Tiêu đề Notes_Short Answers_09.Center Of Mass.pdf ✅

1. Can the centre of mass of a body be at a point outside the body? Ans: Yes [Hint:  for the circle.] 2. If all the particles of a system lie in X-Y plane, is it necessary that the centre of mass be in X- Y plane? Ans: Yes [Sol. As zcm = m m z m z ....... m z 1 1 2 2 n n     All the particles will have zero z coordinate. So zcm = 0.] 3. If all the particle of a system lie in a cube, is it necessary that the centre of mass be in the cube? Ans: Yes 4. The centre of mass is defined as   i i i m r m 1 R   . Suppose we define "centre of charge" as   i C i ir Q 1 R   q where qi represents the i th charge placed at ir and Q is the total charge of the system. a. Can the centre of charge of a two-charge system be outside the line segment joining the charges? b. If all the charges of a system are in X-Y plane, is it necessary that the centre of charge be in X- Y plane? c. If all the charges of a system lie in a cube, is it necessary that the centre of charge be in the cube? Ans: Yes, Yes, No [Sol. (a) Taking example xcm = q a q 4 4   = – a (b) As z coordinate of all charge is zero centre of charge lie in X-Y plane. (c) Because charge can be positive or negative.] 5. The weight Mg of an extended body is generally shown in a diagram to act through the centre of mass. Does it mean that the earth does not attract other particles? Ans: No [Sol. Earth attracts all the particles but the effective force can be assumed to be at the centre of mass.] x – 5q q 4a y COM_MOMENTUM_COLLISION