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#1 2005-11-07 01:19:16
- taragaa
- Guest
Speed-time graph
How to do this qns?? my ans is 3.333, while my teacher's ans is 2.5
A motorist starts from rest, accelerates constantly to a speed of 20m/s after 5 second and then maintan with the constant speed.
A cyclist travels at a constant speed of 5m/s throughout the 20seconds.
a. Calculate after how many seconds will the meet each other again.
My ans:
5t = 0.5 x 5 x 20 + 20(t-5)
t = 3.33333
#2 2005-11-07 03:13:59
- mathsyperson
- Moderator
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Re: Speed-time graph
The cyclist has a constant speed of 5m/s, so its distance is 5t, as you've said.
The motorist is trickier. Your formula only works when t ≥5. If t < 5, then the triangle in the speed-time graph has a variable size, so you can't represent it as a constant, as you have. Its size would be t x 4t /2 = 2t².
If you equate that with 5t then you get 5t = 2t².
5 =2t
t = 2.5
You can tell that that is right because it returned a value that was less than 5. If it gave a value more than 5, you'd have to use the formula that you made.
Why did the vector cross the road?
It wanted to be normal.