Question

Find the moment about the point i+2j-k of a force represented by 3i+k acting through the point 2i-j+3k.

Answer 1

Explanation:

moment is given by M = r×F

a(1,2,-1) b(2,-1,3)

then r, = (2-1,-1-2,3-×-1)

r= (1,-3,4)

r×F =(1i-3j+4k)×(3i+0j+k) N.m

= -j+3k-3i+12j

=-3i+11j+3k N.m

Answer 2

Concept Introduction: Moment in physics is given by multiplying distance and a physical quantity.

Given:

We have been Given: Point P is

$i + 2j - k$

Point Q is

$2i - j + 3k$

Force is given

$3i + k$

To Find:

We have to Find: Moment about the point P.

Solution:

According to the problem, Moment is given by

$m = r \times f$

so we need to first find radius, so we add the values of the vectors P and Q given so,

$q(2 \: - 1 \: 3) - p(1 \: 2 \: - 1) = r(1 \: - 3 \: 4)$

Now,

so the r vector is

$r = i - 3j + 4k$

Therefore, Applying the formula of Moments,

$m = (i - 3j + 4k) \times (3i + k)$

therefore,

$m = - 3i + 11j + 3k \: nm$

Final Answer: The vector of the moment is,

$m = - 3i + 11j + 3k \: nm$

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