To find velocity of the body we use the equation
where "{{v}_{0}}" is the initial velocity of the body, "{{v}_{0}}=0" ,
"{{y}_{0}}" is the initial height, "{{y}_{0}}=20\\,m" ,
y is the height at which we need to find the speed of the body, "y=0\\,m" at the ground, or "y=10\\,m" at a height of 10 m from the ground.
g is the acceleration of gravity, "g=10\\,m\\cdot {{s}^{-2}}" .
Substituting the known values, we get:
1) at the ground
2) at a height of 10 m from the ground
"=10\\sqrt{2}\\,m\/s\\approx 14.14\\,m\/s"
To find the average resistant force we use the work-energy theorem: The net work "{{W}_{net}}" on a system equals the change in the quantity of kinetic energy
"{{W}_{K}}=\\frac{1}{2}m{{v}^{2}}"Calculate kinetic energy of the body near the ground. Substitute "m=5\\,kg" and"v=20\\,m\/s"
The net work of an average friction force f on a system is
where d is the distance it takes to stop, d=3/4m. Equate the kinetic energy and the work of an average friction force
or
Then we get an average friction force
"f=\\frac{1\\,kJ}{3\/4\\,m}=\\frac{4}{3}\\,\\frac{kJ}{m}\\approx 1.33\\,kN"
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