The approximate magnitude of the friction force of the stone is 2N.
Given values:
Mass(m)= 20 Kg
Velocity(v)= 3m/s
Distance(x)= 40m
Kinetic energy is given by the expression:
[tex]Ek=1/2mV^{2}\\ Ek=1/2*20Kg*(3m/s)^{2} \\Ek=90J[/tex]
If we divide the kinetic energy by the distance traveled we get the friction force:
[tex]Ff=Ek/x\\Ff=90J/40m\\Ff=2.25N[/tex]
Approximate magnitude of the friction force is: [tex]Ff=2N[/tex]
Result is 2N.
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