Vidaara.orgClass 9 · Physics
CodeVID-P9-03-TPB-01
Thrust, Pressure & Buoyancy — Assignment
Name: ____________________
Roll No.: __________
Date: ____________
General Instructions
- All questions are compulsory.
- Section A carries 1 mark each, Section B 2 marks, Section C 3 marks and Section D 5 marks.
- Show all working for Sections B, C and D. Take density of water $= 1000\ \text{kg/m}^3$ and $g = 10\ \text{m/s}^2$ unless stated otherwise.
Section A — Multiple Choice Questions
5 × 1 = 5 marks
1.
The SI unit of pressure is the:
- A.newton
- B.pascal
- C.joule
- D.watt
2.
Thrust acts ___ to a surface.
- A.parallel
- B.perpendicular
- C.at $45^\circ$
- D.in a circle
3.
Buoyant force on a submerged body acts:
- A.downward
- B.sideways
- C.upward
- D.in all directions equally
4.
Relative density has the unit:
- A.$\text{kg/m}^3$
- B.$\text{g/cm}^3$
- C.pascal
- D.no unit
5.
An object sinks in water when its density is:
- A.less than water's
- B.equal to water's
- C.greater than water's
- D.zero
Section B — Short Answer (2 marks)
3 × 2 = 6 marks
6.
Define pressure and give its SI unit.
7.
A force of $150\ \text{N}$ acts on an area of $3\ \text{m}^2$. Find the pressure.
8.
State Archimedes' principle.
Section C — Short Answer (3 marks)
2 × 3 = 6 marks
9.
A body of mass $500\ \text{g}$ has a volume of $250\ \text{cm}^3$. Find its density and relative density.
10.
An object weighs $30\ \text{N}$ in air and $25\ \text{N}$ in water. Find the buoyant force and the weight of water displaced.
Section D — Long Answer (5 marks)
1 × 5 = 5 marks
11.
Explain, using pressure and Archimedes' principle, (a) why a sharp knife cuts better than a blunt one and (b) why a ship made of steel floats while a steel nail sinks.
Answer Key
Section A — Multiple Choice Questions
- (B) pascal
- (B) perpendicular
- (C) upward
- (D) no unit
- (C) greater than water's
Section B — Short Answer (2 marks)
- Pressure is thrust (force) per unit area, $P = \dfrac{F}{A}$; SI unit is the pascal ($\text{N/m}^2$).
- $P = \dfrac{150}{3} = 50\ \text{Pa}$.
- A body immersed in a fluid experiences an upward buoyant force equal to the weight of the fluid it displaces.
Section C — Short Answer (3 marks)
- $\rho = \dfrac{500}{250} = 2\ \text{g/cm}^3 = 2000\ \text{kg/m}^3$; R.D. $= \dfrac{2}{1} = 2$.
- Buoyant force $= 30 - 25 = 5\ \text{N}$; weight of water displaced $= 5\ \text{N}$.
Section D — Long Answer (5 marks)
- (a) A sharp edge has a tiny area, so for the same force $P = F/A$ is very large and it cuts. (b) A hollow ship has low average density and displaces enough water for the upthrust to equal its weight, so it floats; a solid nail has density greater than water, displaces little water, and sinks.
Generated by Vidaara.org · Assignment VID-P9-03-TPB-01 · vidaara.org