Answer:
In a two particle system, the center of mass lies on the center of the line joining the two particles.
<span>A wedge is a double sided screw basically. Therefore, the difference is the number of sides.</span>
Answer: electric field
Explanation: when a charge is placed in space, it alters the space around it by creating an electric field.
This electric field has the ability to exert a force (f) on any test charge(q) placed within this vicinity.
This is the reason why a charge can either attract or repel another charge.
Answer: Phillipe wins the race
Explanation:
Given
Length of track=20 m
Velocity of Phillippe=2 m/s w.r.t sidewalk
Velocity of sidewalk=1.5 m/s
Rena velocity=2 m/s
Therefore absolute velocity of P is 2+1.5=3.5 m/s
Time taken by phillippe 
=5.714+10=15.714 s
Time taken by rena

as the time taken by phillippe is less than rena therefore Phillippe wins the race.