Amotorcyclist drove 7 km at 57km/h and then another 7 km at 81 km/h. what was the average speed?

The average speed of the car is 66.9 km/h

Explanation:

Here distance covered with the speed 57 km/h=7 km  

distance covered with the speed of 81 km/h=7 km

Average speed is equal to the ratio of total distance to the total time.

total distance= 7 + 7= 14 km  

time taken to cover the first 7 km= 7/57 h  

time taken to cover the second part of the journey = 7/81 h

average speed =  

Shortcut:

When equal distances are covered with different speeds average speed=2 ab/(a+b) where a and b are the variable speeds in the phases.

electric field is a function of 1/r^2

yes, the force per unit charge for a point charge is inverse square law.

3. dark matter

empty space could also be a simple vacuum