December 23, 2015

Junior Inter Physics Important Questions for 2 and 4 Marks

Junior Inter Physics - System of Particles And Rotational Motion

Q: Is it necessary that mass should be present at the centre of mass of any system? (2 Marks)
A: It is not necessary that mass should be present at the centre of mass of any system. Ex: Circular ring.

Q: Why are spokes provided in a bicycle wheel? (2 Marks)
A: To ensure uniform speed of cycle wheel, it should possess large moment of inertia. To increase moment of inertia, the wheel is provided with spokes.

Q: By spinning eggs on a table top, how will you distinguish a hard boiled egg from a raw egg? (2 Marks)
A: A raw egg has some fluid in it and hard boiled egg is solid from inside. Both eggs are spinning on a table top, the fluid is thrown outwards. Then moment of inertia of raw egg is greater than boiled egg. i.e., Ir > Ib, ωr < ωb. So, raw egg will come to rest soon

Q: Why do we prefer a spanner of long arm as compared to the spanner of shorter arm? (2 Marks)
A: From the definition of torque (τ) = Force × Perpendicular distance.
For long arm spanner, perpendicular distance is large. Thus it provides more torque.

4 Marks Questions:

Q: Explain about the centre of mass of earthmoon system and its rotation around the sun.
A: The earth-moon rotates about common centre of mass. The mass of earth is about 81 times that of moon.
The centre of mass of earth-moon system locate relatively very nearer to the centre of the earth. The interaction between earth - moon does not affect the motion of centre of mass of earth-moon system. The gravitational attraction of the sun is only external force that acts on earth-moon system, centre of mass of the earth-moon system moves in
an elliptical path around the sun.

Q: Distinguish between centre of mass and centre of gravity

Answer:

Centre of mass:

1. Point at which entire mass of the body is supposed to be concentrated.
2. It may or may not lies inside the body.
3. It describes the nature of motion of the body as a whole.
4. It is independent of acceleration due to gravity.

Centre of gravity:

1. The point through which the weight of the body act.
2. It always lies inside the body.
3. It is defined to know the stability of the body.
4. It depends on acceleration due to gravity (g).

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