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# Either the third equation is wrong or I am .

I need help. The question came up in my mind from nowhere. I took u=5 m/s , v=-5m/s and s=10 m. Using the third equation ,I tried to calculate the acceleration which lead to a disaster.

v^2=u^2+2as

-5^2=5^2+2a10

25=25+20a

0=20a

0=a

This is a very surprising result as v is not equal to u. But according to the result, there is no acceleration.

SOS.

Note by Platinum Grieger
2 years, 7 months ago

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The problem is that s cannot equal 10. If you start at 5m/s and end at -5m/s with a constant acceleration then you are going to go forward a certain distance but then go backwards exactly the same distance so s=0 · 2 years, 7 months ago

A good answer but then what about the acceleration . Will it also be zero? · 2 years, 7 months ago

No, from the u and v values you know a has to be non zero and negative but you cannot know anymore about it from this equation. To find a you would need to use one of the other equations of motion and need some extra information like t. · 2 years, 7 months ago

So, I cannot my eyes and use third equation in every case. Thank you. · 2 years, 7 months ago

Well there is a way to use this equation but you have to split the question up into two parts, if you consider the two directions of motion separately. If you take u =5 v = 0 and s = 5 and then u = 0 v = -5 and s = 5 then you can find the value of a you are looking for. · 2 years, 7 months ago