example force. because you can say" I applied 3 newtons downward to the floor".
force has magnitude and direcion
The runner has initial velocity vector
and acceleration vector
so that her velocity at time 
is
She runs directly east when the vertical component of 
is 0:
It's not clear what you're supposed to find at this particular time... possibly her position vector? In that case, assuming she starts at the origin, her position at time would be
so that after 10.4 s, her position would be
which is 19.9 m away from her starting position.
-- There's a force of 240N pushing her backwards.
-- She's maintaining a steady speed (of 2.5 m/s) .
-- In order to maintain a steady speed (no acceleration),
the forces on her must be balanced. So she's maintaining
a steady force of 240N forward.
-- Every time she moves 1 m forward, she does work of
(force) x (distance) = 240 joules.
-- She moves 2.5 meters forward every second.
So she's doing (240 x 2.5) = 600 joules of work every second.
-- 600 joules per second = 600 watts .
F = ma
<span>where </span>
<span>F = frictional force </span>
<span>m = mass of the block = 1.4 kg (given) </span>
<span>a = acceleration of the block = 1.25 m/sec^2 (given) </span>
<span>Substituting values, </span>
<span>F = (1.4)(1.25) </span>
<span>F = 1.75 N </span>
<span>By definition, </span>
<span>F = (mu)(Normal force) </span>
<span>where </span>
<span>mu = coefficient of friction </span>
<span>Normal force = mg = 1.4*9.8 = 13.72 </span>
<span>Again, substituting appropriate values, </span>
<span>1.75 = mu(13.72) </span>
<span>mu = 0.128</span>
