Teaching Relative Motion

Day before yesterday , i start teaching relative motion.It is very topic to teach.For last years i used to teach by dividing this topic in two parts -Resultant and relative velocity.this year i try to do experiment to teach it as a simple concept by using one general equation .Because of this i was able to create interest of students in topic.In physics or more general in life nothing is absolute.Any event which happens in space time may be in physics or in life are relative.

The position, velocity and acceleration of a particle are relative terms and are defined with respect to an assumed reference point. The reference point may be a t rest (that is, stationary on the ground), moving with constant velocity or uniform acceleration. So far, we have studied the motion of a particle with respect to a stationary point. In this section we will modify the equations of kinematics with respect to a moving point.

Suppose two objects A and B are in motion with respect to a fixed origin O as well as with respect to each other. Their position vectors at any time t are rA/O and rB/O respectively and the vector joining B with A is rB/A. By using vector diagram

rB/O = rB/A + rA/O (1)

On differentiating the above equation with respect to time, we get

d/dt rB/O = d/dt rB/A + d/dt rA/O

VB/O = VB/A + VA/O

Velocity of B = Velocity of B + Velocity of A

w.r.t O w.r.t. A w.r.t O

The above relation is so obvious that we could have written it without intermediate steps. In the above discussion we have used double subscripts for velocity because by doing so we can write the final equation quite easily this is because the double subscript form equality under multiplication i.e.

B/O = (B/A)(A/O)

Further, we can extend this concept when number of objects are more than two. For example

VC/D = VC/B + VB/A + VA/O

If we further differentiate equation with respect to time, we get following equation for acceleration

aB/O = aB/A + aA/O


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