1. Dyne - unit of force in CGS system. it is equal to 10⁻⁵ of a Newton.
2. 4 km towards North in one hour:
and then 3 km towards East in next one hour.
Final displacement of the person = √4²+3² = 5 km from the point of start.
The velocity = total displacement / total time = 5 km /2 hours = 2.5 km/hr
if you want the average speed then it is = total distance / total time
= 7 km /2 hours = 3.5 kmhr
3. We could directly say that
The car travels a uniform velocity of v1 over a distance x and a uniform velocity v2 over distance y. let the times of travel be t1 and t2 respectively. We assume that the car travels in the same direction throughout.
average velocity = ( v1 * t1 + v2 * t2 ) / (t1 + t2)
= ( v1 * x/v1 + v2 * y/v2 ) / (x/v1 + y/v2)
= (x + y) (v1 v2) / (v1 y + v2 x)
if x = y, then,
v = 2 x v1 v2 / (v1 + v2) x
= 2 / [ 1/v1 + 1/v2 ]
or, 2/v = 1/v1 + 1/v2