Resposta :
a) Fr=m*a
F - Fat = m*a
8 - Fat = 4*2
8 - 8 = Fat
Fat = 0 (sem atrito).
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b) Formula para achar o coeficiente de atrito
Fat=mi*FN
0=mi*40
mi=0 (coeficiente de atrito)
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c)
P=m*g
P=4*10
P=40N (peso)
FN=P
FN=40N
Fr mesa = FN - P
Fr mesa = 40 - 40
Fr mesa = 0