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A object with mass 0.5 kg slides in straight line for 200 m, where the friction act on it is 0.4 times the normal reaction acting on it. Find its initial speed.

I let $u$ be the speed.

$u^2=2as=2*0.4mg*200=2*0.4*0.5*9.81*200,u=28.01ms^{-1}$.

But the answer should be $40ms^{-1}$, can someone tell me what's wrong with my solution?

Thank you.

EDIT:

Correct answer:

$u^2=-2as=-2(f/m)s=-2(-0.4N/m)s=-2(0.4mg/m)s=2*0.4*10*200=1600$

$u=40ms^{-1}$

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You're solution is wrong. For one, the law is

$v^2 = u^2 + 2aS$

where v is the final velocity and u is the initial velocity.

You've substituted the initial velocity in the wrong place, can't you see? :)


Secondly, why do you think the acceleration is equal to 0.4(N)? It is given in the question that the friction is equal to 0.4(N), not acceleration! Think of a way to inter-relate the two?? ;)


Thirdly, try taking the acceleration due to gravity (g) as 10, it'll simplify stuff.

Try solving it now, and update your question with your answer!

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