# What is the magnitude of the average frictional force on the block?

A $2.0 \space kg$ block, initially moving at $10.0 \space m/s$, slides $50.0 \space m$ across a sheet of ice before coming to rest. What is the magnitude of the average frictional force on the block?

I know the answer is 2.0 N.

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This question seems ambiguous to me. Do you mean the average over time, or the average over distance? If you mean the former, then I don't think there's enough information to solve the problem. You might get a better insight if you ask on physics.stackexchange.com. – user22805 Sep 30 '12 at 2:43

$F_f^2 = F_0^2 + 2a(\Delta x)$

$F_f = 0 m/s$

$F_0 = 10 m/s$

$\Delta x = 50 m$

$a = -1 m/s$

$F = ma = 2 * -1 = -2N$ (negative because opposite to direction of motion)

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Thanks this makes sense. Except, the answer on the answer sheet given by the professor is $2N$, not $-2N$. – user42896 Sep 30 '12 at 2:54
You should be able to convert $F = -2$ to $|F| = 2$. ($|F|$ is what the problem asks for.) – Tunococ Jun 30 '13 at 6:43