Time Interval is Frame Independent ( In Newtonian Mechanics)

In Newtonian mechanics or when frames are inertial ie they are no accelerating wrt to each other the Time interval between two events are independent of the frame of reference

For Example

A motorboat going downstream overcame a raft at a point A; T=60 min later it turned back and after sometime passed the raft at a distance l=6.0 km from point A. Find the flow velocity assuming the duty of the engine to be constant.[Irodov Q1.1]

Solution

This problem can be easily solved using the the above tip

Consider two frames

1. Frame of reference moving with the stream velocity

2. Frame of reference wrt to river bank

In the first frame of reference, the raft is stationary , hence boat will take exactly same time to return to the boat and hence a total time of 2T for its entire motion

In the Second frame of reference, the raft has moved a distance l in the same time 2T( from above tip)

Therefore,

Speed of raft = Flow Velocity = l/2T= 3km/ h