divide by 13? The bus is traveling at speed 9 m/s. In part 2: how can the bus ever cacth the car? (Read the question!)

 

[FactChecker]

It looks to me as though the speeds of the bus and car are swapped. Part 2 would make more sense if they were swapped.

 

[buckybarnes]

It looks to me as though the speeds of the bus and car are swapped. Part 2 would make more sense if they were swapped.

yes i reworded the question, i wrote it wrong unfortunately, how would part 2 make sense now?

 

[buckybarnes]

Part 1: Whose's speed should you be using?
Part 2: Which vehicle is moving faster? What can you conclude from that?
the bus moves faster but would that mean that it is just able to catch up?

 

[FactChecker]

yes i reworded the question, i wrote it wrong unfortunately, how would part 2 make sense now?

Then I think your calculations of both parts are correct.

 

[buckybarnes]

Then I think your calculations of both parts are correct.

okay! could you please explain to me why my calculation to my second part was correct? I am so confused i just guessed that answer. also thank u so very much for helping me it means alot!

 

[FactChecker]

The car is going at 9 m/s and the bus is going at 13 m/s, the car is in front but slower. So the bus is catching up at 13-9 = 4 m/s. It needs to catch up an amount of 9000 m. So it takes 9000/4 seconds for it to catch up.

 

[Merlin3189]

For part two you used their relative velocity.
You probably said, the bus is moving 4 m/s relative to car, which started out 9 km away from bus (ie relative distance at that time.)
That's why relative velocity is useful: you cancel out the movement of one object.

Whether your answer is correct, is irrelevant! You have worked out the time both from the initial observation to reaching the car's original position and from the initial observation to catching up with the car. If they had wanted the difference between these two times, they can just subtract. If they don't like it, it's their own fault for not being clear enough in the question!

Edit: Sorry, I did not notice FactChecker had already replied.

 

[buckybarnes]

For part two you used their relative velocity.
You probably said, the bus is moving 4 m/s relative to car, which started out 9 km away from bus (ie relative distance at that time.)
That's why relative velocity is useful: you cancel out the movement of one object.

Whether your answer is correct, is irrelevant! You have worked out the time both from the initial observation to reaching the car's original position and from the initial observation to catching up with the car. If they had wanted the difference between these two times, they can just subtract. If they don't like it, it's their own fault for not being clear enough in the question!

Edit: Sorry, I did not notice FactChecker had already replied.

dont worry! your response was very informative and helped clear my understanding further :)