Question

A rubber cord catapult has cross-sectional area $$25\,m{m^2}$$  and initial length of rubber cord is $$10\,cm.$$  It is stretched to $$5\,cm$$  and then released to project a missile of mass $$5\,gm.$$  Taking $${Y_{{\text{rubber}}}} = 5 \times {10^8}\,N/{m^2}.$$     Velocity of projected missile is

A. $$20\,m{s^{ - 1}}$$
B. $$100\,m{s^{ - 1}}$$
C. $$250\,m{s^{ - 1}}$$  
D. $$200\,m{s^{ - 1}}$$
Answer :   $$250\,m{s^{ - 1}}$$
Solution :
Young’s modulus of rubber,
$${Y_{{\text{rubber}}}} = \frac{F}{A} \times \frac{\ell }{{\Delta \ell }} \Rightarrow F = YA.\frac{{\Delta \ell }}{\ell }$$
On putting the values from question,
$$F = \frac{{5 \times {{10}^8} \times 25 \times {{10}^{ - 6}} \times 5 \times {{10}^{ - 2}}}}{{10 \times {{10}^{ - 2}}}} = 6250\,N$$
kinetic energy = potential energy of rubber
$$\eqalign{ & \frac{1}{2}m{v^2} = \frac{1}{2}F\Delta \ell \cr & v = \sqrt {\frac{{F\Delta \ell }}{m}} = \sqrt {\frac{{6250 \times 5 \times {{10}^{ - 2}}}}{{5 \times {{10}^{ - 3}}}}} = \sqrt {62500} \cr & = 250\,m/s \cr} $$

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Basic Physics >> Mechanical Properties of Solids and Fluids

Releted Question 1

The following four wires are made of the same material. Which of these will have the largest extension when the same tension is applied?

A. $$length=50\,cm, \,\,diameter=0.5 \,mm$$
B. $$length = 100 \,cm, \,\,diameter= 1 \,mm$$
C. $$length= 200 \,cm, \,\,diameter= 2 \,mm$$
D. $$length=300 \,cm, \,\,diameter =3 \,mm$$
Releted Question 2

A U-tube of uniform cross section (see figure) is partially filled with a liquid I. Another liquid II which does not mix with liquid I is poured into one side. It is found that the liquid levels of the two sides of the tube are the same, while the level of liquid I has risen by $$2 \,cm.$$  If the specific gravity of liquid I is $$1.1,$$  the specific gravity of liquid II must be-
Mechanical Properties of Solids and Fluids mcq question image

A. $$1.12$$
B. $$1.1$$
C. $$1.05$$
D. $$1.0$$
Releted Question 3

A homogeneous solid cylinder of length $$L\left( {L < \frac{H}{2}} \right),$$   cross-sectional area $$\frac{A}{5}$$ is immersed such that it floats with its axis vertical at the liquid-liquid interface with length $$\frac{L}{4}$$ in the denser liquid as shown in the figure. The lower density liquid is open to atmosphere having pressure $${P_0}.$$ Then density $$D$$ of solid is given by-
Mechanical Properties of Solids and Fluids mcq question image

A. $$\frac{5}{4}d$$
B. $$\frac{4}{5}d$$
C. $$4d$$
D. $$\frac{d}{5}$$
Releted Question 4

A large open tank has two holes in the wall. One is a square hole of side $$L$$ at a depth $$y$$ from the top and the other is a circular hole of radius $$R$$ at a depth $$4y$$  from the top. When the tank is completely filled with water, the quantities of water flowing out per second from both holes are the same. Then, $$R$$ is equal to-

A. $$\frac{L}{{\sqrt {2\pi } }}$$
B. $$2\pi L$$
C. $$L$$
D. $$\frac{L}{{2\pi }}$$

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