QUIZ 11

1. How much weight must be put on a 1.5-kg wood block with density of 0.4 x 10³ kg/m³ that the block is just submerged in the water?

Solution.

We find the volume of the block from V = m_wood/r_wood.

When the block is just submerged we have            

                  F_B - m_woodg - W_add = 0,

                 r_watergV_wood - m_woodg  - W_add = 0;

                 r_watergm_wood/r_wood.- m_woodg  - W_add = 0;

                (10³kg/m)(9.80m/s²)(1.5kg)/(0.4 x 10³ kg/m³) - (1.5kg)(9.80m/s²) - W_add = 0,

 which gives W_add = 22.05N.

2.What is the speed of flow of water from a faucet if the pressure head is 12.0m?

   Solution.

If we choose the initial point at the pressure head where is not moving and the final point at the faucet, from Bernoulli's equation we have

                 P₁ + 1/2rv₁² + rgh₁ = P₂ + 1/2rv₂² + rgh₂;       

                 P_atm + 0 + (1.00 x 10³kg/m³)(9.80m/s²)(12.0m) = P_atm + 1/2(1.00 x 10³kg/m³)v₂²,  

which gives v₂ =  15.3m/s.