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#1 2005-11-06 02:19:16

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 2005-11-06 04:13:59

mathsyperson
Moderator
Registered: 2005-06-22
Posts: 4,900

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.

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