Hi All,
Pls help to solve this P6 Speed qn :
A wave of the water current is flowing at a speed of 8 km/h towards Point X from Point Y which was 877 km apart. At 8 am, Johnny rode off with his boat at a speed of 50 km/h towards Y from Point X for 30 mins and then stopped to have a rest for 45 mins before continuing his remaining journey at the same speed. At 9.15 am, Peter set off with his boat at a speed of 120 km/h from Point Y to Point X and both of them travelled for a distance and finally met at Point Z. How long did Johnny take to travel after passing Peter at Point Z? Express your answer in hrs.
Thks.
A wave of the water current is flowing at a speed of 8 km/h towards Point X from Point Y.
Relative velocity question huh ?
P6 maths question ? Sure or not ? Don't anyhow say say hor.
please plot dis-time graph =x
According to the one who post this Math qn, he/she replied that this P6 qn was from one of the assessment books. So, how to solve it ?
Thks.
Originally posted by Chocolate Cake:Hi All,
Pls help to solve this P6 Speed qn :
A wave of the water current is flowing at a speed of 8 km/h towards Point X from Point Y which was 877 km apart. At 8 am, Johnny rode off with his boat at a speed of 50 km/h towards Y from Point X for 30 mins and then stopped to have a rest for 45 mins before continuing his remaining journey at the same speed. At 9.15 am, Peter set off with his boat at a speed of 120 km/h from Point Y to Point X and both of them travelled for a distance and finally met at Point Z. How long did Johnny take to travel after passing Peter at Point Z? Express your answer in hrs.
Thks.
Are you sure the distance is 877km and not 879km? The 2km will have made a lot of difference, especially when this is a P6 question.
This qn was posted by someone else in another blog :-> http://road-to-psle.blogspot.com/
BTW, he/she mentioned that the answer was 6 1/4 h and this was a 5-mark qn.
Thks.
Originally posted by Chocolate Cake:Hi All,
Pls help to solve this P6 Speed qn :
A wave of the water current is flowing at a speed of 8 km/h towards Point X from Point Y which was 877 km apart. At 8 am, Johnny rode off with his boat at a speed of 50 km/h towards Y from Point X for 30 mins and then stopped to have a rest for 45 mins before continuing his remaining journey at the same speed. At 9.15 am, Peter set off with his boat at a speed of 120 km/h from Point Y to Point X and both of them travelled for a distance and finally met at Point Z. How long did Johnny take to travel after passing Peter at Point Z? Express your answer in hrs.
Thks.
The question may be wrong. 877km should be 879km. I got the answer when the distance between Point X and Point Y is 879km.
If the question is :
A wave of the water current is flowing at a speed of 8 km/h towards Point X from Point Y which was 879 km apart. At 8 am, Johnny rode off with his boat at a speed of 50 km/h towards Y from Point X for 30 mins and then stopped to have a rest for 45 mins before continuing his remaining journey at the same speed. At 9.15 am, Peter set off with his boat at a speed of 120 km/h from Point Y to Point X and both of them travelled for a distance and finally met at Point Z. How long did Johnny take to travel after passing Peter at Point Z? Express your answer in hrs.
Water current is 8km/h flowing from X to Y. Hence, when a boat travels from X to Y, an additional 8km/h will have to be added since current is helping to push the boat. However, when a boat travel from Y to X, 8km/h will have to be deducted from the speed as water current is going against the boat.
Speed from X to Y (Johnny) = 50 + 8 = 58km/h
Distance from X to Y in 30min ( Johnny) = 58 x 30/60 = 29km
He had completed 29km
Distance between Johnny and Point Y = 879 - 29 = 850
Speed from Y to X (Peter) = 120 - 8 = 112km/h
Point Z is the point between X and Y. They met there.
Time taken for both of them to meet = 850/(112 + 58) = 5hours
Time taken for Johnny from X to Z = 5h + 30min + 45 min = 6 1/4 hours
Hi,
Does the PSLE student have any concept of relative velocity? This question is atypical and I wonder if it is appropriate for any student to attempt at all.
Shouldn't Johnny's speed be 42 km/h since the current is against him, according to the problem statement of '8 km/h towards Point X from Point Y...'
Thanks!
Cheers,
Wen Shih
Originally posted by wee_ws:Hi,
Does the PSLE student have any concept of relative velocity? This question is atypical and I wonder if it is appropriate for any student to attempt at all.
Shouldn't Johnny's speed be 42 km/h since the current is against him, according to the problem statement of '8 km/h towards Point X from Point Y...'
Thanks!
Cheers,
Wen Shih
I think the answer is correct, but the question has a lot of typing errors. And when I say a lot, I mean A LOT.
1) The distance is pretty suspicious, if it is 877km, no matter how you work it out, the answer will be full of decimal places.
2) The current should be from X to Y, if it is not....have fun plotting your graph.
3) When Johnny stopped down, did he he use an achor or stop at an island? If not, wouldn't his boat drift due to current?
4) Shouldn't the question be 'How long was the journey that Johhny travelled before reaching point Z' ? If it is put in its current way, it sound as if the question wants you to calculate the time for johnny to reach point Y from point Z.
Originally posted by wee_ws:Hi,
Does the PSLE student have any concept of relative velocity? This question is atypical and I wonder if it is appropriate for any student to attempt at all.
Shouldn't Johnny's speed be 42 km/h since the current is against him, according to the problem statement of '8 km/h towards Point X from Point Y...'
Thanks!
Cheers,
Wen Shih
Ohh... I saw the mistake. Despite the mistake, I still got the answer. The question can't be done. wee_ws, you are right, the question has too many typing errors.
Thks everyone for yr replies and help. Fyi, the one that posted this qn had corrected and reposted the qn as below :
A wave of the water current was flowing at a speed of 8 km/h towards Point X from Point Y which was 865 km apart. At 8 am, Johnny rode off with his boat at a speed of 50 km/h towards Y from Point X for 30 mins and then stopped to have a rest for 45 mins before continuing his remaining journey at the same speed. At 9.15 am, Peter set off with his boat at a speed of 120 km/h from Point Y to Point X and both of them travelled for a distance and finally met at Point Z. How long did Johnny take to travel after passing Peter at Point Z? Express your answer in hrs. [ 5 Marks ]
Accordingly to him/her, the answer is still 6 1/4 h and this qn has nothing to do with relative velocity. Any comments and Thks.
Originally posted by Chocolate Cake:Thks everyone for yr replies and help. Fyi, the one that posted this qn had corrected and reposted the qn as below :
A wave of the water current was flowing at a speed of 8 km/h towards Point X from Point Y which was 865 km apart. At 8 am, Johnny rode off with his boat at a speed of 50 km/h towards Y from Point X for 30 mins and then stopped to have a rest for 45 mins before continuing his remaining journey at the same speed. At 9.15 am, Peter set off with his boat at a speed of 120 km/h from Point Y to Point X and both of them travelled for a distance and finally met at Point Z. How long did Johnny take to travel after passing Peter at Point Z? Express your answer in hrs. [ 5 Marks ]
Accordingly to him/her, the answer is still 6 1/4 h and this qn has nothing to do with relative velocity. Any comments and Thks.
I still think the question is incorrect. It should be rephrased as how long it takes for Johnny to meet Peter at point Z from the time he starts off from point X.
Assuming that the point I made above is correct, the workings will be as followed:
Distance travelled by Johnny in half an hour
= 1/2 X 50km - 1/2 X 8km
= 21km
While resting for 45mins, distance the boat drifted back
= 3/4 X 8km
= 6km
As such, after 1h 15mins, actual distance travelled is only
= 21km - 6km
=15km
Distance between Johnny now and Peter
= 865km - 15km
=850km
Actual travelling speed of Johnny
= 50km/h - 8km/h
= 42km/h
Actual travelling speed of Peter
= 120km/h + 8km/h
= 128km/h
Time taken for the two guys to meet each other
= 850km / ( 42km + 128km )
= 850/170 hours
= 5 hours
Total time for the 2 guys to meet
= 5 hours + 45 mins + 30 mins
= 6 and 1/4 hours.
If the question is correct whereas the one I proposed is wrong, the final answer will be over 15 hours, not to mention with some decimal places.
Originally posted by Chocolate Cake:Accordingly to him/her, the answer is still 6 1/4 h and this qn has nothing to do with relative velocity. Any comments and Thks.
Hi,
It is relative velocity, when we add magnitudes of vectors in the same direction and subtract magnitudes of vectors in opposite directions.
Thanks!
Cheers,
Wen Shih
Aiyah, it is so obvious that TS Chocolate Cake is not consistent in the replies.
First, TS claims that the question is from a PSLE assessment book. If it is really from an assessment book, name of the book please.
Then, TS changes to say that it is from another website
Now, forumers pointed out the mistakes and TS says that the poster in another
forum changes the question.
More likely TS is the setter of the question lor.
TS has probably posted the question in another website and no one answers the question in that website and so TS re-post the question in this forum lor.
So, when homework.sgforums.com forumers pointed out the mistakes, TS changes the question in the other website and in this forum too.
The question is DEFINITELY a relative velocity question, though not a difficult
relative velocity question AND THE question is of many mistakes and
inconsistencies.
The moderator Mr Wee has already kindly pointed out that it is a relative velocity
question but TS still tries to say that it is not a relative velocity question.
I totally agree with Mr Wee that this question is atypical and it is inappropriate for
any PSLE student to attempt it at all.
so this is kinna of a sec 4 EM qns........