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A: It is known that p1= 3.5×104pa2 = 5.6×104pas = 200cm2 = 0.02m2ao: ob = 2: 5g a =1000ng =1. ρ= 1.0× 103kg/m3?

Found: (1)η=? (2)ρ=?

Solution:

(1) a. When lifting armor with a moving pulley,

① Take the object A and the moving pulley as the research objects, and the force analysis is as shown in figure 1:

② Take the crown block as the research object, and the stress analysis is shown in Figure 2:

(3) Force analysis of lever point A and point B is shown in Figure 3:

④ The force analysis of counterweight B is shown in Figure 4:

From top to bottom, 2t 1 = Gjia +G0? ①

FA 1=G0+3T 1? ②

f’a 1? lOA = F’b 1? lOB? ③

N 1+FB 1=G B ④

Substitution value

n 1 = p 1S = 3.5× 104 pa×0.02 m2 = 700n

T 1 = T’ 1,fa 1 = F’a 1,FB 1 = F’b 1,

G2-G0 = 1300n;

B. When the object C is immersed in water and rises at a uniform speed,

⑤ Taking the object C and the moving pulley as the research objects, the force analysis is shown in Figure 5:

⑥ Taking the crown block as the research object, the stress analysis is shown in Figure 6:

⑦ Force analysis of lever point A and point B is shown in Figure 7:

Today, the force analysis of counterweight B is shown in Figure 8:

Start from scratch? FA2=G0+3T2? ⑤

f’A2? Loa = B2? lOB? ⑥

N2+FB2=G B? ⑦

The substitution value N2 = P2S = 5.6×104pa× 0.02m2 =1120n.

T2 = T′2,FA2 = F′A2,FB2 = F′B2,

T2=Pv=20W0. 1m/s=200N can be obtained from the image.

That is, 5G b -2G0= 100N.

The solutions are G0= 100N and G B = 1400N.

When lifting the object A, the mechanical efficiency of the pulley block is η=W useful W total =W useful W useful +W quantity =GhGh+G moving h=G A G A +G moving =100N100N+65438×100% ≈ 965438+.

(2) The gravity of object C is G C =5G0=5× 100N=500N.

∫2T2+F floating =G0+G C? ⑧

The buoyancy of ∴c object is f float = G0+GC-2t2 =100n+500n-2× 200n = 200n.

The volume of the object c is V C =V row =F floating ρ water g = 200 n1.0× kg/m3×10n/kg = 0.02m3,

Is the density of object c ρ=m C V C =G C V? g = 500n 0.02 m3× 10n/kg = 2.5× 103kg/m3。

Answer: (1) The mechanical efficiency of pulley block is 91%when lifting object A;

(2) The density of object C is 2.5× 103kg/m3.