3th WEEK:ELASTICITY & HYDROSTATIC

QUESTION 1

1. The height of the mercury column in barometer is 760 mm. Find the pressure of the atmosphere in pascals.

ANSWER 1

1. p = pgh
= ( 13.6 X 10 ³ kg/ m ³ ) ( 9.80 m/s ² )( 0.760 m)
= 1.013 X 10 ⁵ Pa.

QUESTION 2

2. A metal ball weighs 0.096N. When suspended in water it has an apparent weight of 0.071N. Find the density of the metal.


ANSWER 2 ;

2. The desired density is given by p = m/V. But since the volume V of the ball is also the volume of displaced water, the buoyant force is given by B = p water g V. Thus,
p = (mg)p water / B
= (0.096 N)(1 X 10 ³ kg/m ³ ) / ( 0.096-0.071) N
= 3840 kg/m ³ .

QUESTION 3

3. Air has a density of 1.29kg/m ³ under standard conditions. What is the mass of air in a room with dimensions
10m x 8m x 3m?


ANSWER 3


3. m = p V
V = (10 m)(8 m)(3 m) = 240 m ³
m = (1.29 kg/m ³ )(240 m ³ )
= 310 kg.

QUESTION 4

4. Approximately how large a force is required to stretch a 2.0 cm diameter steel rod by 0.01 percent?
Y= 195000 M Pa


ANSWER 4

4. F = AY( ∆ L/L )
= ∏(0.01) ² (195 X 10 ⁹ ) (10 ⁻⁴)
= 6100 N


QUESTION 5

5. A copper wire 2.0 m long and 2 mm in diameter is stretched 1 mm. What tension is needed? Young's modulus for copper is 117600 M Pa


ANSWER 5

5. F/A = Y ( ∆ L/ L)
F = Y ( ∆ L/ L) A
= (117.6 X 10 ⁹ ) 0.001 / 2 ( ∏ ( 0.001 ) ² )
= 184.7 N.