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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
3 years, 2 months ago

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1 vote

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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

Gordon Crawford - 3 years, 2 months ago

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A good answer but then what about the acceleration . Will it also be zero?

Platinum Grieger - 3 years, 2 months ago

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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.

Gordon Crawford - 3 years, 2 months ago

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@Gordon Crawford So, I cannot my eyes and use third equation in every case. Thank you.

Platinum Grieger - 3 years, 2 months ago

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@Platinum Grieger 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.

Gordon Crawford - 3 years, 2 months ago

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@Gordon Crawford That was awesome

Platinum Grieger - 3 years, 2 months ago

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