Phys101
Term:122
Online HW-Ch14-Lec02
Q1:
The principle of fluid pressure that is used in hydraulic brakes or lifts is that:
A.
B.
C.
D.
pressure is the same at all levels in a fluid
increases of pressure are transmitted equally to all parts of a fluid
the pressure at a point in a fluid is due to the weight of the fluid above it
increases of pressure can only be transmitted through fluid
E. the pressure at a given depth is proportional to the depth in the fluid
Ans:
B
Q2:
A hydraulic press has one piston of diameter 2.0 cm and the other
piston of diameter 8.0 cm. What force (in N) must be applied to the
smaller piston to lift an object of mass 700 kg at the larger piston?
Ans:
Pascal′ s principle: ri = 0.01 m ; ro = 0.04 m ; Fo = mg = 700 × 9.8 = 6860 N
Fi
Fo
=
Ai
Ao
⇒ Fi =
Q3:
Ai
πr1 2
⇒ Fi =
F =
mg
Ao o
πr0 2
0.012
× 6860
0.042
= 429 N
An object hangs from a spring balance. The balance indicates 30 N in air and
20 N when the object is completely immersed in water (ρ water = 1000 kg/m3).
The object is then completely immersed in another liquid with a density
ρ liquid and the new reading of the balance is 25 N. Find the density ρ liquid in
kg/m3. (Give your answer in three significant figures form) (Hint: one of the
methods is to find the expression of the volume from the first case and
use it in the second one)
𝐀𝐧𝐬:
Real weight: Wr = 30 N
In water ∶ Ww = 20 N ∶ ρw = 1000 kg/m3
Fb = Wr − Ww = 10 N
⇒ ρw Vg = 10 N ⟶ (1)
In liquid ∶ the new apparent weight is W𝑙 = 25 N
the buoyant force is ρl Vg = Wr − W𝑙
⇒ ρl Vg = 30 − 25 = 5 N ⟶ (2)
Dividing equ. (2) by equ. (1):
⇒ ρ𝑙 = 500 kg/m3
KFUPM-Physics Department
ρl Vg
5
=
⇒ ρ𝑙 = 0.5 ρw = 0.5 × 1000
ρw Vg
10
1