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# Mechanics watch

1. A particle is projected vertically upwards from a point O with speed u ms^-1. The greatest height reached by the particle is 62.5m above O. Find a) The value of u b) The total time for which the particle is 50m or more above O

So i calculate u as 35m/s

Worked out the time which is 5.1686 and 1.9742

I dont understand how the time of flight would be 5.1686 - 1.9742
Because the time it takes to get to point 50m = 5.1686
second time it passes point 50m = 1.9742

Therefore how would the time be 5-2?

2. (Original post by Tynos)

I dont understand how the time of flight would be 5.1686 - 1.9742
Because the time it takes to get to point 50m = 5.1686
second time it passes point 50m = 1.9742
Clearly 5>2 so the first time it passes (on the way up) is at 1.97 seconds and the second time it passes (on the way down) is at 5.17 seconds

I am no sure what you think the answer would be other than subtracting those values
3. (Original post by Tynos)
Because the time it takes to get to point 50m = 5.1686
I assume you mean that the time it takes to reach 50m when it's travelling upwards? (i.e. the first time it passes the 50m mark)
Is this true? See if you can verify it for yourself?
4. (Original post by Tynos)
A particle is projected vertically upwards from a point O with speed u ms^-1. The greatest height reached by the particle is 62.5m above O. Find a) The value of u b) The total time for which the particle is 50m or more above O

So i calculate u as 35m/s

Worked out the time which is 5.1686 and 1.9742

I dont understand how the time of flight would be 5.1686 - 1.9742
Because the time it takes to get to point 50m = 5.1686
second time it passes point 50m = 1.9742

Therefore how would the time be 5-2?

You are confusing what the values of t are I believe. 1.97 is NOT the length of time its is above 50m, it is the time after projection at which the particle is at 50m. So correctly you get 2 values, one as it passes 50m on the way up, and the other as it passes on the way down. The difference in t values (5-2) is the time at which the particles is above 50m.
Here; I have amended your diagram

5. Thank you guys.

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Updated: April 14, 2013
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